Meridional Wind Stress Anomalies over Tropical Pacific and the Onset of El Nino. Part Ⅰ: Data Analysis

By using correlation analysis and singular value decomposition (SVD) methods, diagnostic studies are made to investigate the relationship between the meridional wind stress anomalies over the tropical Pacific and El Nino events. The correlation of the sea surface temperature anomalies (SSTA) in the NINO3 region (150°-90°W, 5°S-5°N) and the leading meridional wind stress anomalies shows that the NINO3 SSTA is significantly correlated with the anomalous meridional wind stress convergence in the equatorial eastern Pacific at the leading time of more than six months. With the reduction of the leading time, the correlation becomes stronger and the convergence area with statistical significance enlarges and extends to the west. The coupling patterns between SSTA in tropic Pacificand the leading meridional wind stress anomalies revealed by SVD show the similar feature to that revealed by the correlation analysis. The converging meridional wind stress about the equator in the eastern equatorial Pacific precedes the increasing of SSTA in the central and eastern equatorial Pacific as early as half a year or more. Compared to the leading zonal wind stress anomalies in the tropical western Pacific, it seems that the increasing of the NINO3 SSTA is more related to the convergence of the leading meridional wind stress anomalies in the eastern equatorial Pacific. It is suggested that preceding meridional wind stress anomalies may play an important role in the occurrence of El Nino events.