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The impact of the AMO on the West African monsoon annual cycle



A combination of observations and re-analysis was used to investigate the mechanisms of the connection between the Atlantic Multidecadal Oscillation (AMO) and Sahel rainfall. A composite technique based on the AMO index was used to identify differences between warm and cold phases of the AMO. A significant summer rainfall increase over the Sahel during warm phases of the AMO was observed, with large increases during the typical monsoon onset period in June. In spring of warm phases of the AMO prior to monsoon onset, strengthening of the Saharan heat-low and its associated shallow meridional overturning circulation is observed. The intensified shallow meridional overturning circulation increases moisture flux into the Sahel from the south during spring while increased westerly winds from the Atlantic increase westerly moisture flux into the Sahel during spring and summer. The strengthening of the heat-low is accompanied by increases in Mediterranean sea-surface temperatures during warm phases of the AMO that lead to increases in moisture flux convergence in the northeast Sahel. During warm phases of the AMO the African easterly jet is farther north than in cold phases, and increased African easterly wave (AEW) activity across West Africa and into the Atlantic is observed. This increased AEW activity particularly in the early hurricane season, as measured by eddy kinetic energy, may be contributing to the increased number of Atlantic tropical storms during warm phases of the AMO.