A study on the physical processes of the formation of the ENSO cycle

Abstract

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.