Impact of warm ENSO events on atmospheric circulation and convection over the tropical Atlantic and West Africa

S Janicot

doi:10.1007/s005850050461

Abstract

Empirical studies have shown that warm El Nino/Southern Oscillation (ENSO) episodes are associated during northern summer with, first, a southward location of the intertropical convergence zone (ITCZ) over the tropical Atlantic, and, second, a weakened convection over West Africa where the ITCZ is near its mean latitude. A modelling experiment presented here is used to help explain this apparent contradiction. In simulated ENSO conditions, the ITCZ is located southwards over the tropical Atlantic. Over West Africa the intertropical front is also displaced southwards, but more slightly; the ITCZ is located at its climatological latitude and the vertical development of convective clouds over West and Central Africa is reduced due to dynamical subsidence in the upper levels.

Highlights

West African rainfall variability has been connected to sea-surface temperature (SST) anomalies in the tropical Atlantic (Lamb, 1978a,b), and in distant oceanic basins (Folland et al, 1986; Palmer, 1986)
These anomalies are signi®cant at the 1% level over the tropical Paci®c between 20°N and 15°S and over the tropical Atlantic between 25°N and 15°S; they are not signi®cant over West Africa
Higher positive pressure departures over the Atlantic along 15°N lead to a northward meridional pressure gradient along 10°N. This is consistent with the simulated anomaly wind ®eld at 900 hPa (Fig. 4): west of 15°W along 10°N the north-easterly trade winds are enhanced, resulting from the positive Paci®c SST anomalies which lead to a larger wind convergence in the eastern Paci®c intertropical convergence zone (ITCZ) located at 10°N in August

Summary

Introduction

Hastenrath et al (1987) computed the correlation pattern between the Southern Oscillation Index of Parker (1983) and the mean sea-level pressures in the tropical Atlantic and Paci®c basins in the July-August periods 1935±1983 (Fig. 1a). On the other hand, Moron (1993, 1995) analysed statistical connections between ENSO events and African convection estimated through outgoing longwave radiation (OLR) records He computed the warm (1976±79±82±83±87) minus cold (1975± 81±84±85±88) ENSO composite of OLR anomaly ®elds over West Africa in July±August. This ®eld (Fig. 2a) displays an overall decreasing convection over West and Central Africa, which is not consistent with a possible southward displacement of the ITCZ over the continent, as Hastenrath et al (1987) observed over the tropical Atlantic. A modelling experiment is presented here to investigate this point

Experimental design

Results

Conclusion