The Structures and Evolutions of the Wintertime Southern Branch Trough in the Subtropical Westerlies

The structures and evolution characteristics of the wintertime southern branch trough in the subtropical westerlies (WEBTISR), as the semi-permanent trough generated by the subtropical southern branch westerlies (SBW) over the Bay of Bengal, to the south of the Tibetan Plateau, were studied for their climatological significance with the following results: 1) The WEBTISR is generated over the northern Bay of Bengal in October, maintained from November to the following February, and active from March to May, then disappears and transforms to the Bay of Bengal trough in June. The fact that the WEBTISR is established in October indicates that the northern hemispheric circulation transforms from summer pattern to winter pattern, while its disappearance in June, together with the Bay of Bengal trough establishment, is one of the important indications of summer monsoon onset. 2) The WEBTISR is noticeable at 700 hPa, in front of which the transmission of the warm-dry advection is favorable for the formation and maintenance of the Kunming quasi-stationary front. And the cold-wet advection behind the WEBTISR has a close relationship with the cold surge in the Bay of Bengal. 3) The descending branches of divergence circulation restrain the development of the vertical motion in front of the WEBTISR in winter, with concurrent convergence/divergence at the lower/middle levels, inducing that the ascending parts extend roughly to the middle troposphere at 600 hPa. Conversely the weakening of the divergence circulation in spring, and appearance of the divergence center on the south side of East Asia jet stream inlet lead to rapid upward extension of the vertical motions. 4) Climatologically, water vapor transport is weak; ascending motion is also restrained and there is not severe convection, therefore the precipitation is unobvious in front of the WEBTISR, while the rainfall is mostly over the areas from the Kunming quasi-stationary front to the southeast of the plateau to South China in winter. In spring, water vapor transport strengthens in the WEBTISR, with increased transport of warm-wet moisture outside the subtropical high as well as developing upward motion and convection, leading to a distinct increase in the rainfall produced by the WEBTISR. Therefore, spring is the period the WEBTISR is most active.