Interannual Variation of the Summer Synoptic-Scale Disturbance Activity in the Western Tropical Pacific

Abstract

The authors computed the occurrence frequency of synoptic-scale disturbances over the western tropical Pacific with a simple synoptic scheme and the Goddard Earth Observing System data for 1980?94 supplemented by the 1979 National Meteorological Center (currently the National Centers for Environmental Prediction) data. The major occurrence of these disturbances exists along the monsoon trough with maximum occurrence frequency over Micronesia. The occurrence frequency distribution undergoes an interannual variation through the influence of large-scale circulation during boreal summer. As a part of the interannual variation of large-scale summer circulation in the Pacific basin, a major anomalous anticyclonic (cyclonic) circulation cell emerges in the western subtropical Pacific (north of 15°N), and a minor anomalous cyclonic (anticyclonic) circulation cell appears in the western tropical Pacific (south of 15°N) during warm (cold) summers. The tropical synoptic-scale disturbance activity is affected by these anomalous summer circulations in the following two ways: 1) the maximum occurrence frequency appears east (west) of 150°E (the east end of the climatological monsoon trough) during warm (cold) summers, and 2) tropical synoptic-scale disturbances are located primarily south of 15°N during warm summers, while their occurrences are enhanced north of 15°N during cold summers. Since the effort made here is a pilot study, the authors suggest some potential investigations of tropical synoptic-scale disturbances and interannual variations of large-scale summer circulation.