Posted on February 15th, 2013 in Isaac Held's Blog
(Left) Sea surface temperature averaged over the North Atlantic (75-7.5W, 0-60N), in the HADGEM2-ES model (ensemble mean red; standard deviation yellow) compared with observations (black), as discussed in Booth et al 2012. (Right) Upper ocean (< 700m) heat content in this model averaged over the same area, from Zhang et al 2013 ( green = simulation with no anthropogenic aerosol forcing, kindly provided by Ben Booth.)
A paper by Booth et al 2012 has attracted a lot of attention because of the claim it makes that the interdecadal variability in the North Atlantic is in large part the response to external forcing agents, aerosols in particular, rather than internal variability. This has implications for estimates of (transient) climate sensitivity but it also has very direct implications for our understanding of important climate variations such as the recent upward trend in Atlantic hurricane activity (linked to the recent rapid increase in N.Atlantic sea surface temperatures) and drought in the Sahel in the 1970’s (linked to the cool N. Atlantic in that decade). I am a co-author of a recent paper by Rong Zhang and others (Zhang et al 2013) in which we argue that the Booth et al paper and the model on which it is based do not make a compelling case for this claim.
The interest results from the figure in the left panel above. This model’s forced response agrees very well with the observed surface temperatures averaged over the North Atlantic, so in this model one doesn’t need to invoke internal multidecadal variability to match these observations. (The forced response is estimated by averaging over multiple realizations of the model with different initial conditions). Zhang et al list several aspects of this simulation that seem problematic, exemplified by the upper right panel, which shows a time series of the ocean heat content down to 700m over this same region. (observations from Levitus, 2009). The model does not produce the upward trend in this N. Atlantic heat content. If one removes the anthropogenic aerosol forcing from the model (green line) it fits these observations better.
[The flatness of the heat content in the N. Atlantic in this model is intriguing. Based on discussions with my colleagues Rong Zhang and Mike Winton, this seems to be a consequence of an AMOC (Atlantic Meridional Overturning Circulation) which builds in strength when the aerosol cooling is strong, trying to balance a part of the cooling at the surface with warm waters advected in from the tropics, but also — by a process that is not particularly straightforward — cools the subsurface waters. It’s as if and (where is the mean temperature between the bottom of the mixed layer and 700m) resulting in out phase with for sinusoidal fluctuations, and with these out-of-phase near surface and subsurface temperatures compensating in the total heat content — or something like that. But there is no particular reason to expect close compensation.]
Another problematic aspect of the N.Atlantic simulation is the co-variability of temperature and salinity. Decadal scale temperature and salinity variations in the subpolar Atlantic tend to be positively correlated in observations. In particular, the cold period in the 70’s was marked by a fresh subpolar Atlantic. This is what one expects when the AMOC is weak, with less transport of more saline waters from the subtropics and more export of fresh waters from the Arctic. The model does not show this correlation, and in the 70’s it has relatively high salinity (presumably due to the stronger AMOC mentioned in the previous paragraph). Our understanding of AMOC variability is admittedly limited, but the temperature-salinity correlations point towards there being a substantial internal component to the observations. These Atlantic temperature variations affect the evolution of Northern hemisphere and even global means (e.g., Zhang et al 2007). So there is danger in overfitting the latter with the forced signal only.
Our lab has a model, CM3 (Donner et al, 2011), that also has strong indirect aerosol effects and that produces simulations of the past century that share many of the features of HAD-GEM2-ES discussed here, including the nice fit to the N. Atlantic SSTs. So this issue is naturally a hot topic of conversation in our lab. The issue has been around for a while. For example, Rotstayn and Lohmann 2002 made a case that strong aerosol forcing could explain the Sahel drought of the 70’s by cooling the N. Atlantic. The same qualitative behavior is seen in many models, but we are left with the quantitative question of how big the aerosol effect is.
Differences of opinion make life interesting and always force us to sharpen our arguments. And there remain strong differences of opinion on the relative importance of AMOC variability and aerosol forcing for the non-monotonic variation of North Atlantic surface temperatures and all the phenomena that we think are affected by it (including hurricanes and African rainfall). But I remain skeptical that one can make a compelling case for aerosol dominance by focusing only on SSTs, without simultaneously considering salinities and sub-surface temperatures that are better able to distinguish between forced and free variations.
(conversations with Rong Zhang, Mike Winton, and Yi Ming have helped me think about this issue)
[The views expressed on this blog are in no sense official positions of the Geophysical Fluid Dynamics Laboratory, the National Oceanic and Atmospheric Administration, or the Department of Commerce.]