Westerly Jet and Seasonal Transition of the East Asian Summer Monsoon

The East Asian summer monsoon has a unique seasonality characterized by several distinct intraseasonal stages with abrupt transitions in between 1) Following the spring rainfall in April-May, the pre-mei-yu starts and intensifies rainfall over southern China. 2) The rain belt then jumps northward in June and forms Chinese mei-yu, the Korean changma, and the Japanese baiu. 3) The second jump occurs in July, marking the end of mei-yu and the onset of midsummer.

The latitudinal position of the westerly jet impinging on the Tibetan Plateau is systematically correlated to the seasonal transitions of the East Asian summer monsoon (e.g., Yeh et al. 1959; Schiemann et al. 2009). Figures on the right provide an overview of the close linkage between the two.

Central to this project is exploring a hypothesis proposed by Chiang et al. (2015) that changes to the latitudinal migration of the westerlies across the Tibetan Plateau alter the timing and duration of the intraseasonal stages of the East Asian summer monsoon. We have been combining observations and climate modeling to test the hypothesis across a range of scenarios.

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Stratospheric Northern Annular Mode and Ural Blocking High

The Ural Blocking High is one of the major weather systems that have significant impacts on winter weather and short-term climate variability of North China. The purpose of this project is to study the influence of Northern Hemisphere winter stratospheric circulation anomalies on the Ural Blocking High. We composited the Ural Blocking High relative to positive and negative stratospheric Northern Annular Mode (NAM) phases throughout 1958-2010. During the negative stratospheric NAM phase, the Ural Blocking High has higher frequencies of occurrence, longer life cycle, stronger amplitude, and exerts stronger influences on Northern China. Three-dimensional Eliassen-Palm (E-P) flux analysis demonstrates a stronger vertical component of E-P fluxes in the Ural Mountain region during the negative NAM phase than the positive NAM phase. It suggests that stratospheric circulations during the negative NAM phase favor upward wave propagation and thus the development of the Ural Blocking High. The results here have important implications for medium-range forecasting of winter weather or short-term climate variability in North China.

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