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 perio d January 1964-December 1989, the interannual variability in the stren gth of the tropical Hadrey cells is investigated. Although several mea sures of the intensity of the zonal-mean cells are discussed, the main focus is on the maximum in the streamfunction ia the northern and sou thern Tropics. The streamfunction was computed from observed monthly m ean latitude versus pressure cross sections of the zonal-mean meridion al wind component. Significant seasonal Variations are found in the st rength, latitude, and height of the maximum streamfunction for both Ha dley cells. Significant correlations are also observed between the Had ley 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 bran ch 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 N ino) and weakened Hadley cells during episodes of cold anomalies. The anomalies in the strength of the Hadley cells are strongly and inverse ly correlated with the anomalies in the strength of the Walker oscilla tion.