by Donald Rapp
Why after 400 years of La Niña precedence, did periods of El Niños dominance start in the 20th century? And why did the two periods of strong El Niño dominance in the 20th century occur during a period when the CO2 concentration was rising? Is there a link between rising CO2 and the El Niño – La Niña balance? But if there is such a link, why did El Niños become less prevalent than La Niñas from 1941 to 1976 and be in balance after 1998?
Introduction
Bob Tisdale is noted for his extensive reviews, discussions and analyses of El Niño and La Niña occurrences and their implications for climate change. He emphasized that the Pacific Ocean covers 1/3 of the earth and its condition impacts the climate of the whole earth. The Pacific periodically goes through transitions from El Niño to neutral to La Niña, and vice versa. In an El Niño, warm waters cover a sizable portion of the Pacific, which heats the atmosphere. Tisdale has shown that at least in the 20th century, one can correlate durations of increase in global temperature with periods of El Niño dominance. Actually, the global warming of the 20th century correlates better with Niño indices than it does with CO2 concentration. He has therefore argued that a substantial part (if not all) of the global warming of the past ~120 years can be attributed periods of imbalance in favor of El Niños, rather than the effect of rising CO2 concentration.
Tisdale pointed out that the establishment view is that rising greenhouse gas concentrations over time cause a steadily increasing forcing that tends to drive earth temperatures upward. According to this viewpoint, the sequence of periodic changes in the El Niño – La Niña balance is superimposed on this continuing trend. During periods of El Niño dominance the warming due to El Niños adds to the warming effect due to rising greenhouse gases, and during periods of La Niña dominance the cooling due to La Niñas reduces the warming effect due to rising greenhouse gases.
A common belief in the climate establishment is that El Niño – La Niña sequences will eventually balance out over long periods of time, and warming due to greenhouse gases will be the dominant trend over longer periods of time. However, when one actually examines the data, one finds that the El Niños and La Niñas were not in balance during the 20th century. Starting in year 1900, and continuing to about 1941, El Niños were more prevalent and stronger than La Niñas. Also during this period, the earth warmed significantly. From about 1941 to about 1976, El Niños and La Niñas were fairly balanced, although there was a slight excess of La Niñas. The earth cooled slightly during this period but the prevalence of La Niñas was weak. From 1976 to 1998, El Niños strongly dominated over La Niñas. This 23-year period included the very strong El Niño of 1982-3 and culminated in the very strong El Niño of 1997-8. The earth warmed rapidly during this period from 1976 to 1998. In fact, about half the warming of the 20th century occurred during those 23 years. With the termination of the great El Niño of 1997-8, we entered a period of balance between El Niños and La Niñas and the earth’s temperature remained essentially unchanged from 1998 to 2014.
From the results reported above, one could infer that the climate of the earth has been controlled by the El Niño – La Niña balance during the past 120 years. When El Niños dominate, the earth warms; and when La Niñas dominate, the earth cools. When they are balanced, the earth tends to remain at a constant average temperature. One could possibly go further (as Tisdale has done) and argue that CO2 had little to do with the earth’s climate these past 120 years; only the state of the Pacific Ocean determined the climate. Unfortunately it is difficult to prove this proposition.
Proxies have been used to estimate the El Niño – La Niña balance over the past ~500 years. Proxies are not as reliable as direct temperature measurements, and tree rings in particular have shown some poor correlation with measured temperatures. In general, comparisons of proxies with measured temperatures during the calibration periods tend to show only moderate correlation. Nevertheless, proxies are the only source we have for estimating the El Niño – La Niña balance over the past longer periods of time. The proxies indicate that for 400 years prior to year 1900, the Pacific remained mainly in a La Niña – dominant mode and earth temperatures were lower than in the 20th century.
Thus one could argue that for 400 years, the earth was cool, in some way due to a prevalence of La Niñas, and this reversed in the 20th century during which there were two major periods of El Niño dominance in which all the global warming of the 20th century took place.
