An Observational Study of Intraseasonal Variations over Southeast Asia during the 1998 Rainy Season

Abstract

In the present study, intraseasonal variations of precipitation over Thailand [Thailand daily rainfall (TDR)] during 1998 and the accompanying variations of the tropospheric circulation are analyzed, mainly with the use of rain gauge data and the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) reanalysis version 1.5. Wavelet analysis reveals that the amplitudes of the intraseasonal variations of the 1998 TDR exhibit two peaks. One peak is categorized as a 30?60-day variation that appeared in the early half of the rainy season, while the other peak is categorized as a 10?20-day variation that appears in the later half of the rainy season. The variation of atmospheric circulation associated with the latter peak of the TDR is analyzed. When the 10?20-day variation of the TDR is in the most active phase, a negative streamfunction anomaly at the 850-hPa level exists over the Bay of Bengal, and a positive anomaly is found in the Southern Hemisphere to the south of the negative anomaly. Both anomalies are associated with cyclonic vortices and are accompanied by negative height anomalies at the same level. Such horizontal structures resemble the horizontal structure of an equatorial Rossby wave of the gravest meridional mode (n1ERW). The cyclonic disturbances move westward with speeds of 6?6.4 m?s?1. In the vertical, the disturbances have a first-baroclinic structure in the troposphere, with a node at the 300?400-hPa levels. In addition, the amplitude of the streamfunction anomaly in the lower troposphere is smaller than that in the upper troposphere over the western Pacific but larger than that in the upper troposphere over the Bay of Bengal. The mechanism for the longitudinal difference in the vertical structure and comparison of the analyzed disturbance with n1ERW-type disturbances over the Pacific are also discussed.