Simulating Multi-Season Past Droughts
Abstract

Abstract Author: Michela Biasutti

Abstract Title: The role of the Sahara Low in Sahel Rainfall Variability and Change in the CMIP3 Models

Abstract: Global sea surface temperatures (SSTs) are the main driver of interannual and interdecadal variations of Sahel rainfall in observations, atmospheric general circulation models, and the CMIP3 coupled climate models. Yet, in the CMIP3 ensemble, twenty-first century trends in Sahel rainfall cannot be expressed in terms of corresponding trends in SSTs. Conversely, different changes in Sahel rainfall across the ensemble models correspond to similar changes in the strength of the Sahara Low (defined as the difference between the Sahara mean and the global mean geopotential height at 925hPa).

The Sahara Low can serve as a predictor for trends in Sahel rainfall. Changes in the Sahara Low can indeed be a cause, rather than an effect, of changes in Sahel rainfall, as can be inferred from the Sahara/Sahel relationship at interannual-interdecadal time scales: variability of the Sahara Low leads that of Sahel rainfall, while the Rossby wave response to anomalies in Sahel rainfall is limited to the western portion of the Sahara.

Variability of the Sahara Low does not respond primarily to local surface temperature as one would expect for a heat Low, but is driven remotely---on one hand, by tropical SSTs and, on the other hand, by the summer North Atlantic Oscillation (NAO) and the position of the jet. The influence of the former is strongest during the core of the rainy season, the latter during its beginning and end, in late spring and early fall.

The fact that mid-latitude jet dynamics can drive variability in Sahel rainfall, through the Sahara Low, suggests a possible role for it at the timescales of climate change---for example in response to the enhancement of Northern Hemisphere Land/Sea contrast or to downward propagation of stratospheric changes in the Northern Annular Mode. We suggest that these aspects of the models' simulations of the twentyfirst century must also be taken into account, alongside the pattern of the SST trend, if one wants to explain the disparate projections for Sahel rainfall.