Impacts of ENSO On The Seasonal Transition From Summer to Winter in East Asia

Abstract The El Niño-Southern Oscillation (ENSO) has seasonally distinct impacts on the East Asian climate so that its seasonal transition depends on the phases of El Niño and La Niña. Here, we investigate the seasonal transition of surface temperature in East Asia from boreal summer to winter based on the warm/cold ENSO developing phases. During La Niña years, from summer to winter the continuous temperature drop in East Asia tends to be faster than that during El Niño, indicating a latter start and earlier termination of fall. This different seasonal transition in East Asia according to phases of ENSO is mostly explained by atmospheric responses to the seasonally-dependent tropical/subtropical precipitation forcings in ENSO developing phases. The anomalous positive precipitation in the subtropical North Pacific exists only in September and leads to the subtropical cyclonic flow during El Niño years. The resultant northerly anomalies on the left side of subtropical cyclone are favorable for transporting cold advection towards East Asia. However, the positive subtropical precipitation disappears and teleconnection to East Asia is strongly controlled by the negative precipitation anomalies in the western North Pacific, modulating the anticyclonic anomalies in East Asia during the early winter (November). Therefore, these seasonally sharp precipitation changes associated with ENSO evolution induce distinctive teleconnection changes from northerly (summer) to southerly (winter) anomalies, which eventually affect seasonal transition in East Asia. Also, the Coupled Model Intercomparison Project Phase 5 models reasonably simulate the relatively rapid temperature transition in East Asia during La Niña years, supporting the observational argument.