A New Look At Sahel Rainfall Variability Over Two Centuries

Florida State University, Tallahassee FL

Investigators

Abstract

This project is a re-examination of the dynamics of Sahel rainfall variability using datasets which provide long-term coverage of the region, in one case going back more than two centuries. The re-examination is motivated by the strong dependence of current understanding on observations over the latter half of the 20th century, when the region experienced prolonged wet (1950-1967) and dry (1968-1997) periods with strong year-to-year persistence. The behavior of Sahel rainfall in the 21st century is in some ways atypical compared to this climatology, thus the context provided by a longer-term record may prove valuable. One significance difference between the recent years and the second half of the last century is that in the recent record the late rains (in August and September) appear to be less stable from year to year than the June-July rains, thus the importance of the two parts of the rainy season may be shifting over time. A further assumption which is challenged in the recent record is that Sahel rainfall variability is spatially coherent over the considerable longitudinal extent of the region. In particular, the recovery from the late 20th century drought was stronger in the western portion of the Sahel. The research focuses on the relationship between rainfal and atmospheric circulation features, including a low-level African Westerly Jet (AWJ) near the equator and the upper-tropospheric Tropical Easterly Jet (TEJ). The monsoon rain belt lies between the axes of the two jets and is thus sensitive to their fluctuations. Variations of the TEJ and AWJ lead to two distinct modes of Sahel rainfall variability: a north-south shift in the monsoon rain belt which is primarily driven by meridional displacements of the AWJ; and an overall change in the intensity of the rains which is governed by the speed of the TEJ. The work has societal broader impacts given the susceptibility of the Sahel to prolonged drought and consequent food shortages and related societal disruption. The PI has a long record of service as an advisor to US agencies and African governments and meteorological services, and the work performed here may be of value for developing empirical drought forecasting techniques. In addition, the work will support and train two graduate students, thereby contributing to the future workforce in this research area.