January 14th, 2013
Key Findings
- The All Forcings HadGEM2-ES historical simulations exhibit no trend in the North Atlantic upper ocean heat content between 1955 and 2004. The Constant Aerosols HadGEM2-ES historical simulations show a clear warming trend. The comparison between these two shows the important role of anthropogenic aerosols in causing the discrepancy between simulated and observed trends.
- The observed sea surface temperature pattern suggests an important role for AMOC variations. The net aerosol response in HadGEM2-ES shows excess cooling in most ocean basins, and cannot explain the observed pattern.
- The simulated subpolar North Atlantic sea surface salinity in HadGEM2-ES shows an unrealistic positive trend, mainly due to the aerosol response.
- The aerosol mechanism cannot account for the observed anticorrelated multi-decadal tropical North Atlantic SST and subsurface temperature variations, regardless of whether the aerosol effects are overestimated. This discrepancy is inconsistent with the interpretation that aerosol forcing drives the bulk of the observed Atlantic multi-decadal variability.
Rong Zhang, Thomas L. Delworth, Rowan Sutton, Dan Hodson, Keith Dixon, Isaac Held, Yochanan Kushnir, John Marshall, Yi Ming, Rym Msadek, Jon Robson, Tony Rosati, MingFang Ting, Gabriel Vecchi. Journal: Journal of Atmospheric Sciences. DOI: 10.1175/JAS-D-12-0331.1
Summary
Identifying the main drivers of the twentieth-century multi-decadal variability in the Atlantic Ocean is crucial for predicting how the Atlantic will evolve in the coming decades and the resulting broad impacts on weather and precipitation patterns around the globe. A paper recently published in Nature from the Met Office Hadley Centre suggested that aerosols are a prime driver of twentieth-century North Atlantic climate variability, based on simulations using the HadGEM2-ES (UK Met Office Hadley Centre Earth System Model).
In this paper, GFDL authors and colleagues from the University of Reading, Columbia University, and MIT, show that there are major discrepancies between the HadGEM2-ES simulations and observations in the North Atlantic upper ocean heat content, in the spatial pattern of multi-decadal sea surface temperature changes within and outside the North Atlantic, and in the subpolar North Atlantic sea surface salinity. These discrepancies may be strongly influenced by, and indeed in large part caused by, aerosol effects.
The authors show that the aerosol effects simulated in HadGEM2-ES cannot account for the observed anti-correlation between detrended multi-decadal surface and subsurface temperature variations in the tropical North Atlantic. These discrepancies cast considerable doubt on the claim that aerosol forcing drives the bulk of this multi-decadal variability.
