Canceling the AMO

by Judith Curry

Conclusion from Michael Mann’s new paper:  “We conclude that there is no compelling evidence for internal multidecadal oscillations in the climate system.”

Michael Mann’s most recent paper:

Multidecadal climate oscilliations during the past millennium driven by volcanic forcing

Abstract. Past research argues for an internal multidecadal (40- to 60-year) oscillation distinct from climate noise. Recent studies have claimed that this so-termed Atlantic Multidecadal Oscillation is instead a manifestation of competing time-varying effects of anthropogenic greenhouse gases and sulfate aerosols. That conclusion is bolstered by the absence of robust multidecadal climate oscillations in control simulations of current-generation models. Paleoclimate data, however, do demonstrate multidecadal oscillatory behavior during the preindustrial era. By comparing control and forced “Last Millennium” simulations, we show that these apparent multidecadal oscillations are an artifact of pulses of volcanic activity during the preindustrial era that project markedly onto the multidecadal (50- to 70-year) frequency band. We conclude that there is no compelling evidence for internal multidecadal oscillations in the climate system.

From the Penn State press release Apparent Atlantic Warming Cycle an Artifact of Climate Forcing:

<begin quote>

“It is somewhat ironic, I suppose,” said Michael E. Mann, distinguished professor of atmospheric science and director, Earth System Science Center, Penn State. “Two decades ago, we brought the AMO into the conversation, arguing that there was a long-term natural, internal climate oscillation centered in the North Atlantic based on the limited observations and simulations that were available then, and coining the term ‘AMO.’ Many other scientists ran with the concept, but now we’ve come full circle. My co-authors and I have shown that the AMO is very likely an artifact of climate change driven by human forcing in the modern era and natural forcing in pre-industrial times.”

The researchers previously showed that the apparent AMO cycle in the modern era was an artifact of industrialization-driven climate change, specifically the competition between warming over the past century from carbon pollution and an offsetting cooling factor, industrial sulphur pollution, that was strongest from the 1950s through the passage of the Clean Air Acts in the 1970s and 1980s. But they then asked, why do we still see it in pre-industrial records?

Their conclusion, reported today (Mar. 5) in Science, is that the early signal was caused by large volcanic eruptions in past centuries that caused initial cooling and a slow recovery, with an average spacing of just over half a century. The result resembles an irregular, roughly 60-year AMO-like oscillation.

“Some hurricane scientists have claimed that the increase in Atlantic hurricanes in recent decades is due to the uptick of an internal AMO cycle,” said Mann. “Our latest study appears to be the final nail in the coffin of that theory. What has in the past been attributed to an internal AMO oscillation is instead the result of external drivers, including human forcing during the industrial era and natural volcanic forcing during the pre-industrial era.”

<end quote>

Mann has a blog post on the paper at RealClimate 

Wow.  In one fell swoop, the pesky problems of the ‘grand hiatus’ in the mid 20th century,  debates over the attribution of 20th century warming and the role of multidecadal internal variability, and the difficulty of attributing the recent increase in Atlantic hurricane activity to AGW, all go away.  Brilliant!  Almost as ‘brilliant’ as the Hockey Stick.

As it happens, I have a draft chapter in my pocket from a report I’m writing, I’ve excerpted the relevant text below (apologies for not having links to the references):

9.Atlantic Multidecadal Variability

 The Atlantic Ocean is particularly important to the global ocean circulation due to the existence of North Atlantic Deep Water (NADW) formation in the northern North Atlantic, a vital component of the Atlantic Meridional Overturning Circulation (AMOC). The AMOC includes the northward flow of warm salty water in the upper Atlantic and the southward flow of the transformed cold fresh NADW in the deep Atlantic, which is a major driver of the substantial northward Atlantic heat transport across the equator.

Basin‐scale multidecadal fluctuations have been observed in the Atlantic sea surface temperature (SST). The large‐scale multidecadal variability observed in the Atlantic has been referred to as the Atlantic Multidecadal Oscillation (AMO). The multidecadal character of the AMO is distinguished from interannual ocean variability forced by the leading mode of atmospheric circulation variability over the North Atlantic, the North Atlantic Oscillation (NAO). The term Atlantic Multidecadal Variability (AMV) is often used, because the observed multidecadal fluctuations in the Atlantic may not be an oscillation at a single frequency but consist of a broader band of low‐frequency signals.

9.1 Index definition and climatology

The Atlantic Multidecadal Oscillation (AMO) is associated with basin-wide SST and sea level pressure (SLP) fluctuations. For the positive AMO phase, this is sometimes presented as an almost uniform warming of the North Atlantic. The traditional AMO index associates the positive AMO phase with a pattern of horseshoe-shaped SST anomalies in the North Atlantic with pronounced warming in the tropical and parts of the eastern subtropical North Atlantic, an anomalously cool area off the U.S. East Coast, and warm anomalies surrounding the southern tip of Greenland.

Figure 9.1. Atlantic Multidecadal Oscillation Index (1880-2018).

The past 100 to 150 years of Atlantic SSTs are characterized by a net century-long rise and periods of multidecadal warming and cooling. However, the rationale for the ‘Trend+AMO’ separation is confounded by lack of linearity in the global warming trend, and hence detrending aliases the AMO index. The nonlinearity is particularly pronounced during the period 1945-1975, when global SSTs showed a slight cooling trend.

To address the ambiguities associated with detrending in formulating the AMO index, Johnstone (2017) has formulated an Atlantic ‘Arc’ Index, based on the leading Principal Component of north Atlantic SST variability (60°N to 0°, 70°W-0°W). The Atlantic ‘Arc’ SST index reflects coherent variability within a basin-scale arc-shaped pattern (sometimes referred to as a ‘horseshoe’), a signature of the AMO that encompasses the tropical North Atlantic, the midlatitude eastern boundary and much of the subpolar north (Fig. 9.2).

Figure 9.2. The Arc pattern is delimited by the bold black line, which encompasses the tropical North Atlantic, the midlatitude eastern boundary and much of the subpolar north Atlantic.  From Johnstone.

The Arc pattern is recognized as a spatial signature of the AMO, identified with coupled ocean-atmosphere variability, and is closely related to a ‘tripole’ pattern of SST response to the NAO. The Arc Index displays a net warming in addition to multidecadal period recognized as a cool phase of the AMO (Figure 9.3). Since the Arc Index combines both the AMO variability and the overall warming trend, it is more usefully interpreted as defining multidecadal regimes and shifts (see section 9.4).

The Arc Index (Figure 9.3) shows abrupt shifts to the warm phase in 1926 and 1995, consistent with the conventional AMO analysis in Figure 9.1. Johnstone’s analysis indicates a shift to the cold phase in 1971, which differs from the analysis shown in Figure 9.1 that indicates the shift to the cold phase in 1964. The AMO index of Klotzbach and Gray (2008) also indicates a shift to the cold phase in 1970.

Figure 9.3. Time series of the Atlantic Arc Index from 1880 through early 2018. From Johnstone.

9.2 Paleoclimate reconstructions

The brevity of available instrumental data limits our understanding of the Atlantic Multidecadal Variability (AMV). Paleoclimate proxy‐derived reconstructions of AMV‐related signals that extend beyond the instrumental era provide an important basis for understanding the nature and stationarity in time of the AMV.

The recent Wang et al. (2017) AMV reconstruction using terrestrial proxy records (tree rings, ice cores, etc.) over the past 1,200 years has both broad spatial coverage and high temporal (annual) resolution. Wang et al. (2017) found that large volcanic eruptions and solar irradiance minima induce cool phases of Atlantic multidecadal variability and collectively explain about 30% of the variance in the reconstruction on timescales greater than 30 years. They isolated the internally-generated component of Atlantic multidecadal variability, which they define as the AMO. They found that the AMO is the largest contributor to Atlantic multidecadal variability over the past 1,200 years.

Zhang et al. (2019) provides a summary of studies that have analyzed paleoclimate data to investigate whether AMV is internally or externally driven. Over the past 12 centuries, the reconstructed solar and volcanic forcing do correlate with the Wang et al. (2017) AMV reconstruction, but their combined contribution explains less than one third (28%) of the total AMV variance; the reconstructed AMV is dominated by internal variability. The internal variability component of the Wang et al. (2017) AMV reconstruction also reveals significant signals at multidecadal timescales above a red noise background and its amplitude during the preindustrial period, especially before the Little Ice Age, is on the same order as that found in the instrumental AMV index. Paleo proxies also supportthe existence of an AMOC‐AMV linkage over the past several centuries.

Knudsen et al (2010) used paleoclimatic data to show that distinct ~55-70 year oscillations characterized the North Atlantic ocean-atmosphere variability over the past 8,000 years (the Holocene). The Holocene AMO signal appears to have been quasi-periodic and the associated climate response to have been of highly variable intensity, both in time and space. In the tropical Atlantic, the AMO response signal was generally relatively weak during the Northern Hemisphere warming of the Holocene thermal maximum (HTM) between 5,500 and 9,000 BP, after which it picked up in intensity. Through the past 8,000 years, minor shifts appear to have occurred in the dominating period within the 55- to 70-year band. The dominant oscillation period in the interval of 5,500–8,000 BP was ~65 years, whereas it shortened somewhat between 5,500 BP (before present) and ~2,700 BP (55–60 years). The period of the dominant oscillations increased slightly again after ~2,700 BP (65–70 years), but the oscillations were generally not as well defined as during the early Holocene, when the AMO period bandwidth appears to have been narrower.

