Formation, Propagation And Evolution Characteristics Of Wintertime Southern Branch Trough In The Subtropical Westerlies And Its Impacts On The Weather And Climate In China

Using NCEP/NCAR reanalysis data and the observational precipitation collected in China which are then treated with various statistical methods, study is performed firstly of the definitions of the wintertime southern branch trough in the subtropical westerlies (WEBTISR) in terms of evolution of the southern branch jet, then the climatology features as the structure, development, propagation and the multi-scale variability of the WEBTISR, and finally its impacts on the weather and climate all over China. Also, a severe snowstorm in Deqin of Yunnan Province on November 14th-17th, 2007 has been taken as an example to investigate the formation process of the strong weather in Southwest China under the combined action of the WEBTISR and the storm over the Bay of Bengal. And the main results of this paper are as follows:(1) The WEBTISR is a semi-permanent low pressure trough brought about by the wintertime southern branch of the subtropical westerlies over the Bay of Bengal on the south of Qinghai-Tibet Plateau. The fact that WEBTISR established in October indicates that the Northern-Hemispheric circulation transits 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. The trough is noticeable at 700 hPa, while the dry and warm advection in its front is favorable for formation and development of the quasi-stationary front in Kunming. In winter, the descending branch of the wintertime divergence circulation inhibits development of the ascending motion in front of the WEBTISR. And in the spring, as the descending branch of the divergence circulation weakened, divergence field is strengthened on the south of the entrance region of the East Asian jet stream, which makes a rapidly upward extension of the vertical movement. In detail, due to the weak moisture transportation, limited upward motion and no strong convection in winter, there is less rainfall in front of the study trough, with the rainfall mainly over regions from the Kunming quasi-stationary front on the southwest of the Plateau to South China, as opposed to the situation in spring. Consequently, the WEBTISR is most significant in spring. (2) There are three southern disturbances under the wintertime subtropical westerlies jet in the Northern Hemisphere, located in the Arabian Sea, the Bay of Bengal and South China, respectively, in which the WEBTISR over the Bay of Bengal has the smallest variability. A teleconnection wave train with negative, positive and negative centers, migrated from North Africa to the Bay of Bengal via Arabian Sea, suggests that the WEBTISR is positive related with the trough over North Africa, but negative related with the trough over Arabian Sea. During the eastward process from North Africa to the Bay of Bengal, the westerlies waves has always stalled or even strengthened in the Arabian Sea. The eastward process costs about 20 days, marked by characteristics of low-frequency oscillations. It is likely to be a major mechanism of WEBTISR strengthening that the stationary Rossby wave originated in North Africa propagates from the Arabian Sea to the Bay of Bengal along the jet waveguide. Besides, the activity of the southward cold air on the west and east of the Plateau is another important factor for the deepening and development of the study trough.(3) The WEBTISR is marked by seasonal periods at 10-30 and 30-60 days, inter-annual periods at quasi-4 and quasi-6 yrs, as well as inter-decadal periods at 10-20 yrs, with an abrupt strong-to-weak change in the late 1970s on the inter-decadal scale. A good correlation between the low-frequency WEBTISR and the rainfall in South China. Till the WEBTISR is joined with the Sri Lanka low vortex, a great number of water vapors from the Bay of Bengal in front of the trough unite with the cross-equatorial transportation of water vapor from the South Indian Ocean. And then the moisture transports northward under the impacts of low-frequency anti-cyclone in Philippines, as in coactions with the South China low vortex, resulting in increased convergence at the lower level and active convection, which is responsible for the fact that the low-frequency rainfall in winter-half year is mainly concentrated in the South China. In the years with stronger(weaker) WEBTISR, the upper-level subtropical westerly jet is extraordinary increased (decreased), which is favorable (unfavorable) to the upstream-to-downstream propagation of the westerlies waves. The increased/decreased meridional circulation the mid-high latitudes is favorable/unfavorable to the southward transmission of cold air, resulting in the stronger/weaker southern branch of the westerlies waves, then the subtropical high is significantly weakened/strengthened along with the enhanced/reduced water vapor transmission and vertical ascension in front of the trough, so that the rainfall is plentiful/scarce in South China. During the period of the increased/decreased WEBTISR on an inter-decadal scale, the North Africa jet is enhanced/reduced and southward/northward than normal, as advantageous/disadvantageous to the upstream-to-downstream propagation of the westerlies waves. Meanwhile, the East Asia jet is reduced/enhanced and northward/southward than normal, which is prosperous/unprosperous to the development of the vertical ascension in front of the trough. The increased (decreased) meridional circulation the mid-high latitude is favorable (unfavorable) to the southward transmission of cold air. The southern branch of the westerlies is stronger (weaker) than the normal years, water vapor transmission is increased (decreased) in front of the trough, and the water vapor is transmitted by winds which are greatly southward (northward) than normal in East China, all these making the precipitation belt northward/southward than normal.(4) The southern branch of the cold air along the southern edge of the plateau to the east when there are the Hengduan Mountains to block the accumulation of dry and cold air, and the northern branch of the cold air has composed the dry and cold air masses on the east of the plateau when it is moving southward along the surface layer from the mid-high latitudes. The southwesterly in front of the WEBTISR has transformed into warm and wet flows when it is bypassing the Hengduan Mountains to the north in lower level. They merge into a structure that is warm and wet in the center and dry and cold on the both sides, Kunming quasi-stationary front on the south of the Qinghai-Tibet plateau, as well as a weak front on the west of the plateau. In winter, the vertical circulation of the front is strengthened by the ascension in front of the study trough. In spring, under the impacts from the South China trough and the upper divergence on the south of the jet's entrance region, the vertical motion along the front has enhanced markedly. The dry advection in front of the WEBTISR always makes decreased rainfall before the front, while the rapid increase in the wet advection at 850 hPa and 700 hPa behind the front is beneficial to the water vapor convergence and rainfall in South China. Cold air is active in South China. In winter, cold air is from the southern branch of the westerly jet and the lower levels of the mid-high latitudes. And in spring, cold air is basically from the southern branch of the westerly jet. The WEBTISR influencing rainfall in South China can be divided into mobile and static patterns, and the latter is the major means. Mobile trough transports moisture, and the ascending convergence in its front provides dynamic conditions which are necessary to the rainfall. And the static trough provides plenty of moisture for precipitation in China.(5) The severe snowstorm in Deqin was formed under the collaboration of the WEBTISR and storm over the Bay of Bengal, while combined with the north cold air and favorable topography there. Isentropic potential vorticity analysis indicates that cold air originates from the upper troposphere near the Caspian Sea, traveling along the southern branch of the westerlies to the Bay of Bengal, which deepens the trough and beneficial to leading the storm over Bay of Bengal to move northward. The southwest lower-level jet occurs in front of the trough, which transports the storm-induced cloud and moisture to the northeast. And then the uplifting topography humidifies first the upper atmosphere. Deqin is located in the belt of strong moisture transport, where the wet level is thick with convergence of a large number of water vapor flux therein. With the northward shift of the storm, the lower jet has strengthened but obstructed by the Plateau, resulting in convergence and shear over Deqin. The combination of the storm and the WEBTISR, and southward movements of cold air from the plateau make rapid enhancement of the ascension in front the trough. Generally, the slantwise vorticity development and conditional symmetric instability are the important factors for the heavy snow formation.