Moisture budget analysis of monsoonal precipitation in the Sahel region of Africa

It has been well documented that characteristics of the African Monsoon, which supplies the Sahel region with the majority of its annual rainfall, have greatly changed throughout Earth’s history. In recent decades alone the region has transitioned from a relatively wet climate to severe prolonged drought, to a partial recovery of monsoonal rains. While many studies have attributed this recent variability in part to oceanic influences on the monsoon, the scale and duration of the late twentieth century drought as well as the previous wet period have yet to be reproduced in coupled ocean-atmosphere general circulation models. Thus our present understanding of variations in the African monsoon, particularly in the Sahel region, is far from complete, making it difficult to estimate the monsoon’s response to climate change. Using data from available observations, the NCEP/NCAR Reanalysis, and ERA-40 Reanalysis, this study aims to understand the moisture budget of the Sahel, specifically focusing on differences amongst the various decades: wet (1949-1967), dry (1968-1989), and “partial recovery” (1990-2011). In breaking down the moisture budget equation, contributions from the mean and transient flow to the overall monsoon characteristics are studied, as is the interplay between differences in moisture transport and circulation dynamics of the atmosphere during the different time periods. Preliminary results suggest that it is the relationship between the upper level air flow and surface winds that may explain the variations in monsoonal precipitation amongst the three time periods. Our diagnostic analysis of the observations guides us to look into a set of general circulation model ensembles, one forced with observed global SSTs and others where SST is prescribed only in various tropical sub-basins. This model analysis allows us to examine in more detail the role of SST forcing in African monsoon variability. Specifically, it points to the roles of the Atlantic, Pacific, and Indian Ocean in causing the 20th century decadal Sahel precipitation variability.