by Judith Curry
Yesterday, I started writing a post on air/sea fluxes. A new paper just published in Nature Climate Change changed my mind about what I want to write about.
Here is the paper just published in Nature Climate Change:
Human-Induced Global Ocean Warming on Multi-Decadal Time Scales
P.J. Gleckler, B.D. Santer, C.M. Domingues, D.W. Pierce, T.P. Barnett, J.A. Church, K.E. Taylor, K.M. AchutaRao, T.P. Boyer, M. Ishii, & P.M. Caldwell
Abstract. Large-scale increases in upper-ocean temperatures are evident in observational records. Several studies have used well-established detection and attribution methods to demonstrate that the observed basin-scale temperature changes are consistent with model responses to anthropogenic forcing and inconsistent with model-based estimates of natural variability. These studies relied on a single observational data set and employed results from only one or two models. Recent identification of systematic instrumental biases in expendable bathythermograph data has led to improved estimates of ocean temperature variability and trends and provide motivation to revisit earlier detection and attribution studies. We examine the causes of ocean warming using these improved observational estimates, together with results from a large multimodel archive of externally forced and unforced simulations. The time evolution of upper ocean temperature changes in the newer observational estimates is similar to that of the multimodel average of simulations that include the effects of volcanic eruptions. Our detection and attribution analysis systematically examines the sensitivity of results to a variety of model and data-processing choices. When global mean changes are included, we consistently obtain a positive identification (at the 1% significance level) of an anthropogenic fingerprint in observed upper-ocean temperature changes, thereby substantially strengthening existing detection and attribution evidence.
Nature Climate Change (2012) doi:10.1038/nclimate1553 [link]
This paper is getting some press, see [here].
This paper looks at several different data sets of volume averaged temperature anomalies for the upper 700 m. They compare several different data sets that use different methods for correcting for XBT biases and infilling where data are not available. The authors state: “ substantial structural uncertainties remain. The impact of different XBT bias corrections is a major source of this uncertainty.” And “Bias corrections have a substantial impact on the time evolution of ΔT, particularly during the 1970s–1980s, when they markedly reduce spurious decadal variability.” CMIP3 20th century model simulations are then compared with these observations, in a detection and attribution exercise similar to that used in the AR4.
A very different perspective on recent ocean warming is provided by Yeager and Large:
On the Observed Trends and Changes in Global Sea Surface Temperature and Air-Sea Heat Fluxes (1984 – 2006)
W.G. Large and S.G. Yeager
Abstract. Global satellite observations show the sea surface temperature (SST) increasing since the 1970s in all ocean basins, while the net air-sea heat flux, Q, decreases. Over the period 1984-2006 the global changes are 0.28°C in SST and -9.1 W/m2 in Q, giving an effective air-sea coupling coefficient of -32 W/m2/°C. The global response in Q expected from SST alone is determined to be -12.9 W/m2, and the global distribution of the associated coupling coefficient is shown. Typically, about one-half (6.8 W/m2) of this SST effect on heat flux is compensated by changes in the overlying near surface atmosphere. Slab Ocean Models (SOMs) assume that ocean heating processes do not change from year to year, so that a constant annual heat flux would maintain a linear trend in annual SST. However, the necessary 6.1 W/m2 increase is not found in the downwelling longwave and shortwave fluxes, which combined show a -3 W/m2 decrease. The SOM assumptions are revisited to determine the most likely source of the inconsistency with observations. The indirect inference is that diminished ocean cooling due to vertical ocean processes played an important role in sustaining the observed positive trend in global SST from 1984 through 2006, despite the decrease in global surface heat flux. A similar situation is found in the individual basins, though magnitudes differ. A conclusion is that natural variability, rather than long term climate change, dominates the SST and heat flux changes over this 23 year period. On shorter time scales the relationship between SST and heat flux exhibits a variety of behaviors.
The implication is that natural variability dominates the SST and the order 10 W/m2 heat flux signals over 1984-2006 time period, with a significant contribution from the 1995-1996 shift from positive to negative NAO index. Also supportive of this possibility are the different ways SST and the various heat flux anomalies behave on decadal and shorter time scales. Although incomplete, the apparent much more steady behavior of SST and heat flux through the 1950s and 1960s is also consistent.
Abstract. Air-sea fluxes from the Community Climate System Model Version 4 (CCSM4) are compared with the Coordinated Ocean-ice Reference Experiments (CORE) dataset to assess present-day mean, variability, and late 20th Century trend biases. CCSM4 is improved over the previous version, CCSM3, in both air-sea heat and freshwater fluxes in some regions; however, a large degradation in net shortwave radiation into the ocean may contribute to an enhanced hydrological cycle. We provide a baseline for assessment of flux variance at annual and interannual frequency bands in future CESM versions and contribute a new metric for assessing any model’s planetary boundary layer (PBL) scheme. Maps of CCSM4 variance ratio to CORE reveal that processes on annual timescales have larger variance error than those on interannual timescales and that different processes cause errors in mean, annual, and interannual frequency bands. Air temperature and specific humidity in the CCSM4 PBL follow the sea surface conditions much more closely than is found in CORE. Sensible and latent heat fluxes are less of a negative feedback to sea surface temperature warming in the CCSM4 than in the CORE data with the model’s PBL allowing for more heating of the 20 ocean’s surface.
doi:10.1029/2012GL051813 [link] to abstract [link] to full paper
This section proposes a nonlinear physical mechanism by which the noise-induced drift effect may operate when the air–sea fluxes contain a stochastic component. The conjectured mechanism permits stochastic fluctuations in the air–sea buoyancy fluxes to modify the time-mean mixed- layer depth. The mixed-layer depth is generally determined by the combined influence of surface buoyancy fluxes and turbulent winds, but only the surface buoyancy fluxes are invoked here, because the response to stochastic winds has been studied elsewhere. Note that the importance of surface buoyancy fluxes is not confined to the polar regions, where surface cooling triggers deep convective events, but extends to all latitudes.
