The Low-frequency Variability of the Ocean Heat Content in the SCS and Its Related Extreme Subsurface Warm Events

Understanding the variability of upper ocean thermal conditions is key to regional climate prediction. In this study, in-situ observations and reanalysis data reveal a regime shift of upper ocean heat content (OHC) in the SCS during the late 1990s, very different from a linear warming trend in the sea surface. A simplified upper layer budget diagnosis reveals that more than half of OHC change results from the advection effect, which is caused by an anomalous SCS anticyclonic gyre associated with an anomalous negative wind stress curl. The anomalous anticyclonic circulation deepens thermocline depth at the basin-scale, and result in the regime shift of SCS OHC. Further analyses show that the regime shift process is attributed to a phase transition of the Interdecadal Pacific Oscillation (IPO) from positive to negative. Based on these conclusions, we found two extreme subsurface warm events during 1998/99 and 2006/07, with no corresponding extreme surface warming except in 1998. 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. Further analyses reveal the importance of the Pacific western boundary currents, especially the Kuroshio Current, in maintaining extreme subsurface warm events in the SCS.