The onset of Tibetan Plateau rainy season and its impact factors

doi:10.12006/j.issn.1673-1719.2024.179

Abstract

Abstract:

The spatiotemporal heterogeneity at the onset of the Tibetan Plateau (TP) rainy season and its related evolutions of atmospheric circulation were investigated from 1979 to 2019 based on precipitation datasets from the surface observations and ECMWF. Results show that: (1) TP’s rainy season exhibits three precipitation centers respectively in the central-eastern, southern, and northern regions, with significant discrepancies in their rain process, precipitation concentration period, and precipitation. (2) There exists a bimodal distribution in the annual variation of precipitation over the central-eastern TP, with a remarkable increase around the 27th pentad. The onset of rainy season is accompanied by the weakened westerly stream in the mid-latitudes region, the convergence of the Southwest Monsoon from the Bay of Bengal and the west coast of Indian on the southern edge of the plateau, and the enhanced water vapor transport under southwestlies. However, an increase in precipitation appears at the 32nd pentad over the southern TP, manifesting an unimodal distribution. The corresponding atmospheric circulation pattern involves the northward movement of westerlies, convection development in the Indian Peninsula, and water vapor intrusion in the middle troposphere. In the northern TP, the annual change in precipitation is unimodal, and the increase in precipitation starts at the 29th pentad. The onset of its rainy season corresponds to the westward- and northeard-stretching of East Asian westerly jet, and intensive wind shear over the plateau. (3) It should be pointed out that water vapor flux at the onset of rainy season is mainly transported across the south and west boundaries in the central-eastern and southern subregions but north and west edges over the northern part of TP. The increases in the northward transport flux of water vapor and regional moisture budgets in the plateau region jointly have contributed to an earlier onset of the TP rainy season in the past decades.

Key words: Tibetan Plateau, Onset of rainy season, Atmospheric circulations, Water vapor budgets

SHI Ren-Rui, JIANG Xing-Wen, WANG Zun-Ya. The onset of Tibetan Plateau rainy season and its impact factors[J]. Climate Change Research, 2025, 21(1): 91-101.

Figures/Tables 11

Fig. 1 Terrain height and distribution of observatory stations on the Tibetan Plateau

Fig. 2 Climatology of precipitation during rainy season (a, shadings) and its proportion to annual precipitation (a, numbers, %); the onset of rainy season in the Tibetan Plateau (b)

Fig. 3 The first to third rotated empirical orthogonal function (REOF) modes (a-c) and the corresponding time coefficient (d-f) of the pentad precipitation in Tibetan Plateau from 1979 to 2019

Fig. 4 Climatological annual cycle of the normalized pentad precipitation (a) and the accumulative rainfall (b) over the subregions of Tibetan Plateau averaged from 1979 to 2019

Fig. 5 Time series of the onset (Julian day) of rainy season over the subregions of Tibetan Plateau from 1979 to 2019. (Doted lines denote the linear trendlines)

Fig. 6 Mean circulation fields for the two pentads before and after the onset of the rainy season over the central-eastern Tibetan Plateau

Fig. 7 Same as Fig. 6, but for the southern Tibetan Plateau

Fig. 8 Same as Fig. 6, but for the northern Tibetan Plateau

Fig. 9 Annual variation of pentad water vapor transport across the four boundaries and their total budgets over the subregions of pentad Tibetan Plateau from 1979 to 2019

Fig. 10 Interannual series of water vapor transport across the four edges and their total budgets over the subregions of Tibetan Plateau from 1979 to 2019. (Doted lines denote the linear trendlines)

Table 1 Linear trend of water vapor transport across the four edges and their total budgets over the subregions of Tibetan Plateau from 1979 to 2019 106 kg/(s?(10 a))

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