The Effect of Boreal Summer Intraseasonal Oscillation on Mixed Layer and Upper Ocean Temperature over the South China Sea

Intraseasonal oscillation of the mixed layer and upper ocean temperature has been found to occur over the South China Sea (SCS) in the summer monsoon season based on the multiple reanalysis and observational data in this study. The method of composite analysis and an upper ocean temperature equation assisted the analysis of physical mechanisms. The results show that the mixed layer depth (MLD) in the SCS has a significant oscillation with a 30-60 d period over the SCS region, which is closely related to boreal summer intraseasonal oscillation (BSISO) activities. The MLD can increase (decrease) during the positive (negative) phase of the BSISO and usually lags behind by approximately one-eighth of the lifecycle (5 days) of the BSISO-related convection. The BSISO may cause periodic anomalies at the air-sea boundary, such as wind stress and heat flux, so it can play a dominant role in modulating the variation in MLD. There also are significant intraseasonal seawater temperature anomalies in both the surface and subsurface layers of the SCS. In addition, during the initial phase of the BSISO, the temperature anomaly signals of the thermocline are obviously opposite to the sea surface temperature (SST), especially in the southern SCS. According to the results from the analysis of the temperature equation, the vertical entrainment term caused by BSISO-related wind stress is stronger than the thermal forcing during the initial stage of convection, and it is more significant in the southern SCS.