Interpretation of interannual variability of the zonal contrasting thermal conditions in the winter South China Sea

The distinct winter temperature difference between the eastern South China Sea (ESCS) and western South China Sea (WSCS) has a crucial impact on regional air–sea interactions. By utilizing satellite and reanalysis data, the zonal contrasting winter thermal conditions and their formation mechanisms are investigated. The second empirical orthogonal function (EOF) mode of winter sea surface temperature (SST) anomalies is responsible for this east–west contrasting temperature pattern (EWCTP), with warming (cooling) in the ESCS (WSCS) and cooling (warming) in the WSCS (ESCS) during the positive (negative) phase events. A mixed layer heat budget analysis reveals that the net heat flux plays a primary role in the WSCS. In the ESCS, the temperature variation is instead mainly dominated by the horizontal heat advection term. In the positive phase events, an anomalous cyclonic circulation promotes an  eastern boundary current (EBC) anomaly, which enhances the northward heat transport and thus warms the ESCS. In contrast, an anomalous anticyclonic circulation pattern weakens the heat transport by the southward EBC anomaly and cools the ESCS in the negative phase events. The water exchange through the Mindoro Strait and the vertical entrainment term also contribute to the ESCS SST anomalies. Further analyses show that although there are many EWCTP events that co-occur with the central Pacific El Niño-Southern Oscillation (CP ENSO) events, they have a complex relationship. The EWCTP could appear without CP ENSO events and some CP ENSO events do not lead to the EWCTP. It is because of the different temperature state in the WSCS and ESCS during October–December months and the different contributions of net heat flux and ocean processes to the temperature changes during October–February months.