But then the question arises: Why after 400 years of La Niña precedence, did periods of El Niños dominance start in the 20th century? And why did the two periods of strong El Niño dominance in the 20th century occur during a period when the CO2 concentration was rising? Is there a link between rising CO2 and the El Niño – La Niña balance? But if there is such a link, why did El Niños become less prevalent than La Niñas from 1941 to 1976 and be in balance after 1998? These periods were periods of rising CO2. What other variables were changing during these periods? These might include solar intensity, cloud cover, wind patterns, etc. The data are sketchy and do not seem to lead to any clear answers.
Is nature teasing us? Coincidental with the period when CO2 started rising rapidly, did nature decide to have two periods of strong El Niños that increased the earth’s average temperature? Or did the occurrence of strong El Niños require some sort of CO2 trigger?
Meanwhile some alarmists are pinning their hopes of a new period of rising earth temperatures on emergence of a new El Niño later this year. But in doing so, are they implicitly admitting that the earth does not warm unless there is an excess of El Niños?
In an NPR interview, Kevin Trenberth was quoted as follows:
“The oceans can at times soak up a lot of heat. Some goes into the deep oceans where it can stay for centuries. But heat absorbed closer to the surface can easily flow back into the air. That happened in 1998, which made it one of the hottest years on record.”
Trenberth says since then, the ocean has mostly been back in one of its soaking-up modes.
“They probably can’t go on much for much longer than maybe 20 years, and what happens at the end of these hiatus periods, is suddenly there’s a big jump [in temperature] up to a whole new level and you never go back to that previous level again,” he says.
You can think of it like a staircase. Temperature is flat when a natural cool spell cancels out the gradual temperature increase caused by human activity. But when there’s a natural warm spell on top of the long-term warming trend, the story is dramatically different.
“When the natural variability or when the weather is going in the same direction as global warming, suddenly we’re breaking records, we’re going outside of the bounds of previous experience, and that is when the real damage occurs,” Trenberth says.
Trenberth’s view is that the earth is out of balance (acquiring more heat from the Sun than it can reject to space) and this excess heat finds its way into the oceans. When enough heat is stored in the surface waters, it eventually comes out as in the form of an El Niño. But, we have had El Niños on and off for hundreds of years without rising greenhouse gases, and there was very significant El Niño activity between 1900 and 1942 when CO2 concentrations were much lower. The regime shift at year 1900 was just as dramatic as the regime shift of 1977, and the persistence of El Niños from 1900 to 1942 was just as pervasive as that from 1977 to 1998. Any proposed explanation would have to deal with this widely ignored early phase of high El Niño activity.
And once the heat is released from the ocean to the atmosphere via an El Niño, why can “you never go back to that previous level again”? In fact, during the long hiatus in El Niño dominance from 1942 to 1976, the earth did cool somewhat. Would a really good dose of La Niñas, such as prevailed prior to 1900, bring earth temperatures down significantly? Or is Trenberth’s “staircase” analogy correct – that we can only go up from here?
Now, Trenberth seems to be pinning his hopes for further warming on the emergence of a new era of El Niño dominance to release all the heat he believes is stored in the surface waters of the Pacific. He might have a good point here but since we are dealing here with the crucial process by which the earth warms as a result of some sort of supposed interaction between rising greenhouse gases and their presumed impact on the El Niño – La Niña balance in the pacific, surely this topic requires a great deal more explanation, explication and filling out of details than these brief sound bites by Trenberth?
Indeed, all the climate scientists in the world ought to halt their myriad little ventures and come to grips with this all consuming aspect of climate change. Is the mechanism for global warming that rising greenhouse gas concentrations produce a heat imbalance that warms surface waters of the Pacific, leading to periodic episodes of El Niño dominance that acts as THE mechanism for warming the earth? If so, why do we have these extended periods (1942-1976 and 1998-2014) where the Pacific is supposedly biding its time while storing up energy?
Wang and Picaut (2004) wrote an excellent review of proposed mechanisms for ENSO phenomena. They pointed out “The issue of ENSO as a self-sustained oscillation mode or a stable mode triggered by random forcing is not settled… Since 1988, four concepts have been proposed for the oscillatory and self-sustained nature of ENSO. They also represent the negative feedbacks of a growing ENSO stable mode triggered by stochastic forcing, and are unified in a single concept. More data and model diagnoses are needed to test these concepts or to discover others.” They also went on to say:
“The relationship between ENSO and global warming is largely unknown. We are not even sure if greenhouse warming will result in an El Niño-like or La Niña-like pattern in the tropical Pacific. To understand the relationship between anthropogenic and natural climate variability, global coupled ocean-atmosphere models must be greatly improved and simulate both ENSO and the response to greenhouse warming.”
