The abrupt changes of zonal circulation in the Tibetan Plateau(TP) region and their likely causes are derived from National Centers for Environmental Prediction and the National Center for Atmospheric Research reanalysis data.The zonal circulation over the TP abruptly changed in summer(31st pentad) and winter(59th pentad).The switch from summer to winter circulation is characterized by a sudden northward shift of the westerlies and the zero-velocity curve and disappearance of the westerly jet.The winter-summer switch is characterized by the reverse pattern.Therefore,the circulation conversion between summer and winter can be judged from the position of the zero-velocity curve.Curves located north of 20 °N indicate summer circulation over the TP and vice versa.The abrupt change of zonal circulation is mainly caused by the thermodynamic effect of the TP.In June,this effect causes a huge monsoon circulation cell extending from the TP to low latitudes.Consequently,the westerlies jump to the north as easterlies develop.This process,which is enhanced by the strong northerly in Coriolis,establishes the summer circulation.In October,the Hadley cell recurs as the thermal effects of the TP diminish,the westerlies rush southward,and the winter circulation is established.
