Impact of Foehn Clearance Effect on the Formation of Spring Warm Pool in the South China Sea

Abstract

AbstractThe spring warm pool (SWP) in the South China Sea (SCS) is a region west of Luzon Island with a sea surface temperature nearing 30°C in late spring, significantly higher than the surrounding waters. Understanding its formation aids in reliable prediction of the summer monsoon onset in the SCS. The current explanation for the SWP formation is the wake effect, which claims that the orographic blocking of Luzon Island forms a wake zone west of Luzon Island and so considerably reduces sea surface latent heat (LH) flux release during the winter monsoon season in comparison to adjacent waters. In this study, we enhance this explanation by incorporating the foehn clearance effect. Our statistical analysis indicates that the SWP evolution is strongly associated with frequent foehn clearance west of Luzon Island and that, in the SWP domain relative to its adjacent domains, the increase of surface short‐wave (SW) radiation induced by the foehn clearance effect is almost equal to the decrease of surface LH loss resulting from the wake effect. The ocean mixed layer heat budget analyses not only confirm that the zonal difference of surface heat flux dominates the SWP formation, but also demonstrate that this zonal difference is largely contributed by the zonal difference of SW radiation caused by the foehn clearance effect. Furthermore, sensitivity assessments demonstrate that the SWP would not emerge if the foehn clearance effect was excluded, indicating that both the wake effect and the foehn clearance effect contribute jointly to the SWP formation.