They closed with a statement: describing ENSO as a “fascinating puzzle of nature”.
Clearly, Trenberth’s sound bites don’t answer the mail on this puzzle.
Eight years later, Wang et al. (2012) concluded
“The issue of ENSO as a self-sustained oscillation mode or a stable mode triggered by random forcing is not settled. It is possible that ENSO is a self-sustained mode during some periods, a stable mode during others, or a mode that is intermediate or mixed between the former and the latter.”
In regard to the effect of global warming on ENSO, Wang et al. (2012) concluded:
“ENSO changes under global warming are uncertain. The tropical Pacific response to global warming has been suggested to be neither El Niño-like nor La Niña-like since the mechanisms for these changes are different from that of ENSO events – the Bjerknes feedback. Increasing greenhouse gases changes the background mean states in the tropical Pacific Ocean and atmosphere which in turn induce ENSO changes. However, the response of the mean states to increasing greenhouse gases is uncertain. For example, the tropical Pacific zonal SST contrast under global warming is reported to be either strengthened or weakened. The uncertainty in the eastern Pacific warming may be also caused by the Atlantic warming. Due to the fact that the change in tropical mean condition under global warming is quite uncertain even during the past few decades, it is hard to say whether ENSO is going to intensify or weaken, but it is very likely that ENSO will not disappear in the future.”
Cobb et al. (2013) used corals to estimate ENSO variance throughout the Holocene. They concluded:
“Twentieth-century ENSO variance is significantly higher than average fossil coral ENSO variance but is not unprecedented. Our results suggest that forced changes in ENSO, whether natural or anthropogenic, may be difficult to detect against a background of large internal variability.”
1. Nino Indices and Running Sums 1870 – Present
1.1 Cane (2004)
Cane (2004) provided Nino3 data as shown in Figure 1. The data in this figure is “commonly used index of El Niño, the sea surface temperature (SST) anomaly in the NINO3 region of the eastern equatorial Pacific (90°W-150°W, 5°S-5°N)”.
Figure 1. The Nino3 index according to Cane (2004).
This data is presented in slightly different form in Figure 2.
Figure 2. The Nino3 index according to Cane (2004), with color added.
Figure 3 suggests that over the time period 1866 to the present, there have been five eras of El Niño activity:
- From 1866 to 1900, La Niñas were more prevalent and stronger than El Niños. Also during this period, the earth was cooler than it is today..
- Starting in year 1900, and continuing to about 1941, El Niños were more prevalent and stronger than La Niñas. Also during this period, the earth warmed significantly.
- From about 1941 to about 1976, El Niños and La Niñas were fairly balanced, although there was a slight excess of La Niñas. The earth cooled slightly during this period.
- From 1976 to 1998, El Niños strongly dominated over La Niñas. This 23-year period included the very strong El Niño of 1982-3 and culminated in the very strong El Niño of 1997-8. The earth warmed rapidly during this period from 1976 to 1998. In fact, about half the warming of the 20th century occurred during those 23 years.
- With the termination of the great El Nino of 1997-8, we entered a period of balance between El Ninos and La Ninas and the earth’s temperature remained essentially unchanged from 1998 to 2014.
Figure 3. The Nino3 index according to Cane (2004) showing five eras.
To clarify the various periods of time with differing El Nino and La Nina ratios, we plot a running sum of the data from Figure 2 in Figure 4. (Bob Tisdale presents many such running sums in his various postings).
Figure 4. Running sum of Nino3 index according to Cane (2004) showing five eras.
A comparison of the running sum of the Nino3 index and the global average temperature is given in Figure 5.
Figure 5. Comparison of the running sum of the Nino3 index and the global average temperature.
1.6 Douglass (2010)
Douglass (2010) refined the Nino3.4 index. His results are shown in Figure 6. Douglass’ results are very similar to those of Cane (2004).
