Observed interannual variability in the Hadley circulation and its connection to ENSO

Based on a 26-yr set of daily global upper-air wind data for the period January 1964-December 1989, the interannual variability in the strength of the tropical Hadrey cells is investigated. Although several measures of the intensity of the zonal-mean cells are discussed, the main focus is on the maximum in the streamfunction ia the northern and southern Tropics. The streamfunction was computed from observed monthly mean latitude versus pressure cross sections of the zonal-mean meridional wind component. Significant seasonal Variations are found in the strength, latitude, and height of the maximum streamfunction for both Hadley cells. Significant correlations are also observed between the Hadley cells and the El Nino-Southern Oscillation phenomenon. During the extreme seasons, only one ''winter'' Hadley cell dominates the Tropics, with the rising branch in the summer hemisphere and the sinking branch in the winter hemisphere. Superimposed an this ''normal'' one-cell winter Hadley circulation in the Tropics are two strengthened direct (i.e., energy releasing) Hadrey cells found during episodes of warm sea surface temperature anomalies in the eastern equatorial Pacific (EI Nino) and weakened Hadley cells during episodes of cold anomalies. The anomalies in the strength of the Hadley cells are strongly and inversely correlated with the anomalies in the strength of the Walker oscillation.