The Jet Stream and the Origin of the Meiyu-Baiu Rainband.

Meiyu-Baiu is the single most important climate phenomenon for highly populated East Asian regions from east China through Japan, but what maintains this climatological rainband has been a long-standing mystery. The difficulty lies in the strong interaction between convection and the low-level circulation. The latter supplies the moisture, but the rainband itself creates heating and can cause the low-level circulation. Although moist, low-level southerly winds blow throughout the summer, the Meiyu-Baiu rainband appears only from June to mid-July.

This study avoided the circularity problem by analyzing events in the mid-troposphere. At 500 hPa, a characteristic large-scale circulation is seen from June to mid-July, in which a band of warm air is advected eastward along the southern flank of the jet stream and downstream of the temperature maximum anchored by the Tibetan Plateau. Along this warm advection band, low-level air is sucked upward, and the induced upward motion triggers the heavy Meiyu-Baiu rainfall and diabatic heating.

Takeaki Sampe and Shang-Ping Xie

Upward motion (warm colors) follows closely warm horizontal temperature advection in the mid- troposphere (contours) during the Meiyu-Baiu season.

Thus, while the summer southerly winds provide the low-level moisture, the jet stream’s advection of the mid-level warm air determines the location and timing of the Meiyu-Baiu rainband. This more complete, dynamical picture should help in making better regional rainfall predictions.

Meiyu/Baiu rain band

anticyclone

Meiyu/Baiu rain band

anticyclone

Meiyu/Baiu rain band

anticyclone