Weakening and Shifting of the Saharan Shallow Meridional Circulation During Wet Years of the West African Monsoon

TL;DR

This study challenges the idea that increased Sahel rainfall is caused by a stronger Saharan shallow meridional circulation, showing instead that during wet years, the circulation weakens and shifts poleward, which correlates with increased rainfall.

Contribution

It demonstrates that the Saharan SMC weakens and shifts poleward during wet Sahel years, contradicting previous assumptions of a strengthening circulation causing rainfall increases.

Findings

Saharan SMC shifts poleward during wet years.

Weakening of the SMC correlates with increased Sahel rainfall.

Idealized model reproduces the SMC behavior observed in reanalyses.

Abstract

The correlation between increased Sahel rainfall and reduced Saharan surface pressure is well established in observations and global climate models, and has been used to imply that increased Sahel rainfall is caused by a stronger shallow meridional circulation (SMC) over the Sahara. This study uses two atmospheric reanalyses to examine interannual variability of Sahel rainfall and the Saharan SMC, which consists of northward near-surface flow across the Sahel into the Sahara and southward flow near 700 hPa out of the Sahara. During wet Sahel years, the Saharan SMC shifts poleward, producing a drop in low-level geopotential and surface pressure over the Sahara. Statistically removing the effect of the poleward shift from the low-level geopotential eliminates significant correlations between this geopotential and Sahel precipitation. As the Saharan SMC shifts poleward, its mid-tropospheric divergent outflow decreases, indicating a weakening of its overturning mass flux. The poleward shift and weakening of the Saharan SMC during wet Sahel years is reproduced in an idealized model of West Africa; a wide range of imposed sea surface temperature and land surface albedo perturbations in this model produce a much larger range of SMC variations that nevertheless have similar quantitative associations with Sahel rainfall as in the reanalyses. These results disprove the idea that enhanced Sahel rainfall is caused by strengthening of the Saharan SMC. Instead, these results are consistent with the hypothesis that the a stronger SMC inhibits Sahel rainfall, perhaps by advecting mid-tropospheric warm and dry air into the precipitation maximum.