The relationship between the boreal spring (or the austral autumn) Antarctic Oscillation (AAO) (March- April) and the West African summer monsoon (WASM) (June-September) is analyzed based on NCEP/NCAR reanalysis data. The results show that the linkage of the boreal spring AAO to the WASM exhibits decadal-scale variations: a strong connection between the two appears over the period 1985-2006 and a weak connection over the period 1970-1984. Further analysis indicates that such an unstable relationship between the two results from the modulation by ENSO events to a large extent.
A possible mechanism for the impacts of the boreal spring AAO on the WASM is also discussed. The variability of the boreal spring tropical South Atlantic sea surface temperature (SST) appears to serve as a bridge linking these two systems. The boreal spring AAO produces an anomalous SST over the tropical South Atlantic by exciting an equatorward Rossby wave train over the western Southern Hemisphere (SH). This AAO-related SST anomaly modulates the meridional gradient of moist static energy (MSE) between the Sahel and the Guinea-tropical Atlantic region in the boreal spring. The MSE gradient is of paramount importance for the changes from spring to summer in the West African monsoon because its relaxation along the seasonal cycle is linked to the northward excursion of the WASM system into the African continent. Therefore, an anomalous AAO-related MSE gradient can lead to anomalous Sahel rainfall in the early summer. When this rainfall occurs over the Sahel, the local positive soil moisture-rainfall feedback plays a crucial role in sustaining and prolonging this rainfall anomaly throughout the whole summer.
2010 by Meteorological Society of Japan