Figure 6. Nino3.4 index and running sum according to Douglass (2010).
1.3 Wolter and Timlin (2011)
Wolter and Timlin (2011) developed a Multivariate ENSO Index (MEI) that combines inputs from six atmosphere–ocean variable fields in the tropical Pacific basin. It is claimed that it provides a more complete and flexible description of the ENSO phenomenon than single variable ENSO indices such as the Niño indices. Their result is shown in Figure 7. The results of Wolter and Timlin (2011) indicate that the entire period from 1872 to 1918 was primarily dominated by La Niñas with a few exceptions (1877-8, 1897-1907). The periods from 1918 to 1942 and from 1977 to 1998 were dominated by El Ninos. While their running sum differs quantitatively from that of Cane (2004) it still suggests cold prior to 1920, roughly neutral from 1942 to 1977, and warm from 1977 to 1998.
1.4 Yu and Kim (2013)
Yu and Kim (2013) distinguished between El Niño events of the Central-Pacific (CP) type, the Eastern-Pacific (EP) type, or the Mixed type. But totaling up all types of events, their results are shown in Figure 8. Their results differ in detail from those of Cain (2004) and Wolter and Timlin (2011). In particular they show the period prior to 1942 to be dominated by La Niñas.
Figure 7. The Multivariate ENSO Index (MEI) of Wolter and Timlin (2011).
Figure 8. El Niño events according to Yu and Kim (2013).
1.5 Giese and Ray (2011)
Giese and Ray performed a new ocean reanalysis that assimilated all available hydrographic and sea surface temperature data into a model of the global ocean forced with surface boundary conditions from an atmospheric reanalysis, covering the period from 1871 to 2008. Their result is shown in Figure 9.
Figure 9. El Niño – La Niña balance according to Giese and Ray (2011).
1.6 Minobe et al. (2004)
Minobe et al. presented Figure 10 that gives their estimate of the standard deviation from average of the Pacific Decadal Oscillation. This clearly shows the era of El Niño events dominating between 1900 and 1941, except for 1916-1920, La Niña events dominating between 1940 and 1976, and El Niño events again dominating from 1976 to 1998. These results are plotted in Figure 11.
Figure 10. Standard deviation from average of the Pacific Decadal Oscillation according to Minobe et al. (2004).
Figure 11. Standard deviation from average of the Pacific Decadal Oscillation according to Minobe et al. (2004).
2. Regime Shifts in the 20th Century
It has been widely acknowledged in the literature that the Pacific Ocean underwent a rather sudden change in the period around 1976-1977, in which all relevant indices (PDO, SOI, ENSO) turned sharply toward prevalence of El Niño events compared to La Niña events. For example, Minobe et al. (2004) said:
“One of the interesting features of the decadal variability over the Pacific Ocean is the so-called climatic “regime shift”, the 1976/77 regime shift being a prime and well studied example.”
Trenberth and Hoar (1997) concluded that “the tendency for more El Niño and fewer La Niñas events since the late 1970s is highly unusual and very unlikely to be accounted for solely by natural variability.” However they admitted “at present, it is not possible to make such an attribution” [whether this ENSO behavior is linked to or a consequence of global warming due to increased greenhouse gas concentrations].
Frauenfeld et al. (2005) mentioned a “Pacific Climate Shift with negative anomalies prior to 1976/77 and almost exclusively positive anomalies since…”
Guilderson and Schrag (1998) said:
“Several studies have noted that the pattern of El Niño–Southern Oscillation (ENSO) variability changed in 1976, with warm (El Niño) events becoming more frequent and more intense. This ‘1976 Pacific climate shift’ has been characterized as a warming in SSTs through much of the eastern tropical Pacific.”
DiLorenzo et al. (2007) said:
“Particularly dramatic physical and biological excursions occurred during the 1976–77 change in the Pacific Decadal Oscillation.”
Hare and Mantua (2000) said:
“It is now widely accepted that a climatic regime shift transpired in the North Pacific Ocean in the winter of 1976–77. This regime shift has had far reaching consequences for the large marine ecosystems of the North Pacific. Despite the strength and scope of the changes initiated by the shift, it was 10 to 15 years before it was fully recognized. Subsequent research has suggested that this event was not unique in the historical record but merely the latest in a succession of climatic regime shifts.”
