Westerlies Affecting the Seasonal Variation of Water Vapor Transport over the Tibetan Plateau Induced by Tropical Cyclones in the Bay of Bengal

This study investigates the activity of tropical cyclones (TCs) in the Bay of Bengal (BOB) from 1979 to 2018 to discover the mechanism affecting the contribution rate to the meridional moisture budget anomaly (MMBA) over the southern boundary of the Tibetan Plateau (SBTP). May and October–December are the bimodal phases of BOB TC frequency, which decreases month by month from October to December and is relatively low in May. However, the contribution rate to the MMBA is the highest in May. The seasonal variation in the meridional position of the westerlies is the key factor affecting the contribution rate. The relatively southern (northern) position of the westerlies in November and December (May) results in a lower (higher) contribution rate to the MMBA. This mechanism is confirmed by the momentum equation. When water vapor enters the westerlies near the trough line, the resultant meridional acceleration is directed north. It follows that the farther north the trough is, and the farther north the water vapor can be transported. When water vapor enters the westerlies from the area near the ridge line, for Type-T (Type-R) TCs, water vapor enters the westerlies downstream of the trough (ridge). Consequently, the direction of the resultant meridional acceleration is directed south and the resultant zonal acceleration is directed east (west), which is not conducive to the northward transport of water vapor. This is especially the case if the trough or ridge is relatively south, as the water vapor may not cross the SBTP.