Knudsen et al (2010) provides the following additional insights. The AMO response signal exhibits a general shift in its pattern within the last 8,000 years, as the signal was most pronounced in the Arctic during the HTM, whereas in the tropics its maximum was generally reached after the HTM. Between 2,000 and 3,500 BP, there was a statistically significant recurrence of multidecadal oscillations in the Arctic. This interval overlaps with a part of the neo-glaciation between 2,000 and 3,000 BP, which was characterized by relatively high SST and generally warmer, and particularly unstable, climate conditions in parts of the northern North Atlantic region. Such conditions meant that the Arctic sites temporarily became more sensitive to multidecadal SST oscillations, possibly due to an associated reduction in Arctic sea-ice cover. However, the major changes in North Atlantic circulation patterns that followed the neo-glaciation in both hemispheres led to a generally weakened AMO response signal after ~2,000 BP. This change was accompanied by a distinct SST decline between 2,000 and 500 BP in some parts of the northern North Atlantic.

9.3 Climate dynamics

Despite ongoing debates about the climate dynamics of the AMV, it is generally accepted that the AMV represents a complex conflation of natural internal variability of the Atlantic Meridional Overturning Circulation (AMOC), natural red-noise stochastic forcing of the ocean by the atmosphere (primarily the NAO), and external forcing from volcanic events, aerosol particles and greenhouse gases.

9.3.1 AMO

The Atlantic Multidecadal Oscillation (AMO) is the most prominent mode of multi-decadal Atlantic variability; however the AMO’s physical origins remain a topic of ongoing debates. Observed AMO SST changes have long been attributed to slow variations in northward upper-ocean heat transport by the AMOC. Ocean processes offer a plausible mechanism for large multidecadal climate variations; such inferences are based largely on climate model simulations due to the short record of AMOC circulation that begins only recently in 2004.

Several additional hypotheses for AMV mechanisms have been proposed. Anthropogenic aerosols have been hypothesized to be a prime driver of the observed AMV. The argument is that an increase in the linearly detrended AMV SST index is forced by the increased downward shortwave radiative heat flux induced by the decreased anthropogenic aerosols through their interaction with clouds. However, the observed decline in the subpolar AMV SST signal over the most recent decade is inconsistent with the recently observed change (a slight decrease) in anthropogenic aerosols over the North Atlantic region. As summarized by Zhang et al. (2019), the hypothesis that changes in external radiative forcing is a prime driver of AMV disagrees with many observed key elements of AMV.

Using observations and models, Delworth et al. (2017) examined the relationship between the North Atlantic Oscillation (NAO) and Atlantic decadal SST variations. Consistent with many previous studies, on short time scales NAO-related surface heat flux anomalies drive a tripole pattern of SST anomalies in the Atlantic. On decadal and longer time scales, there is a lagged response of the ocean to the NAO fluxes, with the AMOC playing a prime role in modulating meridional oceanic heat transport and generating an AMO-like SST response. A prolonged positive phase of the NAO enhances the AMOC after a decadal-scale delay. Delworth et al. (2017) found that decadal-scale SST variability in the subpolar and tropical North Atlantic are well correlated. While ocean dynamics plays a crucial role for decadal-scale SST variability in the extratropical North Atlantic, the results of this study suggest that its direct influence in the tropical North Atlantic appears to be smaller, with local air–sea fluxes playing a larger role.

Lin et al. (2019) argues for two different sources for AMO variability, identifying 50–80 year and 10–30 year AMOs that are associated with different underlying dynamics. Associated with a positive AMO at 50–80 year period is enhanced westerlies north of 60N but weakened between 40-60N, which is dynamically consistent with an enhanced polar vortex and linked to variability in the Pacific. The atmospheric variability associated with the 10–30 year AMO is a zonally asymmetric pattern with blockings prevailing over high latitude North Atlantic and cyclonic anomaly over subtropical North Atlantic, which is independent from the variability over Pacific sector. While the 10–30 year AMO may be linked directly to the dynamics over the tropical Atlantic, the 50– 80 year AMO is heavily related to the cross-basin interaction between the North Atlantic and the Greenland-Iceland-Norwegian Seas. (Note: consistent with the stadium wave.)

Willis et al. (2019) identified a tripolar SST anomaly between the Gulf Stream, the subpolar gyre, and the Norwegian seas that varies on 8–20 yr time scales. Their results suggest that the AMO is confined to the subpolar North Atlantic, while the tropical Atlantic varies primarily on shorter (intradecadal) time scales. In another study, Muller, Curry et al. (2013) identified a strong narrow peak in the AMO with period of ~ 9 yrs.

Nigam et al. (2018) showed that the decadal component of the AMO is closely related to the Gulf Stream variability: the northward shift of the Gulf Stream (GS) path coincides with the cold AMO phase with cold SST anomalies in the subpolar gyre. The GS’s northward shift is preceded by the positive phase of the low-frequency NAO and followed by a positive AMO tendency by 1.25 and 2.5 years, respectively. The temporal phasing is such that the GS’s northward shift is nearly concurrent with the AMO’s cold decadal phase (cold, fresh subpolar gyre).

Kwon et al. (2019) found that the evolution of SST anomalies is very different in the warm versus the cold phase of the AMV. For the AMV warm phase, the warm SST anomalies in the western subpolar gyre are damped by the surface heat flux, and thus pose anomalous heating in the lower troposphere and reduce the overall meridional gradient of the atmospheric temperature. Consequently, the storm track activity weakens. As the blocking substantially influences the seasonal mean atmospheric circulation, the negative phase of NAO dominates at the same time.

RuprichRobert and Cassou (2014) found that the full life cycle of AMOC/AMV events relies on a complex time-evolving relationship with both North Atlantic Oscillation (NAO) and East Atlantic Pattern (EAP) (Figure 9.8). The AMOC rise leading to a warm phase of AMV is statistically preceded by wintertime NAO+ and EAP+ from lag -40/-20 yrs. Associated wind stress anomalies induce an acceleration of the subpolar gyre (SPG) and enhanced northward transport of warm and saline sub- tropical water. Concurrent positive salinity anomalies occur in the Greenland–Iceland–Norwegian Seas in link to local sea-ice decline; those are advected by the Eastern Greenland Current to the Labrador Sea participating to the progressive densification of the SPG and the intensification of ocean deep convection leading to AMOC strengthening. From lag -10 yrs prior to an AMOC maximum, the opposite relationship is found with the NAO for both summer and winter seasons. NAO- acts as a positive feedback for the full development of the AMV through surface fluxes but, at the same time, prepares its termination through negative retroaction on AMOC. Relationship between EAP- and AMOC is also present in summer from lags -30/+10 yrs, while winter EAP- is favored around the AMV peak.

All together, the combined effect of NAO and EAP are responsible for an irregular and damped mode of variability of AMOC/AMV that takes about 35–40 years to build up and about 15–20 years to dissipate. In addition to the direct NAO-/EAP- action, the termination of AMOC/AMV events is also induced by the advection of anomalous fresh water from the subtropical North Atlantic basin along the mean western boundary ocean circulation, and also from the Arctic due to considerable ice volume loss associated with overall atmospheric warmer conditions when AMOC is enhanced.

Figure 9.8 Schematic diagram for an AMOC/AMV positive event. RuprichRobert and Cassou (2014)

Update:  An excellent new publication was pointed out to me on twitter that supports the general conclusions of my write-up

9.4 Recent shifts

As summarized by Robson et al. (2012), in the mid-1990s the subpolar gyre of the North Atlantic underwent a remarkable rapid warming, with sea surface temperatures increasing by around 1.8oC in just 2 years. This rapid warming followed a prolonged positive phase of the North Atlantic Oscillation (NAO), but also coincided with an unusually negative NAO index in the winter of 1995/96. By comparing ocean analyses and carefully designed model experiments, they showed that this rapid warming can be understood as a delayed response to the prolonged positive phase of the NAO and not simply an instantaneous response to the negative NAO index of 1995/96. Furthermore, they inferred that the warming was partly caused by a surge and subsequent decline in the meridional overturning circulation and northward heat transport of the Atlantic Ocean.

Robson et al. (2016) showed that since 2005, a large volume of the subpolar 
North Atlantic Ocean has cooled significantly, reversing the previous warming trend. By analyzing observations 
and a state-of-the-art climate model, they showed that this cooling is consistent 
with a reduction in the strength of the ocean circulation and heat transport, 
linked to record low densities in the deep Labrador Sea. The low density in the deep Labrador Sea is primarily due to deep ocean warming since 1995, but a long-term freshening also played a role. They inferred that the observed cooling of a large region of the upper North Atlantic Ocean since 2005 cannot be explained as a direct response to changes in atmospheric circulation over the same period.

Johnstone (2017) describes a ‘coupled shift model’ of low-frequency North Atlantic climate 
change, based on abrupt transitions between quasi-stable sea surface temperatures and coupled 
atmospheric circulations. This hypothesis describes recurrent step-like changes in North Atlantic SST, wherein high-amplitude SST perturbations are occasionally maintained as anomalous multidecadal climate states by positive atmosphere-ocean feedbacks. Statistical evidence is presented that low-frequency SST changes were not gradual processes as commonly described, but through a series of short, discrete events, characterized by abrupt ~1 year step-like shifts that separate longer multidecadal periods of relatively little change.

