Regional Drivers of the Sahel Precipitation Response to Anthropogenic Aerosol Forcing

Abstract:

Understanding the roles of past external forcing on Sahel climate is key for evaluating the possible future evolution of the region’s climate. Global circulation models (GCMs) show that external forcing has had a substantial role in driving late-20th century multidecadal variability of Sahel precipitation. Recent work, such as Undorf et al., 2018, has found a detectable signal of anthropogenic aerosol (AA) forcing in this variability. In analyses of Large Ensembles of CESM1 and CanESM2 simulations, we also find that AA forcing is the main external forcing driving a forced multidecadal swing in Sahel precipitation, though the models underestimate the magnitude of multidecadal variability relative to observations.

We seek to study the mechanisms underlying the AA-forced signal by conducting a set of atmosphere-land GCM (AGCM) simulations in CAM5 and CanAM4 (the atmospheric components of CESM1 and CanESM2 respectively). In these simulations, we isolate the atmospheric response directly due to the forcing (the “fast” direct-atmospheric response) from the atmospheric adjustments to AA-forced sea surface temperature (SST) changes (the “slow” ocean-mediated response) for the 1950s to 1970s and the 1970s to 2000s. We find that the 1950s to 1970s Sahel drying is a direct-atmospheric response to AA-forcing while the 1970s to 2000s Sahel wetting is predominantly an ocean-mediated response.

To understand these signals, further CAM5 experiments are carried out to determine the effect of regional emission sources and the effect of SST anomalies in different ocean basins. Notably, these experiments suggest that Asian AA emissions drive the 1970s to 2000s direct-atmospheric drying in the Sahel in spite of declining European and North American emissions. On the other hand, a weak 1950s to 1970s ocean-mediated response is the result of opposing responses to SST anomalies in the Atlantic versus Pacific.