Southern-Northern Hemispheres Symmetric and Asymmetric Effect of El Niño Events on General Circulation and Analysis on Its Mechanism

This article composed seven El Niño events for the period 1971－2003, and analyzed the Southern－Northern Hemispheres symmetry of general circulation anomalies during the El Niño events. The analysis shows that the tropical (15°S－15°N) general circulation have mainly symmetric anomalies, but less antisymmetric anomalies during El Niño events. And antisymmetry of height and zonal wind anomalies is week; oppositely that of meridional wind anomalies are much stronger. In addition, there are more varieties in height and zonal wind anomalies than in meridional wind anomalies. Comparing the anomalies at 200 hPa and 700 hPa, it can be seen that the anomalies at these two levels are obviously out-of-phase. Furthermore, the Southern-Northern Hemispheres asymmetric effect of El Niño events on general circulation was studied. The Hough function was utilized to expand the observational ERA-40 daily data to equatorial waves for analyzing the evolutive characteristics of equatorial waves during El Niño events. The result shows that the reason of strong tropic symmetric anomalies of general circulation is the presence of strong symmetric Rossby waves anomalies. On the other hand, the mixed Rossby-gravity wave presents the antisymmetric anomalies, especially the meridional wind anomalies. In addition, during the El Niño events the positive sea surface temperature anomalies over the tropical eastern Pacific generated the Gill-type circulation, which consists of symmetric Rossby wave and Kelvin wave. Finally, the Hough function has advantages in analyzing the symmetry and asymmetry of the tropical atmospheric waves.