The strong Atlantic warming of the mid-1990s (Figure 9.1), which is represented by filtered AMO indices as a gradual process lasting a decade or more, can be traced to an abrupt and remarkably continuous rise in basin-scale SST, beginning in October-November 1994, and essentially accomplished as a +0.8°C SST change across most of the North Atlantic by July 1995 (Figure 9.3). The abrupt warming of 1994-95 went undamped in successive months, during the next few years, and fully through the subsequent two decades up to the present,  rapidly introducing a new warmer climate state. The basin-scale expanse of the 1995 shift can be seen in the abrupt shift in monthly SST anomalies over both the subpolar North Atlantic (50-60oN) and the subtropical margins of NW Africa, which warming together in nearly simultaneous fashion (Arc Index, Figure 9.2).

Shifts appear in the annual Arc SST record (Fig. 9.3) as pronounced year-to-year jumps in 1925-26 (+0.5°C), 1970-71 (-0.3°), and 1994-95 (+0.6°) that were followed by multidecadal persistence of similar 
anomalies with respect to prior years (1926-1970: +0.5°C, 1971-1994: -0.2°C, and 1995-2014: +0.5°). Each of these intervals lacks a significant linear Arc SST trend, suggesting that large 
transient climate changes were followed by restabilization of the upper-ocean heat balance and persistence of new anomalous conditions over years to multiple decades. Arc SST changes 1926, 1971 and 1995 occurred with moderate same-signed anomalies of winter (October-March) Niño 3.4 SST, suggesting a systematic role for 
ENSO in the generation of low-frequency North Atlantic climate changes.

A more specific regional indicator appears in the correspondence of Arc SST shifts with high-amplitude SST changes off northwest Africa, which peaked September 1925, August 1970 and November 1994. West African 
SSTs are a prominent component of the Atlantic Multidecadal Mode (AMM), which may serve as a bridge across time scales, sustaining SST perturbations as 
sustained climate anomalies.

A physical implication of the shift model is that low-frequency climate changes occur through occasional pulses of upper-ocean heat uptake and release, rather than gradual or cumulative processes.

Atmosphere-ocean conditions leading to 1994-95 warming share notable similarities with the warming of 1925-26 and (oppositely) with the cooling of 1970-1971, suggesting predictability of major North Atlantic climate shifts. All three events were preceded during the prior 2-3 years by uniquely strong sea level pressure (SLP) anomalies of opposite sign around the Norwegian Sea within a broader NAO-like pattern. In each case, the transitional winter featured moderate ENSO conditions favorable to the developing temperature change, and each shift was distinguished by extreme local SST changes off NW Africa.

Historically, Atlantic shifts have been marked by extreme short-term SST changes off NW Africa: behavior that is 
not currently evident, as subtropical and tropical areas of the Arc remain in a warm state begun in 1995. 
However, it is notable that subpolar SSTs from 50-60N show evidence of abrupt cooling since 2015 (Fig. 9.10), behavior suggestive of a ‘partial’ shift that might soon involve the broader North Atlantic, including the tropics. The current divergence between subpolar and tropical North Atlantic SST is potentially analogous to 
behavior seen during the late 1960s-early-1970s, when rapid subpolar cooling in 1969-70 slightly 
preceded the sharp 1971 drop in tropical SST. Based on historical patterns, an abrupt shift to cooler conditions may be imminent, although the unusually long regime from 1926 to 1970 suggests that a substantial delay of up to 10-20 years may also be plausible.

Figure 9.10. Annual SST anomalies for the subpolar and tropical North Atlantic. Subpolar SST (blue, 60°-50°N) 
displays a sharp drop and persistently cool conditions since 2015 (20°N- 0°, red). Similar divergence around 1970 might provide an early indication of tropical and broader North Atlantic 
cooling within the next few years.

To what extent was the 1995 shift in the AMO predictable by climate models? Msadek et al. (2014) summarize the decadal prediction experiments conducted using the GFDL Climate Model. Initializing the model produces high skill in retrospectively predicting the mid-1990s warming, which is not captured by the uninitialized forecasts. All hindcasts initialized in the early 1990s show a warming of the SPG (subpolar gyre); however, only the ensemble-mean hindcasts initialized in 1995 and 1996 are able to reproduce the observed abrupt warming and the associated decrease and contraction of the SPG. The enhanced Atlantic decadal prediction skill is achieved primarily by initializing AMOC anomalies, instead of predicting AMOC anomalies at northern high latitudes.

In contemplating a possible future shift to the cold phase of the AMO, it is instructive to consider the prior shift to the cold phase that occurred in the 1960’s and early 1970’s, when the sea surface temperatures in the North Atlantic Ocean cooled rapidly. Hodson et al. (2014) demonstrated that the cooling proceeded in several distinct stages:

  • 1964–68: The initial cooling is largely confined to the Nordic Seas and the Gulf Stream Extension. There are no notable atmospheric circulation anomalies during this period, aside from a small low MSLP anomaly over the Arctic in October–June.
  • 1968–72: As the cooling progresses, cool anomalies extend to cover much of the subpolar gyre (SPG) and northern midlatitudes. There is a hint of low SLP anomalies over North Africa, but the most prominent evidence of circulation anomalies is an anti-cyclonic anomaly in July–September, which extends over northern Europe and into Asia.
  • 1972–76: The cool anomalies reach their maximum magnitude and spatial extent during this period. The western part of the subtropical North Atlantic does not show a significant cooling, resulting in a tripole (or horseshoe) pattern. The pattern of SLP anomalies projects on the positive phase of the NAO.

9.5 Climate model simulations

Many coupled climate models simulate Atlantic Decadal Variability that is consistent in some respects with the available observations. However, the mechanisms differ strongly from model to model, and the inadequate observational database does not allow a distinction between ‘realistic’ and ‘unrealistic’ simulations (Latif and Keenlyside, 2011). Ruiz-Barradas et al. (2013) examined historical simulations of the AMO in CMIP3 and CMIP5 models. Variability of the AMO in the 10–20/70–80 year ranges is overestimated/ underestimated in the models.

Cheng et al. (2013; 2015) examined the Atlantic Meridional Overturning Circulation (AMOC) simulated by 10 models from CMIP5 for the historical and future climate. The multimodel ensemble mean AMOC exhibits multidecadal variability with a 60-yr period; all individual models project consistently onto this multidecadal mode.

As summarized by the NCA (2017), the simulated AMOC-AMV linkage varies considerably among the coupled global climate models, likely resulting from the spread of mean state model biases in the North Atlantic. The AMOC-AMV linkage depends on the amplitudes of low-frequency AMOC variability, which is much weaker in climate models than in the real world owing to the underestimated low-frequency AMOC variability that amplifies the relative role of external radiative forcing or stochastic atmospheric forcing in AMV.

The timing of a shift to the AMO cold phase is not predictable; it depends to some extent on unpredictable weather variability. However, analysis of historical and paleoclimatic records suggest that a transition to the cold phase is expected prior to 2050. Enfield and Cid-Serrano (2006) used paleoclimate reconstructions of the AMO to develop a probabilistic projection of the next AMO shift. Enfield and Cid-Serrano’s analysis indicates that a shift to the cold phase should occur within the next 13 years, with a 50% probability of the shift occurring in the next 6 years.

Evaluation of the Mann et al. paper

With that context, you can see why I am not accepting the aerosol explanation (pollution and/or volcanoes) for an explanation of what causes the AMO.  There is substantial discussion and disagreement in the climate dynamics community on this topic, which isn’t surprising given the apparent complex interactions between ocean circulations and the AMOC, weather and interannual climate variability, and external forcing from the sun and volcanoes.

So, what exactly is wrong with Mann’s analysis? He relies on global climate models, which are inadequate in simulating the AMO.  This was most recently emphasized by Kravtsov et al. (2018), who concluded that:

“While climate models exhibit various levels of decadal climate variability and some regional similarities to observations, none of the model simulations considered match the observed signal in terms of its magnitude, spatial patterns and their sequential time development. These results highlight a substantial degree of uncertainty in our interpretation of the observed climate change using current generation of climate models.”

Relying on global climate models, which don’t adequately simulate the multi-decadal internal variability, to ‘prove’ that such multi-decadal internal variability doesn’t exist, is circular reasoning (at best).  How does this stuff get published in a journal like Science?  Peer review is sooooo broken.

What Mann is seeing in the climate model simulations is the shorter period tropical expression of the AMO that is limited to the North  Atlantic – not the multi-decadal variability that is linked to the global oscillation.  This tropical expression may very well be forced to some extent by tropical volcanic eruptions, but says little about global multi-decadal variability that is of greatest interest.

The true multi-decadal climate variability is mostly internally generated, although solar variations can help set the tempo and major volcanic eruptions can mask the variations or help trigger a shift.

Why does this matter?  The different phases of the AMO are linked to:  Atlantic hurricane activity, mass balance of Greenland and weather patterns influencing North America and Europe (notably droughts); this is not to mention global temperature change.

From the Penn State press release, a primary motive for cancelling the AMO appears so that Mann can attribute the increase in Atlantic hurricane activity since 1995 to AGW.  Cancelling the AMO isn’t going to help much here.  As discussed in my recent post on the  AIR insurance sector report, the number of major hurricanes (Cat 3+) in the Atlantic during the 1950’s and 1960’s (previous warm phase of the AMO) was at least as large as for the last two decades, when SSTs were significantly cooler.

A further key issue with the AMO is that all of the acceleration in sea level rise in recent decades is coming from Greenland melt, which is heavily influenced by the AMO.

Assuming that nature continues to behave as it has for the past 8 millennia, at some point (possibly in the next decade), we will see a shift to the cold phase of the AMO, with a slow down in Atlantic hurricane activity and Greenland mass loss.

