The Influence of ENSO on the Seasonal Convection Evolution and the Phase of 30-60-Day Oscillations during Boreal Summer

Abstract

We examined the composite seasonal evolution of atmospheric convection over the tropical western North Pacific and northern Indian Ocean during April-August, following the wintertime warm, cold ENSO events, and neutral cases. It was found that the warm (cold) wintertime ENSO events correspond to three times of anomalous convection suppressions (enhancements) in the South China Sea and tropical western North Pacific, separately in April and May, in middle and late June, and in late July and August. The total seasonal evolution and its annual cycle component exhibit nonlinear responses to the different ENSO phases. The annual cycle, and 30-60-day oscillations, all contribute to this ENSO-related seasonal evolution.The phases of composite 30-60-day oscillations for the different ENSO phases were further examined. We found the out-of-phase feature of summertime 30-60-day oscillations in convection over the tropical western North Pacific, between the warm ENSO cases and cold or neutral cases in the preceding winter. The phase of the composite oscillations for the warm cases is three or four pentads lagged to that for the cold and neutral cases. The phase-locking feature of 30-60-day oscillations shown by previous studies is essentially contributed by the cold and neutral cases. Over the northern Indian Ocean, in contrast, the phase of composite oscillations for the warm cases is similar to that for the neutral cases, and is about ten days lagged to that for the cold cases. These features of ENSO-related phase of 30-60-day oscillations are explained by the annual cycle of local zonal winds. The 30-60-day oscillations appear to be more closely phase-locked to the annual cycle of zonal winds, which is substantially influenced by the ENSO phase in the preceding winter, than they are phase-locked to the calendar annual cycle.