Water mass heaving enhances subsurface marine heatwaves in the South China Sea monsoon region

Marine heatwaves have profoundly impacted marine ecosystems. Water masses are expected to regulate ocean heat content, however, their contributions to subsurface marine heatwaves remain difficult to assess when heat anomalies are measured at fixed depths. In this study, we combine reanalysis products and in situ observations to classify four water masses from the subsurface to the deep layers in the western South China Sea using a $${{\\rm{\\sigma }}}-{{\\rm{\\pi }}}$$ coordinate and examine marine heatwave variability from 1993 to 2022. The upper two water masses became lighter and fresher, concurrent with stronger marine heatwaves. Heat-budget analysis shows that horizontal heat advection enhances marine heatwaves in deep water. By contrast, vertical heat advection linked to water-mass heaving dominates subsurface warming, contributing about 60% of advective heat transport. This heaving is associated with deepened water-mass centers, enhanced anticyclonic eddies, and atmospheric anticyclonic circulation. Analyzing marine heatwaves in water-mass coordinates highlights the contributions of water-mass vertical motion to the intensification of subsurface marine heatwaves. Vertical heat advection linked to water-mass heaving dominates subsurface warming in the South China Sea, according to analyses of reanalysis products and in situ observations