Extreme subsurface warm events in the South China Sea during 1998/99 and 2006/07: observations and mechanisms

Conductivity–temperature–depth observations, objectively analyzed data, and model output are used to investigate the variability of subsurface temperature in the South China Sea (SCS) during 1948–2010. Two extreme subsurface warm events are identified during 1998/99 and 2006/07, with no corresponding extreme surface warming except in 1998. Mixed-layer heat budget analysis reveals that the lack of significant heat input from surface net heat flux or from current advection is responsible for the absence of extreme surface warming during 1999, and 2006/07. The surface net heat flux alone cannot explain the first phases of subsurface warming during 1998/99 and 2006/07. Warm advection from the southern SCS in 1998/99 and from the Kuroshio intrusion in 2006/07, induced by anomalous ocean currents, is likely the major contributor to warming of the subsurface water. During the second phase of warming, the surface net heat flux plays a damping role to cool the subsurface water, and the warm advection induced by anomalous SCS western boundary currents from the southern SCS leads to extremely warm subsurface water anomalies. The results show the importance of the Pacific western boundary currents, especially the Kuroshio, in maintaining extreme subsurface warm events in the SCS.