Study On Relationships Between The Northern Hemisphere Westerlies And Tibetan Plateau Monsoon

In this paper, the abrupt seasonal change phenomenon of zonal circulations in the Tibetan Plateau(TP) region was studied with the NCEP/NCAR reanalysis data acquired during 1951–2012. Additionally, the main causes of the abrupt seasonal changes were explored, the relationships between the intensity of TP monsoons and the location change of the westerlies in the Northern Hemisphere was reanalyzed, and the causes of the location change of the westerlies were discussed. Accordingly, the following conclusions were drawn:The zonal circulations over the TP have an significant abrupt seasonal change phenomenon. When transitioning from winter circulation to summer circulation, the zonal circulations exhibit abrupt northward jump of zero-velocity curve, countinuous intensification and northward advance of the easterlies, and sudden disappearance of westerly jet; when transitioning from summer circulation to winter circulation, the zonal circulations exhibit southward retreat of zero-velocity curve, recurrence of westerly jet, and sudden southward movement of the easterlies and westerlies.The 600 hPa zero-velocity curve index can be used as a criterion for transition of zonal circulations between winter and summer circulations. 20 °N is the critical value. When the zero-velocity curve is on the north of 20 °N, there are summer circulations over the TP, and when the zero-velocity curve is on the south of 20 °N, there are winter circulation over the TP. The abrupt change times between winter and summer circulations are the 31 st pentad and the 59 th pentadThe main cause of the abrupt seasonal change of zonal circulations is the thermal effect on the TP. With the enhancement of heating in the TP in June, a huge monsoon circulation is formed from the TP to low latitude zone, so that the easterly flow in the south of the TP is enhanced, forcing the westerly jet to retreat northwards, thus completing the transition from winter circulation to summer circulation. The heating effect weakens in the TP in October, the Hadley circulation from the TP to low latitude zone recurs and the westerly jet enhances gradually so that the zonal circulation transitions to winter circulaiton. Whether during transition from winter circulaiton to summer circulaiton or from summer circulaiton to winter circulaiton, the change of thermal wind-formed easterlies and westerlies is always earlier than that of zonal wind-formed easterlies and westerlies. In other words, the change of zonal wind over the TP is from the development of thermal wind. Therefore, this further demonstrates from the angle of thermal wind that the thermal effect over the TP plays a decisive role in the seasonal change of zonal circulations.The establishment and end times of the TP monsoon agree with the formation and end times of the low pressure over the TP. When the TP summer monsoon is in the most powerful period, it is just the seasonal transition period of summer circulation. The end period of TP summer monsoon is consistent with the seasonal transition time of winter circulation over the TP.The time of location change of the westerlies is related to the intensity of the TP summer monsoon. The times of northward jump and southward retreat of the westerlies are the 30 th pentad(1 pentad earlier), and the 59 th pentad, respectively, in a year with strong monsoon, and the 33 rd pentad(2 pentads later), and 63 rd pentad(4 pentads later), respectively, in a year with weak monsoon. This phenomenon is also global.During the northward jump of the westerlies, the South Asian High enhances and moves northwards, the west trough of Lake Baikal enhances, and the trough within the longitude range of the TP deepens, causing earlier northward jump time of the westerlies in a year with strong TP monsoon, compared with a normal year. The cases are reversed in a year with weak monsoon. During the southward retreat of the westerlies, the location of the subtropical high is relatively southerly, the East Asian Trough deepens. This is favorable for the south retreat of the westerlies in a year with strong TP monsoon, compared with a normal year. The cases are reversed in a year with weak monsoon.