GFDL - Geophysical Fluid Dynamics Laboratory

Have Aerosols Caused the Observed Atlantic Multi-decadal Variability?

January 14th, 2013

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.

North Atlantic upper ocean heat content anomaly. Red line: area-averaged North Atlantic upper ocean heat content anomaly (0-700m, 75-7.5W, 0-60N) from ensemble mean of HadGEM2-ES All Forcings simulations. Yellow shading: 1 std of ensemble spread of All Forcings. Green Line: ensemble mean from Constant Aerosols historical simulations. Black Line: observations. All anomalies are relative to 1955-2004 mean. The dash lines are linear trends for the respective variables.
North Atlantic upper ocean heat content anomaly. Red line: area-averaged North Atlantic upper ocean heat content anomaly (0-700m, 75-7.5W, 0-60N) from ensemble mean of HadGEM2-ES All Forcings simulations. Yellow shading: 1 std of ensemble spread of All Forcings. Green Line: ensemble mean from Constant Aerosols historical simulations. Black Line: observations. All anomalies are relative to 1955-2004 mean. The dash lines are linear trends for the respective variables.