The proposed mechanism relies on a fundamental asymmetry in the physics of the ocean mixed layer, as follows. In a statically stable water column in the mixed layer, dense anomalies at the surface (created by evaporation or cooling) can destabilize the water column, initiate convection and vertical mixing, and deepen the mixed layer. However, in contrast, buoyant anomalies at the surface (created by precipitation or heating) simply further stabilize the water column and cannot shoal the mixed layer. Therefore, the ocean mixed-layer depth responds asymmetrically to positive and negative surface buoyancy fluctuations. In short, positive fluctuations cannot undo the vertical mixing caused by negative fluctuations of equal magnitude. The mechanism has much in common with the mixed-layer demon of Stommel , who proposed that “there is some process at work that selects only late winter water for actual net downward pumping”, “a process much like that performed by Maxwell’s Demon”.
Stochastic fluctuations in the air–sea buoyancy flux are expected to have various impacts, in sequence, on the time-mean climate. First, because of the above mechanism, the mixed layer is expected to deepen systematically. Then, contact with deeper water is expected to cool the mixed layer, especially on the thermal equator where the mixed layer is shallowest. Then, the atmospheric Hadley circulation in each hemisphere is expected to weaken, because warm equatorial surface water provides the thermal energy for these circulations [e.g., Bjerknes, 1966]. Finally, the weakened Hadley circulations are expected to decrease precipitation in the inter-tropical convergence zone (ITCZ), because of decreased horizontal convergence and decreased upward flow, and to increase precipitation in the subtropical high pressure regions at around 30 N and 30 S, because of decreased downward flow.
I have long been concerned by the issue of subgrid stochastic ocean surface fluxes, and the asymmetric effect of cool vs warm anomalies on the ocean. A recent paper by my research group illustrates this issue:
High-resolution Satellite Surface Latent Heat Fluxes in North Atlantic Hurricanes
Jiping Liu, Judith Curry, Carol Anne Clayson, Mark Bourassa
Abstract. This study presents a new high-resolution satellite-derived ocean surface flux product, XSeaFlux, which is evaluated for its potential use in hurricane studies. The XSeaFlux employs new satellite datasets using im- proved retrieval methods, and uses a new bulk flux algorithm formulated for high wind conditions. The XSeaFlux latent heat flux (LHF) performs much better than the existing numerical weather prediction reanalysis and satellite-derived flux products in a comparison with measurements from the Coupled Boundary Layer Air–Sea Transfer (CBLAST) field experiment. Also, the XSeaFlux shows well-organized LHF structure and large LHF values in response to hurricane conditions relative to the other flux products. The XSeaFlux dataset is used to interpret details of the ocean surface LHF for selected North Atlantic hurricanes. Analysis of the XSeaFlux dataset suggests that ocean waves, sea spray, and cold wake have substantial impacts on LHF associated with the hurricanes.
[Link] to paper
For hurricane domain comparisons, the XSeaFlux shows well-organized LHF structures and large LHF values in response to the hurricanes. By contrast, the other flux products (NCEP2, OAFlux, HOAPS3, and ERA-Interim) produce no discernible or weak LHF signals, and no distinct structure in response to the hurricanes.
The issue is this. The reanalysis products, even at a resolution of 100 km or so, miss the high latent heat fluxes associated with severe convective weather (even on the scale of a hurricane). Climate models presumably do an even worse job of capturing the high latent heat flux situations, which are associated with the largest buoyancy fluxes.
So, why does this matter? The issue is the feedbacks associated with changes in ocean mixed layer depth and also the surface latent heat fluxes, which have an important influence on sea surface temperature and upper ocean heat content. Feedbacks associated with the latent heat flux are also tied to the water vapor feedback. For further info on the feedbacks influencing upper ocean temperatures, see section 13.6 in the feedback chapter to my text Thermodynamics of Atmospheres and Oceans.
Yeager and Large have made a valiant attempt to pull together a lot of observations, but this effort highlights the deficiencies that we have in the data sets. Focusing on the period since 1983 makes sense because this is the period where we have the best satellite data sets, but there are still substantial problems with the ocean temperature data sets. Many people I talk to have little confidence in the ocean temperature analyses prior to 1980, and most people agree that there are very substantial problems prior to to 1960. ”Infilling” is big problem; IMO it would be better just to compare models with observations in locations where there is reliable data.
I suspect that the reasoning behind the Gleckler et al. Nature Climate Change article will carry the day in the forthcoming IPCC AR5. However, in light of these other papers, climate models have documented deficiencies in simulating the relevant surface fluxes. Multi-decadal natural internal variability (which is poorly simulated by the climate models) may be the dominant cause of the recent ocean warming (in terms of changes in ocean mixed layer depth and changes in sensible/latent heat fluxes).