In closing, Mann’s quest to cancel the Medieval Warm Period and now the AMO, in the interests of showing that recent warming is 100% anthropogenic, is not at all convincing to scientists who understand anything about climate dynamics and global climate models.


140 responses to “Canceling the AMO

  1. The AMO is and was always all-natural. There is no man-made annual forcing signal that can remotely reproduce the annual AMO natural variation that hasn’t changed since before the alleged man-made forcing era, 1856.

    Solar activity above ~78 v2 SN drives positive AMO anomalies.

    No, Michael Mann didn’t cancel the AMO.

  2. Sounds like a load of BS from Michael Mann.

    So what’s the reason for the quasi periodic volcanism over the last 1000 years? It still fits the new physics earth-tide forcing hypothesis.

    • Also, has he not noticed that worldwide volcanism appears to be on a upward trend?

      • Jeremy Poynton

        Ah but that would mean he would have to recognise the sun as the major mover of climate. And that would NOT do would it?

      • “Ah but that would mean he would have to recognise the sun as the major mover of climate. And that would NOT do would it?” – Jeremy

        No, I don’t see the connection there. The Sun’s variability is too small to account for climate change imo, including the glacial cycles.

        Counter intuitively, it would mean that he would have to recognise new physics gravitational forcing as a possibility.

        It’s only a matter of time before volcanic & seismic activity is officially announced to be increasing imo. It’d be difficult to attribute that to manmade CO2 emissions.

      • The Sun’s variability is too small to account for climate change

        The Sun’s small changes accumulate into climate changes.

        Climate change defined by 30ya HadSST3 is governed by the cumulative effect of 11 solar cycles (109y + 11y lag) to an extremely high level of statistical significance, p<.ooo1.

        That fact overwhelms all arguments for aerosols or CO2 forcing.

      • “Also, has he not noticed that worldwide volcanism appears to be on an upward trend?”

        Not only in Iceland but the Philippines Batangas volcano has increased alert levels. I’m predicting similar worldwide headlines will continue as a general trend.

    • Richard Greene

      “Sounds like a load of BS from Michael Mann.”
      To be more accurate, delete the “a”,
      and replace it with the words “yet another”.

    • Jeremy Poynton

      “Sounds like a load of BS from Michael Mann.”

      Kudos to Mann, however, he is always consistent.

    • If the facts don’t fit the theory, change the facts.

  3. I suppose Mann will next cancel the sun as an annoying climate anomaly. He is part of the 97% Climate Collusion …

  4. Isn’t Greenland in the North Atlantic? Here’s an example of what the actual observations show. 2 deg cyclic range of multiple convolved frequencies. He is such a stupid arse. His life has been built around denying observed reality. with models. How can any University allow such an overt science denier on their payroll?

  5. “In closing, Mann’s quest to cancel the Medieval Warm Period and now the AMO, in the interests of showing that recent warming is 100% anthropogenic, is not at all convincing to scientists who understand anything about climate dynamics and global climate models.” – Dr. Curry

    I feel your frustration. Glacial cycles will be cancelled next, as hinted at by Burl’s attempt.

  6. Taking the GISP2 data set, filling in the given numbers into millimeter-squares- paper will show a consistent temperature cycle over millenia, which is exactly 61.9 years long. You can use the latest peak of 2004 and go backwards 7,000 years and AMO-peaks (sometimes higher, sometimes less – less in times of stronger warming and cooling changes, which overpower the AMO-signal) will appear in this exact rhytm and will appear at the exact calculated cycle date. Try it out. This visual method will also do away with this “””about 50 to 70 years”-nonsense, which only shows that authors never checked the multi-millenial time scale but only using cherry-picked shorter centennial scales.

  7. @bobweber: It is not obvious what a graph of annual temperature changes has to tell about variations that take place over multiple decades.

    • The upper panel plots invariably exhibit annual timing and variation within the narrow range shown, encompassing all decades from 1856-2020, without exception, following the annual solar insolation cycle. It tells us to expect similar future annual AMO cycle timing and the AMO anomaly will likely vary within +/-0.5C from zero annually.

      Recent studies have claimed that this so-termed Atlantic Multidecadal Oscillation is instead a manifestation of competing time-varying effects of anthropogenic greenhouse gases and sulfate aerosols. That conclusion is bolstered by the absence of robust multidecadal climate oscillations in control simulations of current-generation models.

      Where do they find the man-made change in forcing to the annual AMO cycle that can also preserve the annual AMO cycletiming that is implied in the above assertions, that mimics the sun’s effect?

      Did the authors produce a monthly model that satisfies both requirements? Did they explain how the annual cycle timing is preserved each month by using just aerosols and CO2 instead of solar forcing?

  8. So what am I supposed to do with all those papers citing the AMO, toss them in the trash with the hundreds of papers that cited the non existent MWP and the non existent LIA?

    Given the science has turned against the CAGW dogma, and the models are stinking up the place, and the predictions have become flops, and the charges of big oil interference are getting stale, it appears a new public relations strategy is unfolding to keep the faithful holding on to their hymnals

  9. joe - the non climate scientist

    “My co-authors and I have shown that the AMO is very likely an artifact of climate change driven by human forcing in the modern era and natural forcing in pre-industrial times.””

    M Mann also created the regional nature of the MWP –

    • joe - the non climate scientist

      Mann’s statement – “My co-authors and I have shown that the AMO is very likely an artifact of climate change driven by human forcing in the modern era and natural forcing in pre-industrial times.””

      Its a good thing that man (Mann) was able to continue the work of mother nature when mother nature became too tired to continue the AMO on her own.

  10. Paul Aubrin

    Temperature variations compared with AMO variations.

  11. joe - the non climate scientist

    This is a timely posting by JA Curry.
    M Mann filed a “motion for partial summary judgement …” in the CEI/Simberg/NR/Steyn lawsuit on January 21, 2021.

    In Mann’s statement of facts, there are 2-3 lies/mistatements/outright fabrications on each page (to be expected from Mann) .

    One of the most astonishing claims is that Dr Curry’s testimony (deposition?) should be excluded under the Daubert standard. See page 12 of motion.

    Apparently, Mann has filed multiple motions to exclude testimony of most every defense witness.

  12. Frank from NoVA

    What does Mann have against natural variability? Looking forward to his next paper that attributes diurnal temperature variations to anthropogenic activity.

  13. If Mann thinks the variations prior to the modern industrial era are primarily due to volcanoes and not the AMO, wouldn’t that simply raise the question of why the volcanoes were in such a regular cycle? And why did they then stop, if a combo of industrial aerosols and CO2 emissions took over?

    • Hi. If you care to check the cyclic variability of volcanoes and its effects… the majority of volcanoes are under the ocean, where they also have over 6 times the specific average output (White et al) , so cab significantly warm the oceans directly with hot rocks, BUT… the cyclic variability of their actual observed emissions is on MIlankovitch periodicities, as already observed in the emission records by Kutterolf. so, while that can possibly account for the ice age cycles interglacial events and the 41Ka and 23Ka variability of those cycels, it doesn’t have a significantly large effect on anything else, according to the actual emissions records. I am sure you could create a model that proved whatever it was you wanted to though, like Man does, especially if it allowed the UN to create a another phoney existential problem that allows them to control the developmental activity of the peoples of the World. I would suggest this is best done by tossing sacrificial IPCC scientists into volcanoes to appease the Earth gods,as they used to when people believed daft idea. No, not toss, lowering them slowly – like in Terminator 2 or Indiana Jones. Dead men do no bad science.

      • ” BUT… the cyclic variability of their actual observed emissions is on MIlankovitch periodicities, as already observed in the emission records by Kutterolf.” – brianrlcatt

        Here’s the paper:

        We propose that this variability in volcanic activity results from crustal stress changes associated with ice age mass redistribution.

        I call BS that the change in insolation from the tilt cycle causes volcanism due to melting or accumulating snowfall.

        New physics gravitational forcing is a better fit imo. Also, the so-called lag is very suspect with such a large error range that they’re basically saying “we’re not sure whether there’s a lag or not”.

        It’s reminiscent of the CO2 lag against temperature change. The timings of such assumed drivers doesn’t fit the data.

  14. Temperature’s rising / Fever is high / Can’t see no future / Can’t see no sky

  15. Tell it to the fish…

    “Clearly, independent of fishing activity, stocks will fluctuate in the short and
    long run due to natural causes. For pelagic resources, major stock fluctuations
    occurred even prior to human exploitation (Soutar and Isaacs, 1974). These
    fluctuations have been best documented in relation to the El Niño-Southern
    Oscillation (ENSO) climatic phenomenon, especially as it affects the production
    of small pelagic fishes in the eastern Pacific (e.g. Lluch-Belda et al., 1989), but
    also as it impacts other resources and other geographic areas. Similar climatic
    forcing factors have been affecting marine production systems on the global
    level (Kawasaki, 1992; Klyashtorin, 2001), and long-term fluctuations will be
    reinforced by climate change (Kelly, 1983). Thus, although ‘decadal’ periodicities
    are frequently mentioned in the fisheries literature (e.g. Zwanenberg et al., 2002),
    Klyashtorin (2001) suggests that natural cycles in productivity of around 50 to 60
    years duration are likely to be dominant.
    Coastal fishery resources are also vulnerable to other human activities that may
    affect critical habitats and/or biological and biophysical processes (e.g. Spalding
    and Kramer, 2004). With respect to the latter, the long-term role of environmental
    change in fisheries has become easier to observe in recent years now that fisheries
    data series more commonly exceed a half century in duration. However, our ability
    to discriminate between natural environmental changes, the effects of fishing, and
    the impact of other human activities remains poor.

