Convection Patterns in the Tropical Pacific and Their Influence on theAtmospheric Circulation at Higher Latitudes

Abstract

Patterns of outgoing longwave radiation (OLR) have been analyzed over the tropical Indian and Pacific Oceans in order to identify the varying influence of their associated convective anomalies on the circulation at higher latitudes. Particular attention has been given to the changes related to El Niño?Southern Oscillation (ENSO) events. The two leading EOFs (emperical orthogonal functions) of monthly OLR anomaly patterns for the region between 20°N?20°S and 70°E?120°W, express complementary variations between centers located 1) near 170°W just south of the equator and over the Philippines, and 2) slightly south of the equator near 145°W and slightly north of the equator near 165°E. Cluster analysis over a smaller area between 10°S?10°N and 140°E?140°W has highlighted ENSO-related changes with two of the six clusters associated with ?moderate? (EN) and ?strong? (EN+) El Niño events, and a third including most La Niña (LN) events. The OLR anomaly patterns associated with the 1986/87 and 1991/92 warm events fell within the moderate category, whereas those for the mature and decaying phases of the 1982/83 and 1997/98 events were associated with the strong pattern. For the EN cluster, composites of the global 1000-hPa height and 300-hPa streamfunction showed wave trains propagating poleward and eastward in each hemisphere from the main area of enhanced convection. These originated approximately 20° farther east for the EN+ composites. Apart from a deeper Aleutian low in the EN+ composites, the differences over North America were comparatively small. Significant changes were observed over New Zealand, where EN events were associated with weak southwesterly anomalies, but strong west-southwesterly anomalies were observed for the months included in the EN+ class. The overall circulation anomaly patterns associated with La Niña clusters were generally weak, and similar to those of EN but with the opposite sign. The composite patterns show little change between summer and winter months in the Southern Hemisphere, but the influence of the anomalous tropical convection on the Northern Hemisphere during the boreal summer is weak. The responses to the anomalous convection indicated by the OLR anomalies have also been modeled by applying a linearized version of the barotropic vorticity equation at the 300-hPa level. The results obtained support a number of the key differences observed in the streamfunction composites and highlight the lack of summertime Rossby wave sources in the Northern Hemisphere. For regions with climate variability sensitive to the location of tropical convection, these results suggest that a single index of tropical circulation, such as the Southern Oscillation index, is not sufficient on its own to specify ENSO-forced climate anomalies.