Interannual and longer term variability of upper-air circulation over the tropical Atlantic and West Africa in boreal summer

Abstract

The 1958–1997 National Center for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) upper-air data set and surface ship observations have been analysed for the July–August extremum of the annual cycle in the tropical Atlantic–West African sector and the core of the Sahel rainy season. (i) In the long-term mean, the Intertropical Convergence Zone (ITCZ) sits over the warmest surface waters in the Atlantic near 8°–11°N, with another lower-tropospheric convergence maximum near 20°N over West Africa. The Tropical Easterly Jet (TEJ) is found at 200 hPa and 5°–8°N, and the West African Mid-Tropospheric Easterly Jet is found at 600 hPa and 15°–18°N. (ii) During years of extreme drought in the Sahel, the band of warmest surface waters is displaced southward and concomitant with this is a southward shift of the strongest lower-tropospheric convergence, ascending motion and upper-tropospheric divergent outflow, as well as a weakened TEJ. (iii) During the low/warm as opposed to the high/cold Southern Oscillation (SO) phase, the Sahel tends to be drier, but differences in circulation are not pronounced except for westerly wind departures in the upper troposphere. (iv) Over the 40-year record, the decreasing Sahel rainfall has been accompanied by statistically significant trends towards increasing upper-tropospheric convergence, mid-tropospheric subsidence and lower-tropospheric divergence and outflow, a southward shift of the wind confluence, as well as the weakening of the lower-tropospheric westerly monsoon flow and the TEJ. The study thus substantiates the characteristics of upper-air circulation and its variability, as captured by the NCEP-NCAR Reanalysis. Copyright © 2000 Royal Meteorological Society