    See, Coastal fisheries of
    Latin America and
    the Caribbean

  16. Dr. Judith, thanks as always. There is a longer instrumental AMO from NOAA here, with details here. It shows a very different pattern from the AMO data you show above.

    Do you know the reason for the difference in the data?

    Best regards,


  17. I agree. AMOC is the trickster of climate. Warm and carbon rich – then the current slows and ice expands. Not technically an oscillation – more ergodic chaos with persistent. emergent regimes and transitions between state space. At scales of moments to aeons. Even in a simple set of equations.

    Ghil’s climate model shows that climate sensitivity (γ) is variable. It is the change in temperature (ΔT) divided by the change in the control variable (Δμ) – the tangent to the curve as shown above. Sensitivity increases moving down the upper curve to the left towards the bifurcation and becomes arbitrarily large at the instability. The problem in a chaotic climate then becomes not one of quantifying climate sensitivity in a smoothly evolving climate but of predicting the onset of abrupt climate shifts and their implications for climate and society.


    Whatever else COVID19 is a technology miracle. Multiple vaccines in short order. Technology is driven by abundant power. I like these helium gas driven atomic engines. Safe, reliable and with mass factory production – cheaper power. This is General Atomics – it’s General Atomics ffs – 250 MWe concept. They are working on a 50 MWe plug and play model with an Italian firm of industrial designers.

    There was a point about these fast neutron designs have difficulty dissipating flow fast enough. These things are designed to operate at 850 degrees C. General Atomics have made a carbon silicone ‘fuel safe’ cladding that is safe to 2000 degrees C. There is a heat sink shown on the left – for process heat supply.

    I took some time off to think about a new physics of dark energy. I’m calling it
    gravitational entropy. It involves the mass of many universes on an infinite pallet of big bangs – gravitational time dilation – and granddaddy black holes. Entropy draws everything to where time travels slowest.

    • How can anyone possibly put a time-stamp on predictions from a chaotic climate model? To make a leap into such uncertainty as a justification for changing society is pretty thin logic. Why not just push making the todays environment (as opposed to a distant construct) better because we all live on the same planet?

      As far as your EM2 is concerned, fast reactors are not remotely economically justified The cost of the machines and the required reprocessing facilities are stunningly (as in multiples of tens of billions of dollars) expensive. Further, the net amounts of high level waste are more than the once-thru fuel cycles currently in use. That being said, the much higher efficiencies of gas reactors would reduce wastes and costs. However, such machines are rare and will likely remain so as long as exotic but not particularly practical fast reactors are pursued over simpler thermal gas reactors. The latter reactors are not elegant enough for the scientific community, but would better serve cost effective power production.

    • “I took some time off to think about a new physics of dark energy. I’m calling it
      gravitational entropy. It involves the mass of many universes on an infinite pallet of big bangs – gravitational time dilation – and granddaddy black holes. Entropy draws everything to where time travels slowest.” – RIE

      I can’t believe I took time off to read such drivel.

      • LOL. “Everything wants to live where time moves most slowly – and gravity pulls us there.” Kip Thorn

      • An apple falls towards the ground because it is attracted to the inner core of the Earth.

      • A Newtonian force in which everything falls to the center of mass – or a general relativity gravity well – same deal for most intents and purposes. For evolutionary cosmology there is the big bang and galaxies wheeling and receding at increasing velocities apparently. The Hubble bubble. Other universes are out there. God’s palette is infinite and eternal. And if we are expanding drawn out by dark energy – sometime we may come into visual range of another universe expanding to meet us. It may have something to do with the ratio of the mass of photons and the mass of electrons. Slightly different cosmological constants would have different cosmological evolutionary paths.

        Universes inexorably colliding over billions of years – entropy growing – local time slowing. As matter falls into orbits. In this new physics – of gravitational entropy – entropy is the absolute measure of time.

      • Question: Re the ‘centre of mass’. Is this just a mathematical construct? Considering that “an ideal point mass” at the centre of mass is being pulled gravitationally away from that point from all sides.

      • According to Matt O’Dowd at PBS SpaceTime, gravity comes from mass borrowed from time.

      • Matt O’Dowd also recognises that the foundations of physics have been in extreme crisis over the last decade, primarily due to the inability to dovetail Einstein’s GR with the Standard Model.

        A paradigm shift is required, yet he and his well-intentioned colleagues are suffering from groupthink.

      • The lapse rate raised such a questions for me. In fact the majority of the lapse rate is simply caused by thermodynamic pressure in the atmosphere, PV=nRT. GHE is a small part pf this effect s added by scatterinf IR returning to space from the surface. Nothing significant within the total. The pressure on the atmosphere comes from the gravitational accelaration of the gas molecules towards the surface. IF this is doing work, then gravity is doing work, so where does the energy come from. It has to be from the variability in the space time continuum that causes gravity, as must be the case with orbital centripetal forces. So is the fabric of space time doing work pulling stars together and generally causing masses to attract, keep water on the planet, compress gasses, etc?

        IF physics is as advertised, this means the fabric of space time is losing energy and hence the gravitational energy required to contain our Universe, hence expansion….. or not. It seems unlikely a contraction is possible if space time is running down. etc. Possibly?

      • Relativity doesn’t need quantum mechanics to explain gravity. Time and gravity are synchronous and the measure is of course entropy. The ‘standard model’ is of an expanding universe since the big bang. It posits dark matter or dark energy or both to explain the rate of expansion. Unhappily – like string theory as a fundamental explanation of quantum mechanics – unobserved.

        What has been observed is Einstein’s gravitational time dilation. Its time to get rid of dark matter or dark energy. Its my new physics of gravitational entropy. Gravitation increases with mass as entropy increases and time slows. Time might stop entirely in black holes.

  18. It’s hard for a high energy physicist to keep all the acronyms in a row and follow which effect is called what, but even though M. Mann seems to be just a Shaman rather than a scientist, I have wondered for a long time about the gorilla in the room, which is the very hot, high specific heat player in the physics of the earth which is the magma under everything, the obviously thinner crust in the Arctic and the thicker crust in the
    antarctic. Newton knew the non spherical shape from his gravity studies in the 1600s. Then there is a measurement lately by a Japanese group that new magma plumes have been identified on the west side of Greenland. I got involved in Climate change discussions due to cosmic rays which I know more about. Everybody dissed them as being too ineffective. But many sun based forcing correlations have essentially been based on a cosmic ray mechanism. No one mentions magma forcing except some comments about volcanoes. A large one is to the east of Greenland. I don’t know the time variation, but I do know that I don’t hear anyone try to explain why magma heat is not important. Sorry for the incomplete hasty argument, but does anyone know why the effects are not included? Lack of clear data? Clear data but too small to affect the problem?

    • Geothermal has been ruled out because the W/m2 is so much lower than the energy received by the Sun at the surface. It has been deemed negligible.

      The counter argument is that although small, the abyssal plane deep oceans have been warming over time. This is counter intuitive to known science and climate modelling.

      Also, the bottom of Lake Tanganiyka at around 800m+ has been found to be warming significantly, although this is again dismissed by IPCC climate scientists.

      My interpretation is that mantle convection is increasing, bringing extra heat from the inner Earth towards the underside of the crust. Because this would require new physics, the possibility is simply ignored by the mainstream.

    • I’m not sure this is your specific interest, but there is significant literature on geothermal activity under the ice sheets affecting basal melt and thus movement of glaciers and ice discharge and sea level rise. Whether it is significant or not, seems to miss the point, at least from a scientific perspective. It is a known part of the physical process with vast uncertainties, including the hydrology under the Ice Sheets, and as such should always be part of the inquiry.

      But true to form, the IPCC6 report, which has nothing to do with science and has everything to do with politics and power, is completely mum on the subject. Why would we expect anything else.

      I’ve compiled quite a list of studies if you are interested.

    • “Lack of clear data?” – deepinelastic

      Here’s some very specific data which shows that the abyssal ocean bottoms are warming:

      • “Here’s some very specific data…” that shows nothing of the sort.

        From the abstract:

        ” we quantify the warming trend of Antarctic Bottom Water as 0.002°C yr−1 over that time period, which is about one seventh the global sea surface temperature warming trend from 1989–2019.

        …showing that it’s a. nothing to do with geothermal and b. not “ocean bottoms”, but a tiny portion near Antartica and c. neglible compared to the heat coming from above.

        Did you cut and paste without reading it or is there some “very specific data” behind the paywall you’d like to share?

      • It’s all about interpretation. I’m combining this data with that of Lake Tanganiyka and coming to the conclusion that mixing with surface waters doesn’t make intuitive sense, whilst geothermal increase does.

        It at least warrants the possibility and therefore further research into the hypothesis.

  19. With Mann’s paper wand and Twitter, Facebook and Google magic AMO will be HunterBidenized, presto change

  20. The Moon’s monthly orbit caused the oceans tides up until pre-industrial times but then was taken over by sea level rise due to manmade CO2 emissions.

    • Not only. See what is happening now. The earth is near equinox, and the moon is in quadrature to the sun-earth line. Unequal gravitational forces on an oblate earth. We have swarm earthquakes around Greece (and Etna furious) and globally opposite in NZ.

