Testing the Role of Westerly Jet in the Termination of Meiyu

Meiyu is characterized by a tilted elongated quasi-stationary subtropical front over East Asia. Previous studies have suggested that the seasonal migration of the westerly jet impinging on the Tibetan Plateau is essential for the seasonal transition of the East Asian Summer Monsoon. Here we further examine the role of the westerly jet in the demise of Meiyu, and seek quantitative understanding of how the changes in the latitudinal position of the upper tropospheric maximal westerlies over the Tibet region modulate the transition from Meiyu to Midsummer. We advance a hypothesis – argued from both observational diagnostics and model evidence – that attributes the demise of Meiyu to the weakening of the tropospheric northerlies over central-northern eastern China during the northward jet transition. In particular, we argue that northward migration of the jet relative to the Plateau weakens the orographic forced stationary waves and thereby the downstream northerlies, which reduces the meridional convergence, weakens the Meiyu fronts and allows monsoonal southerlies further penetrates into northern eastern China.

To test the hypothesis, we examine the evolution of the atmospheric circulation during the transition from Meiyu to Midsummer in reanalysis datasets, and find that the demise of Meiyu is accompanied by 1) shift of the upper tropospheric maximal westerlies impinging on the Tibet to the north of the Tibet (~40N) and 2) weakening of the northerlies adjacent downstream of the Tibet. We then employ a general circulation model with fixed SST to test the response in the transition from Meiyu to Midsummer to various meridional scales of the Tibet. Our simulations suggest delayed termination of Meiyu with a wider Tibet. To further understand the evolution of the orographic forced stationary waves during the jet transition, we also embed a Gaussian-shaped mountain with similar dimension to the Tibet in a dry dynamical core with the ‘Held-Suarez’ configuration, and mimic the seasonal transition of the jet via shifting the mountain latitudinally.