Dynamical effect of the zonal wind anomalies over the tropical western Pacific on ENSO cycles

Abstract

The circulation and zonal wind anomalies in the lower troposphere over the equatorial western Pacific and their roles in the developing and decaying processes of the 1982—1983, 1986—1987, 1991—1992 and 1997—1998 El Nino events and the occurrence of La Nina events are analyzed by using the observed data in this paper. The results show that before the developing stage of these El Nino events, there were cyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies brought the westerly anomalies over the Indonesia and the tropical western Pacific. However, when the El Nino events developed to their mature phase, there were anticyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies made the easterly anomalies appear over the tropical western Pacific. A simple, dynamical model of tropical ocean is used to calculate the response of the equatorial oceanic waves to the observed anomalies of wind stress near the sea surface of the equatorial Pacific during the 1997/98 ENSO cycle, which was the strongest one in the 20th century. It is shown that the zonal wind stress anomalies have an important dynamical effect on the development and decay of this El Nino event and the occurrence of the following La Nina event.