  21. Interesting that Michael Mann claims to have ‘brought the AMO into the conversation’ two decades ago. In 1994, a paper by Michael E Schlesinger and Navin Ramankutty stated: “Here we apply sipngular spectrum analysis to four global-mean temperature records, and identify a temperature oscillation with a period of 65-70 years. Singular spectrum analysis of the surface temperature records for 11 geographical regions shows that the 65-70-year oscillation is the statistical result of 50-88-year oscillations for the North Atlantic Ocean and its bounding Northern Hemisphere continents. These oscillations have obscured the greenhouse warming signal in the North Atlantic and North America. Comparison with previous observations and model simulations suggests that the oscillation arises from predictable internal variability of the ocean-atmosphere system.”.

    • Yes, the first comment in RealClimate makes a similar point.

      “oscillation is the statistical result of 50-88-year oscillations for the North Atlantic Ocean and its bounding Northern Hemisphere continents.” – Mike Jonas

      I couldn’t help but make the connection with the ~88-year paleoclimate cycle, which along with the ~215-year cycle, is reportedly found isotopically far back into the Cretaceous, for at least 98mya.

  22. Pat from kerbob


  23. The next target of Mann’s denial will presumably be ice ages?

    • Agreed. That way there’s no need to defend the Milankovitch insolation theory against the alternative view of new physics & orbital inclination.

      Burl has already spearheaded this whitewashing of past climate cycles by insisting there is no clear-cut 100kyr cycle, therefore random volcanic activity is the answer.

      It’s quite surreal how Mann operates.

  24. Can Mike Mann explain 60-100 year periodicity in volcanoes over the last 500 years?

    [1] We present a tree‐ring based reconstruction of the Atlantic Multidecadal Oscillation (AMO) which demonstrates that strong, low‐frequency (60–100 yr) variability in basin‐wide (0–70°N) sea surface temperatures (SSTs) has been a consistent feature of North Atlantic climate for the past five centuries. Intervention analysis of reconstructed AMO indicates that 20th century modes were similar to those in the preceding ∼350 yr, and wavelet spectra show robust multidecadal oscillations throughout the reconstruction.

    • 500 years too short?
      How about 1200 years?

      • Try 980 yrs – the Eddy cycle- ,with flips every half cycle.

      • Melita
        I meant study period, not oscillation wavelength.
        The wavelength of the AMO is not fixed btw due to the chaotic element.
        This is true I believe of oceanic osscillations in general.
        Asserting that such oscillations are monotonically regular (which they’re not) makes it easier to find spurious grounds to refute their existence.

      • My mistake there.

        Allow me to add something though. From mpov it seems any time period is too short. The 980(+/-) Eddy cycle seems to have a bearing but no one cycle has been the same over the last 8k years.
        This paper has some related info
        Quote “The timberline
        is a sensitive indicator of climatic conditions and ecological
        situation on the whole. It corresponds to a mean July
        isotherm of +11.5C. A number of authors reported on multiple
        variations in the latitudinal and altitudinal timberline
        in Scandinavia and North European part of Russia derived
        from palynological and dendrochronological data [Bjune et
        al., 2004; Kultti et al., 2006; MacDonald et al., 2000]. The
        maximum northward extent of forest was observed in the interval
        4300–4000 years BP. Beginning from 4000 years BP, a
        southward retreat of the timberline associated with cooling
        has been taking place everywhere.”
        In similar fashion one Eddy peak occurred around 5500BP, when the Sahara abruptly dried. It is still dry today.

    • Phil Salmon | March 7, 2021 at 3:38 am | Reply

      Can Mike Mann explain 60-100 year periodicity in volcanoes over the last 500 years?

      The problem is that, as old Joe Fourier showed, any dataset can be broken down into a series of underlying waves.

      Now, I much prefer the CEEMD analysis to Fourier analysis, because the CEEMD analysis doesn’t break a dataset into sine waves. Instead, it breaks a dataset into a group of waves which can change over time.

      With that as prologue, here is a graph showing the CEEMD analysis of the underlying cycles in the tropical volcanoes used in the Mann analysis.

      Yes, there is a ~ 50 year cycle in the data … and?

      Also, that cycle doesn’t fit well at all with modern (1850 – present) AMO data, nor with paleo AMO data.

      Of course, M. Mann doesn’t deal with any of that.


      • Thanks Willis
        It seems that Fourier type frequency can easily be fudged to “cover a multitude of sins” – such as this use by Mann to force volcanoes to fit the AMO.

        The better performance of CEEMD would be of no interest to Mann, his goal here is just to have a paper to hold up saying that there’s no intrinsic climate variability (and no ocean). Mann writes these papers for politicians and the media, not scientists, whom he clearly feels free to ignore with impunity.

  25. This took about 10 seconds to find on Google Scholar.

    The North Atlantic experiences climate variability on multidecadal scales, which is sometimes referred to as Atlantic multidecadal variability. However, the relative contributions of external forcing such as changes in solar irradiance or volcanic activity and internal dynamics to these variations are unclear. Here we provide evidence for persistent summer Atlantic multidecadal variability from AD 800 to 2010 using a network of annually resolved terrestrial proxy records from the circum-North Atlantic region. We find that large volcanic eruptions and solar irradiance minima induce cool phases of Atlantic multidecadal variability and collectively explain about 30% of the variance in the reconstruction on timescales greater than 30 years. We are then able to isolate the internally generated component of Atlantic multidecadal variability, which we define as the Atlantic multidecadal oscillation. We find that the Atlantic multidecadal oscillation is the largest contributor to Atlantic multidecadal variability over the past 1,200 years. We also identify coherence between the Atlantic multidecadal oscillation and Northern Hemisphere temperature variations, leading us to conclude that the apparent link between Atlantic multidecadal variability and regional to hemispheric climate does not arise solely from a common response to external drivers, and may instead reflect dynamic processes.

  26. Judy, one more very recent paper : . They argue, that the “classical” AMO definition ( linear detrended NA SST) gives room for some (wrong) estimations of a mostly forced response of the North Atlantic. They suggest a regression of the NA SST on the radiative forcing as it was done here in 2016: .
    They find a mostly internal generated AMV index. There is also a hint on this paper where the authors find a too strong volcano response in the CMIP5’s . Perhaps these studies are helpful to set the latest MM et al paper in context.

    • MM claims that State of the art GCM/ECM do not show an AMV. This makes no wonder:
      This is the correlation between the observed SST and the CMIP5 model mean. In most parts of the extratropical oceans there is a zero-correlation, also in the NA. It’s a verx bad idea to conclude that the AMV is not real because Climate models dont’t show it, they don’t schow so much more!

  27. Reblogged this on Tallbloke's Talkshop and commented:
    ‘So, what exactly is wrong with Mann’s analysis? He relies on global climate models, which are inadequate in simulating the AMO.’
    ‘Not at all convincing’…

  28. Ed Zuiderwijk

    Mann is a quack. His latest effusion puts it beyond doubt.

  29. Pingback: Canceling the AMO – Climate-

  30. I would like to know what the authors who have papers identifying the AMO think about Mann’s paper. I saw Nic’s analysis over at Climate Audit of the Mann paper from several years ago. But I would like to know the reaction to this paper from others who have done more recent work.

    I don’t want to know their public reaction. That might be different from their private thoughts, given the atmosphere in climate science. What do they say over a beer with their closest confidants.

  31. It’s not merely the AMO that Mann is denying – it appears to be a denial of the entire scientific literature. Here is one of hundreds if not thousands of peer reviewed papers analysing the AMO and the closely related AMOC:

    The AMO emerges inevitably from the AMOC as a nonlinear oscillation linked to the excitability of the AMOC resulting from the salinity-downwelling feedback. Mann’s antics make it ever clearer that to maintain an alarmist narrative it is necessary to commit an outright denial of chaotic nonlinear dynamics, including internal emergent oscillations, that are a fundamental aspect of most natural systems.

    By synthesizing recent studies employing a wide range of approaches (modern observations, paleo reconstructions, and climate model simulations), this paper provides a comprehensive review of the linkage between multidecadal Atlantic Meridional Overturning Circulation (AMOC) variability and Atlantic Multidecadal Variability (AMV) and associated climate impacts. There is strong observational and modeling evidence that multidecadal AMOC variability is a crucial driver of the observed AMV and associated climate impacts and an important source of enhanced decadal predictability and prediction skill. The AMOC‐AMV linkage is consistent with observed key elements of AMV. Furthermore, this synthesis also points to a leading role of the AMOC in a range of AMV‐related climate phenomena having enormous societal and economic implications, for example, Intertropical Convergence Zone shifts; Sahel and Indian monsoons; Atlantic hurricanes; El Niño–Southern Oscillation; Pacific Decadal Variability; North Atlantic Oscillation; climate over Europe, North America, and Asia; Arctic sea ice and surface air temperature; and hemispheric‐scale surface temperature. Paleoclimate evidence indicates that a similar linkage between multidecadal AMOC variability and AMV and many associated climate impacts may also have existed in the preindustrial era, that AMV has enhanced multidecadal power significantly above a red noise background, and that AMV is not primarily driven by external forcing. The role of the AMOC in AMV and associated climate impacts has been underestimated in most state‐of‐the‐art climate models, posing significant challenges but also great opportunities for substantial future improvements in understanding and predicting AMV and associated climate impacts.

  32. Ms. Curry,

    Are there any meteorologists who are studying the effects of volcanic eruptions on WEATHER? It strikes me that volcanic eruptions are highly unlikely to have much of an impact on climate, given that in order to talk about climate, by definition, one must consider a minimum of a 30-year period.

    But, I wonder if the regular and often ongoing eruptions on the Kamchatka Peninsula, for instance, are capable of impacting the tropospheric polar vortex and producing the kind of weather that recently caused Texas’s power grid to fail.

    Do volcanic eruptions EVER figure into weather forecasts?


