Propagating Mechanisms of the 2016 Summer BSISO Event: Air-Sea Coupling, Vorticity, and Moisture

During the summer of 2016, a boreal summer intraseasonal oscillation (BSISO) event was observed over Southeast Asia and the South China and Philippine seas. Precipitation anomalies associated with this event propagated northward at a speed of 0.5-1 degrees per day from July to August. To understand the mechanisms, a regional atmosphere-ocean coupled system with the Weather Research and Forecasting (WRF) model and the Hybrid Coordinate Ocean Model (HYCOM) is used to study this BSISO event. The 50-day-long coupled simulations reasonably capture large-scale northward propagation of the event. Coupled simulations with altered air-sea interaction and atmosphere-only simulations with prescribed sea surface temperature illuminate the insignificant role of air-sea interaction within the computation domain in the northward propagation the for this event. Diagnostics of the coupled simulation as well as the ECMWF-Interim reanalyses indicate that convection and barotropic vorticity are largely in phase north of 5 degrees N during the event. The BSISO convection is accompanied by moisture anomalies whose magnitudes increase as the BSISO propagates northward. Analysis of the moisture budget shows that positive horizontal advection leads positive moisture anomalies on the intraseasonal time scale north of 10 degrees N. The vorticity-convection relationship, the lead-lag relationship between moisture and its horizontal advection, and the latitude dependence of each for this BSISO event are consistent with general features of BSISO events composited with ECWMF-Interim reanalysis and satellite precipitation data sets.

Automated summary

The study investigates a boreal summer intraseasonal oscillation (BSISO) event over Southeast Asia and the South China and Philippine seas in 2016, using a regional atmosphere-ocean coupled system and diagnostic analyses. The results show that convection, vorticity, and moisture anomalies are closely related during the event, with positive horizontal advection leading to moisture anomalies on the intraseasonal time scale north of 10 degrees N.