In the summer of 2013, an unprecedented heat wave was experienced over a vast area of southern China. The great areal extent, duration, and strength of this high temperature are very rare. For the 2013 hot spell, the major and direct influence mostly came from the anomaly of the western Pacific subtropical high(WPSH). The abnormally strong and stable WPSH was associated with specific surrounding circulations. The eastward extension of a stronger Qinghai-Xizang high favored the westward extension of the WPSH. The weaker cold air activity from the polar region led to the northward shift of the WPSH and helped it to remain stable. In the tropics, the western segment of the ITCZ was abnormally strong in the period, and supported the maintenance of the WPSH from the south. In addition, the interdecadal variation of the WPSH provided a decadal background for the anomaly variation of the WPSH that summer.
This study objectively defined an extreme precipitation event(EPE)over southern China and investigated the associated typical circulation pattern on the intraseasonal time scale.The occurrence of the EPE resulted from the joint operations of anomalous circulations over the subtropics and mid-high latitudes.During the EPE,simultaneous enhancements of the trough over the Bay of Bengal(BBT)and the Western Pacific subtropical high(WPSH)facilitated the abundant water vapor conditions over southern China,whereas the weakened East Asian major trough(EAT)led to a moderate cold air invasion to that region,producing a persistent convergence zone over southern China.Wave train pattern in North Atlantic and Europe may be viewed as precursory signals for the EPE over southern China,and it contributed to the Rossby wave propagation in association with the EPE.These wave packets propagated toward East Asia mainly along the North African-Asian subtropical westerly jet waveguide.Low-frequency disturbances along this waveguide contributed to the enhancement of the BBT and WPSH and the weakening of the EAT,constituting favorable circulation conditions for the EPE.
The physical processes involved in the formation of the ENSO cycle,as well as the possible roles of the Hadley circulation (HC),Walker circulation (WC),and the propagating waves of the Southern Oscillation/Northern Oscillation (SO/NO) in its formation,were studied using composite and regression methods.The analysis showed that the convection and heat release triggered by ENSO in the central-eastern equatorial Pacific are the primary drivers for the 3-5 year cycle of the HC,WC and the meridional/zonal circulation.The HC plays a key role in the influence of ENSO on the circulation outside the tropics through angular momentum transportation.Meanwhile,the feedback effects of the anomalous circulation in the mid-high latitudes on ENSO are accomplished by the propagating waves of SO/NO associated with the evolutions of HC and WC.These propagating waves are the main agents of the connections among the meridional/zonal circulation outside the tropics,the Asian/Australian monsoon,the anomalous easterly/westerly winds over the tropical Pacific,and ENSO events.It was found that the 3-5 year cycle of the meridional/zonal circulation forced by ENSO is quite different from the several-week variation of the circulation index triggered by the inner dynamic processes of the atmosphere.The former occurs at the global scale with a definite flow pattern,while the latter occurs only in a wide area without a definite flow pattern.Finally,a physical model for the formation of the ENSO cycle composed of two fundamental processes at the basin and global scale,respectively,is proposed.