    Volcanic Cyclogenesis

    … The evidence for cyclogenesis caused by volcanic explosive events is strong based on the facts presented herein. The shear mass (47 Megatonnes) and kinetic energy (176PJ kinetic energy) of the plume at 12.2 km of event 9 was sufficient to disturb the Jetstream and set the Rossby waves into a meridional flow pattern. The power generated by this single event was 115TW equivalent to nearly 4 super typhoons. …

    Volcanic mesocyclones

    … Here we show that the updraught of the rising column induces a hydrodynamic effect not addressed to date—a ‘volcanic mesocyclone’. This volcanic mesocyclone sets the entire plume rotating about its axis, as confirmed by an unprecedented analysis of satellite images from the 1991 eruption of Mount Pinatubo2,3,4.

    Atmospheric effects in Scotland of the AD 1783–84 Laki eruption in Iceland

    … The summer of 1783 was dominated by long-lasting anticyclonic circulation that led to it being one of the warmest summers across Europe during recent centuries (Kington, 2010; Manley, 1974). Equally, the winters of 1783–84 and 1784–85 were two of the coldest and were associated with extensive sea ice cover around Iceland and the Greenland Sea (Brazdil et al., 2010; D’Arrigo et al., 2011; Lamb, 1977; Parker et al., 1992; Yiou et al., 2014). …

    Climate MODELS [i.e., mathematical fabrications] suggest that the warm summer of 1783 in Europe was unrelated to volcanic radiative forcing (Highwood and Stevenson, 2003; Oman et al., 2006; Schmidt et al., 2011; Zambri et al., 2019b). … [Models strike me as much the way in which Spell Check works on my iPad. It is an abysmal function that is apparently utterly unrelated to an English dictionary.]

    • Ulric Lyons

      Large tropical eruptions with stratospheric aerosols have a positive influence on the NAO so they typically warm the winters slightly.
      The Laki eruption was persistent low altitude dust and fumes, that exacerbates heatwaves. Like in the Moscow 2010 heatwave with forest fire smoke, or record maximum temperatures in the UK 21-27 Feb 2019, and during Easter 2011, when there were very high air pollution levels which had drifted over from Europe, combined with a dusty Saharan plume.
      The 1783-84 is a best heliocentric analogue of the 1962-63 winter, and winters 829 and 1010 when the Nile froze. They are down to monthly changes in indirect solar forcing effects on the NAO/AO. It was hot again by April.

  33. joe - the non climate scientist

    Here is Nic Lewis’s critique of a Mann’s AMO study published in geophysics research letters back in 2014

  34. Rick Pfizenmayer

    Wow! Maybe Mann would like to have another “debate” like the one in West Virginia a couple of years ago. He might come away bloodied again, but at least he’d get a chance to flog whatever book he’s peddling now.

  35. Robert Girouard

    Who are the real deniers ?

  36. Syun-Ichi Akasofu informs us of, two natural components of the currently progressing climate change. The first one is an almost linear global temperature increase of about 0.5°C/100 years. The second one is oscillatory (positive/negative) changes, which are superposed on the linear change. One of them is the multi-decadal oscillation…

  37. Michael Mann is blinded by something / but it’s not the light

  38. Pingback: The Gulf Stream is weakening - says science. - Naval War changes Climate

  39. Tibor Nemeth

    Thanks for the swift post.
    It is clear, that there is certainly a “Mann made climate change”, and “Mann made global warming”. This is a crisis, even if it is not the same as the ‘climate crisis’ echoed by the climate alarmists.

    We need better understanding in the causes of the continual change in climate before vast amount of limited resources are spent by the society in the hope to change the current trend and the currently expected outcome, which outlook might be far away from the real world, but a good business for the elite.

  40. Pingback: Canceling the AMO – Watts Up With That?

  41. Reblogged this on Climate Collections.

  42. Pingback: Canceling the AMO |

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  44. “Some hurricane scientists have claimed that the increase in Atlantic hurricanes in recent decades is due to the uptick of an internal AMO cycle,” said Mann. “Our latest study appears to be the final nail in the coffin of that theory. What has in the past been attributed to an internal AMO oscillation is instead the result of external drivers, including human forcing during the industrial era and natural volcanic forcing during the pre-industrial era.”

    That statement by Mann is not what one would expect to hear from a scientist. The finality of “final nail in the coffin” is more from politics than science.

  45. Ulric Lyons

    “Wang et al. (2017) found that large volcanic eruptions and solar irradiance minima induce cool phases of Atlantic multidecadal variability..”

    Large tropical eruptions have a positive influence on the NAO so that’s true. But the AMO was cooler around sunspot minimum in the mid 1970’s and mid 1980’s because of stronger solar wind states driving positive NAO regimes. And warmer at sunspot maximum around 1969 and 1979-80, because that’s where the major lows in the solar wind were in those solar cycles, with negative NAO periods.
    Then during the warm AMO phases, AMO anomalies are nearly always warmer around sunspot minimum, because of weaker solar wind states driving negative NAO regimes.

    The frequency of warm AMO phases will depend on the timing of centennial solar minima, when the AMO is normally warmer because of an increase in negative NAO. With 130 years between the late 1800’s and the current centennial minimum, there has been a 60yr and then a 70yr AMO envelope. But the long term mean should be around 54 years, when including an additional warm phase between each centennial solar minimum.

  46. I analyzed North Atlantic sea surface temperatures and found a clear AMO cycle of 66 years and steady warming since 1860. However, contrary to global warming science, I didn’t succeed to find any correlation to the logarithm of atmospheric CO2 concentration.

  47. Thanks Dr Curry for this magisterial review of the real science of the AMO or AMV – Atlantic multidecadal variability. Mann indeed crosses into self-parody when he says:

    That conclusion is bolstered by the absence of robust multidecadal climate oscillations in control simulations of current-generation models.

    So all you have to do to disprove something now is write a computer model that fails to replicate it??

    The core of this issue is Mann’s denial of internal intrinsic oscillations and essentially of the entire field of chaos and nonlinear dynamics. The scant respect paid by many scientists to this field – ignoring it where it is highly relevant – clearly gives Mann confidence for this brazen and defiant act of denial.

    In the end this is the rock on which the ship of climate alarm will wreck and founder. With Mann at the helm as it goes down.

    FWIW my thoughts on how nonlinear dynamics bring about oscillations in the strength of the AMOC that results in the AMO are given in these posts, admittedly a bit disorganised and in need of improving.

    In short, it’s not about ocean driven climate starting linear and being moved into a chaotic state. But the opposite – having its dimensionality reduced from turbulence down to borderline chaos. Ocean driven climate is by default highly chaotic and turbulent – not too surprising for water flowing through complex topography. Turbulence is high dimensional chaos and alarmists defending simple linear models can justifiably write it of as “noise” that doesn’t affect long term forced trends. But they miss the point. Certain factors in a chaotic system can reduce dimensionality. This theory has been worked out by chemical engineers such as Matthias Bertram. Specifically, internal positive feedback (“excitability”) and external periodic forcing, can both reduce dimensionality of a chaotic system, moving it from turbulence toward the borderline chaos where emergent spatiotemporal pattern and oscillation emerge. This is what Bertram calls entropy-exporting structure and what Ilya Prigogine called dissipative structures. The AMO is a dissipative structure.

    In the case of the AMOC excitability comes from the salinity-downwelling feedback. And external periodic forcing is provided by solar and tidal rhythms.

  48. It seems that the abrupt climate change decadal variability thing was needed to explain the Younger Dryas and then it survived into the AGW era where it served a purpose in creating fear of human caused ice melt fresh water entering the North Atlantic. In the current situation this decadal time scale became a burden for climate science because it allows deniers to use these brief time scales in their critical attacks on climate science and so maybe climate science decided they are better off without it. The bigger issue here maybe the frivolous use of volcanoes as the explainer of all things that need explaining not only in this issue but also in the ETCW and a few other things. The logic here appears to be that climate scientists know it all and where they don’t there’s always the volcano thing that can explain anything that needs explaining ehether warming (co2) or cooling (aerosol).
    Here’s some info on the younger dryas abrupt climate change issue.

    • Volcanoes do indeed seem to have the role as a “joker” in the pack – they can be whatever is needed. Either warm by CO2 or cool by aerosols, as required. You could call them the control knob on the side of the CO2 control knob.

      However volcano effects on climate are surprisingly transient. Therefore the least credible use of volcanoes as the universal “fixer” of climate is in palaeo climate reconstructions – such as the Younger Dryas in your article – is where they are “stretching” volcanoes to have an effect over thousands or in other cases even millions of years. This is not credible. For instance around the time of the Permian-Triassic boundary (and extinction event) and also the end of the Ordovician there are severe discrepancies between atmospheric CO2 levels and global temperatures, and these are fixed by the universal sticking plaster of volcanoes, where they are incredibly stretched out to alter climate in whatever way is needed – either warming or cooling – over millions of years. Just popping off one after the other in a choreographed sequence controlled by who knows what. It’s the stuff of a Dr Seuss cartoon.

    • As ever people miss the point that the largest number of main cycle volcanoes, whose specific emissions are over 6 times those on continental volcanoes because they sit on a much thinner crust, have a much larger effect on the climate than sub aerial surface volcanoes tiddly bit of CO2 and some aerosols. Submarine volcanoes simply heat the oceans directly with 1200 deg magma/1,000 degree delta while releasing far more CO2 than surface volcanoes into the oceans from where it is later released to the atmosphere.

