Impacts of Two Types of El Nio on Atmospheric Circulation in the Southern Hemisphere

Based on NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data from 1979 to 2010, the impacts of two types of El Nio on atmospheric circulation in the Southern Hemisphere (SH) are analyzed. It is shown that, when a warming event occurs in the equatorial eastern Pacific (EP El Nio), there is a negative sea level pressure (SLP) anomaly in the eastern Pacific and a positive one in the western Pacific. Besides, there exists a negative anomaly between 40oS and 60oS and a positive anomaly to the south of 60oS. When a warming event in the central Pacific (CP El Nio) occurs, there appears a negative SLP anomaly in the central Pacific and a positive SLP anomaly in the eastern and western Pacific, but the SLP anomalies are not so evident in the SH extratropics. In particular, the Pacific-South America (PSA) pattern induced by the CP El Nio is located more northwestward, with a weaker anomaly compared with the EP El Nio. This difference is directly related with the different position of heating centers associated with the two types of El Nio events. Because the SST anomaly associated with CP El Nio is located more westward than that associated with EP El Nio, the related heating center tends to move westward and the response of SH atmospheric circulation to the tropical heating changes accordingly, thus exciting a different position of the PSA pattern. It is also noted that the local meridional cell plays a role in the SH high latitudes during EP El Nio. The anomalous ascending motion due to the enhancement of convection over the eastern Pacific leads to an enhancement of the local Hadley cell and the meridional cell in the middle and high latitudes, which in turn induces an anomalous descending motion and the related positive anomaly of geopotential height over the Amundsen-Bellingshausen Sea.