Wu, Lee, and Liu (2005) said:
“The 1970s North Pacific climate regime shift is marked by a notable transition from the persistent warming (cooling) condition over the central (eastern) North Pacific since the late 1960s toward the opposite condition around the mid 1970s…This large-scale decadal climatic regime shift has produced far-reaching impacts on both the physical and biological environment over the North Pacific and downstream over North America.”
Kim and Miller (2007) studied “the 1976/1977 climate regime shift.” They concluded that the thermocline warmed but did not deepen.
Power and Smith (2007) emphasized that “the lowest 30-year average value of the June–December SOI just occurred in 1977–2006” along with “the highest tropical sea-surface temperatures on record [in] what appears to be a concurrent period of unprecedented El Niño dominance.”
Indeed, a casual glance at Figures 1 to 11 would confirm that such a change did indeed take place during the 1976-1977 period.
Other regime shifts have been mentioned in the literature. Minobe et al. (2004) mentioned regime shifts in the 1920s, the 1940s and the late 1990s.
Gedalof and Smith (2001) said: “the step-like climate shift that occurred in 1976-1977 is not a unique event, with similar events having occurred frequently during the past 400 years”. They also concurred that regime shifts occurred in the 1920s (a trend favoring El Niños) and the 1940s (a trend favoring La Niñas). While there are some significant differences in details in the various studies presented in Figures 1-10, all the figures suggest (in one form or another) a trend favoring El Niños starting in the 1920s and a trend favoring La Niñas starting in the 1940s. The ending of the warming trend after the El Niño of 1998 has been a source of extensive discussion on the Blogs, although the IPCC side-stepped it.
Figures 4 and 6 suggest a strong regime shift around year 1900, of about the same magnitude as the regime shift at 1977. This occurred when the CO2 concentration was lower than 300 ppm.
3. The balance Between El Niños and La Niñas Since 1500
D’Arrigo et al. (2005) provided an estimate of the Nino3 index over the past six centuries based on tree ring data but the comparison of their model with data during the calibration period (1860 to 1980) is unimpressive. Nevertheless they found La Niñas dominant in the late 1800s and from about 1940 to 1980, while El Niños were dominant from about 1900 to 1940. From 1400 to about 1800, El Niños and La Niñas were roughly equally balanced.
Li, et al. (2013) produced a seven-century-long ENSO reconstruction based on 2,222 tree-ring chronologies from both the tropics and mid-latitudes in both hemispheres. They claimed that inclusion of tropical records enabled them to achieve unprecedented accuracy in this estimate. Their result is shown in Figure 12. However, one disturbing aspect of this figure is that during the period 1900-1940, the Pacific was predominantly in the El Niño mode, whereas their figure suggests this period to be predominantly in the La Niña mode. Nevertheless, the main point of their result is that it indicates that from 1300 to about the middle of the 20th century, the state of the Pacific meandering between neutral and predominantly La Niña, with El Niños generally less frequent than La Niñas.
Figure 12. Reconstructed sea surface temperatures in the Nino3.4 region. (Li, et al. (2013)).
Gergis and Fowler (2008) used proxies to estimate the number of strength of La Niñas and El Niños from the 1520s to recent times. They categorized these events as extreme, very strong, strong, moderate or weak. They tabulated the number of events for each intensity, per ten-year period, starting in the 1520s. Their results indicate that La Niñas predominated from 1520 to 1900 (with a short period of strong El Niños from 1700 to 1725). After 1900 there was a slight preference for El Niños. Their results for the 20th century do not agree with results of other studies, so their entire set of results is of uncertain veracity.
Gedalof and Smith (2001) found a number of regime shifts during the period 1600-2000. They found:
Preference for El Niños: 1680-1696; 1712-1734; 1758-1796; 1816-1840; 1923-1940; 1977-1998
Preference for La Niñas: 1696-1712; 1798-1816; 1946-1977
Link to References
JC note: This post was submitted to me via email. This is a guest post, please be civil. This is also a technical thread; keep your comments strictly on topic