      The observed emissions increases are possibly a large enough sustained perturbation to the climate system to cause an interglacial when at a known maximum due to Milankovitch 100Ka solid gravitational tide maximums, but not enough to cause more than local effects in between.

      QUESTION: Ask yourself, how come, as we know, the oceans kept steadily rising and melting the ice caps in the NH, with presumably ever more rain , while the land surface temperatures returned to glacial temperatures during the Younger and Older Dryas periods of the last interglacial warming?
      ANS: Because the heat is coming from below, not from above, as Lili von Shtupp famously sang. The oceans were warm enough to make the rains.

      There are also substantial, more powerful but less energetic, so ultimately lesser 41Ka and 23Ka emissions peaks, also observed and reported in the literature. But not multi-decadal effects identified, yet, except perhaps El Nino, which occurs between the Galapagos hot spot and the Indonesian Equatorial Pacific margin… right where crustal movement due to diurnal solid tides is at a maximum? Also correlated with seismic activity.

      All discussed in my hard to publish per pub paper. It’s the volcanoes under the oceans that do all the emitting, of magma and CO2, and can deliver a significant long term sustained effect, by heating the oceans directly at a low rate. Lots of heat is required. 6×10^24Joules per deg. Simples! Just not instant. Not a qualitative statement. Data, quantification and references here:

  49. Reblogged this on WeatherAction News and commented:

    Wow. In one fell swoop, the pesky problems of the ‘grand hiatus’ in the mid 20th century, debates over the attribution of 20th century warming and the role of multidecadal internal variability, and the difficulty of attributing the recent increase in Atlantic hurricane activity to AGW, all go away. Brilliant! Almost as ‘brilliant’ as the Hockey Stick.

  50. Don’t build your world around / volcanoes melt you down

  51. What, like the interglacial events are “transient”? Volcanic effects vary on periods that make humans lifetimes look vanishingly transient. Take a look under the oceans and the known cyclic nature of volcanic emissivity under orbital forcing. Bigger than you thought?

  52. Matthew R Marler

    Judith Curry,

    Thank you for this review.

  53. We’re a bunch of warring animist tribes here, fighting for our own deities – the carbon god, the sun god, the chaos god, the volcano god etc. Mann has made a smart move to get skeptics fighting against each-other by giving some homage to the volcano god and feigning an alliance between the carbon and the volcano gods. But it won’t last. Don’t trust any promises of the carbon god – they’ll go up in smoke.

    • The new physics god has emerging data to support it.
      Strangely, when waves pass through the core from north to south, they travel faster than waves passing through the core parallel to the Earth’s equator. No one knows why this is, Stephenson said, but it’s a consistent finding. The technical term for this oddity is anisotropy.
      But at the very center of the inner core, something seems to be different, scientists noticed in the early 2000s. At this depth, the anisotropy seemed not to match that of the rest of the inner core.

      “For the last two decades it has been very, very unclear what this signal in the center of the Earth in the data is and why we see it,” Stephenson said.

      The anisotropy of the inner inner core lends itself to the nucleic density matter hypothesis, thought to emanate a planar strong gravitational force which interacts with similar exotic-type cores of planetary bodies.

      • There’s evidence of a millennial cycle disturbance of the inner inner core imo, attributable to a strong gravitational interaction:
        The findings by the researchers show that approximately 800 years ago there was a regional magnetic anomaly in Southeast Asia. They suggest the weakening they observed was likely part of a wider anomaly that stretched all the way to the equator—a phenomenon that has been described as the ‘flux expulsion’ at low latitudes. They acknowledge that they were unable to find any explanation for the anomaly but suggest it could have been due to interference resulting from turbulence occurring at the Earth’s core/mantle boundary. They also note that many such anomalies have been found and studied—one of them is occurring today below southern parts of the Atlantic Ocean.

      • At the same time as the magnetic anomaly, there was volcanic activity in Iceland:

        Unusual seismic activity in a volcanic zone near Iceland’s capital Reykjavik that has been dormant for almost 800 years has left experts stumped and searching for clues as to whether an eruption is imminent.
        Geological studies show that the tiny peninsula is home to five volcanic systems which all appear to show signs of life about every 800 years.

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  55. Question from a layman: how do those who reconstruct paleoclimate ocean temps know that they are measuring the same thing? I assume that scientists’ knowledge of the fluctuations in sea-surface temperature came only from actual measurements. Could they have derived this from measuring whatever the reconstructors (such as Wang et al. and Knudsen et al.) measure as proxies? If not, why trust the proxies for the past?

  56. Scientists speak out against mainstream obsession with anthropogenic climate change. There’s some wonderful quotes:

  57. “Lin et al. (2019) argues for two different sources for AMO variability, identifying 50–80 year and 10–30 year AMOs that are associated with different underlying dynamics.”

    There’s a clear correlation with the proposed 10-30 year variability and the Earth’s core, rotation rate cycle and earthquake/volcanic activity:
    Geophysicists are able to measure the rotational speed of Earth extremely precisely, calculating slight variations on the order of milliseconds. Now, scientists believe a slowdown of the Earth’s rotation is the link to an observed cyclical increase in earthquakes.
    Specifically, the team noted that around every 25-30 years Earth’s rotation began to slow down and that slowdown happened just before the uptick in earthquakes. The slowing rotation historically has lasted for 5 years, with the last year triggering an increase in earthquakes.

  58. So Mann (2021) suggests that Delworth & Mann (2000), Knight et al (2005, Mann was a contributor) & Mann (2011) didn’t know what Mann was talking about…

    I like it, isn’t it good to know Mann has no confidence in Mann :-)

  59. As usual, poor physical knowledge of the oceans coupled with wrong-headed notions of Fourier composition of structured random signals provides a convenient means of dismissing what good data tells us.

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  65. The proposal of a recent nucleic density impact event of Venus can explain the mysterious moons of Mars:

    With their irregular shapes, cratered surfaces, and diameters of just 22 km and 13 km respectively, the pair are commonly believed to be asteroids pulled in by Mars’ gravitational field. However, they also follow highly circular orbital paths, which lie almost perfectly in Mars’ equatorial plane.
    Yet one further aspect of their orbits, relating to their widely differing orbits, mean that this theory is also problematic. Notably, Phobos orbits well beneath Mars’ synchronous radius – the point at which a moon’s orbital period perfectly matches the rotation of its host planet. Meanwhile, Deimos orbits well beyond this point. These factors make it difficult for astronomers to explain how Phobos and Deimos could have formed at the same time as Mars.

    The eclipsing Phobos causes marsquakes during it’s equatorial orbit. This suggests it has a high proportion of nucleic density matter which has an especially *strong* gravitational interaction when on the same plane.

    Hopefully the upcoming Japanese mission, due to launch in 2024, will show evidential hints that lend itself to the notion that they are from the interior of Venus herself.

    • It would be amazing if the Mars Perseverance rover, soon to study the multitude of rocks on it’s surface, concluded that most of them were meteorites.

      • Wow, there’s already evidence to suggest Mars’ meteorites came from a recent nucleic density Venusian impact before it’s present thick & dense atmosphere formed. A slower moving meteorite could remain intact during it’s decent through the thin Martian atmosphere:

        For example, the “Block Island” meteorite (pictured) is much too large to have landed intact given the current thinness of the Martian atmosphere. A thicker atmosphere would be needed to cushion its fall. With this information, scientists suspect that the Block Island meteorite fell billions of years ago when the atmosphere of Mars was much thicker.

        The Mars meteorites also display very little surface alteration. This confirms that the atmosphere and surface soil of Mars contain very little moisture or free oxygen.

      • The Chinese Mars mission Taiwen-1 has just entered orbit and will spend the next 2-3 months observing and searching for a possible landing site in May.

        I predict that they have contemplated the deep enigma and paradox of the Block meteorite. If it entered Mars’ much thicker atmosphere billions of years ago but lasted without surface alteration, this means the thick atmosphere was devoid of water vapour. It contradicts the idea that water flowed on Mars.

        I suspect the Chinese mission is looking for an iron meteorite which is much larger than the 2ft wide Block meteorite, which was easily found by the Curiosity rover.

        They will pip the US in headlining worldwide news of Mars discoveries with the touchdown next to a giant meteorite which ‘shouldn’t exist’.

        Watch this space.

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  70. How long before Planet Nine is considered to be a small core of nucleic density matter eg. hyperon matter?

  71. A fascinating, in depth and realistic look at the complexity, the uncertainties and the disagreements between scientists in the field of the AMO/AMV and other suites of indicators which affect climate dynamics in the north Atlantic.

    If I thought that any European politicians could understand the first word of this, I would highly recommend that they should read it.

    For them, however, it is imperative that they understand that Mann does not represent the concensus opinion amongst relevant experts in the field and, as a result, under no circumstances should they take potentially fateful policy decisions based on work which is at best preliminary, at worst inappropriately accepted for publication and which has clear politically charged issues associated with what should be a rigorous assessment of scientific data, including whatever levels of uncertainty are intrinsic to the analysis.

  72. I’m making the prediction that when another comet impacts Jupiter, the stratospheric wind speeds will be found to have increased due to the hypothesized ‘dark moon’ shepherding the Trojans becoming ‘flatter’.

    By analyzing the aftermath of a comet collision from the 1990s, the researchers have revealed incredibly powerful winds, with speeds of up to 1450 kilometers an hour, near Jupiter’s poles.
    However, astronomers were provided with an alternative measuring aid in the form of comet Shoemaker-Levy 9 , which collided with the gas giant in spectacular fashion in 1994. This impact produced new molecules in Jupiter’s stratosphere, where they have been moving with the winds ever since.

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