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

The 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. Plain Language Summary: The largest island in the Philippines is Luzon Island, which is located in the western rim of the Pacific Ocean. The existence of north‐south mountain ranges on Luzon Island results in the occurrence of blocking effects, such as wake and foehn effects. This leads to frequent leeward wake and foehn phenomenon to the west of Luzon Island when easterly winds prevail, especially during the winter monsoon season. One of the foehn effects is foehn clearance effect, which is characterized by a cloudless sky and intense solar short‐wave (SW) radiation. Foehn clearance is commonly observed on the leeward side of Luzon Island during episodes of foehn phenomena, as evidenced by satellite photography. The spring warm pool (SWP) formation mechanism has been investigated predominantly from an oceanic perspective in previous studies. However, the role of SW radiation and its zonal difference on the SWP formation is poorly understood. We recognize that the cloudless situation of foehn clearance can promote a relative increase in SW radiation to the west of Luzon Island, consequently contributing to the SWP formation. Following this idea, we do a number of investigations and subsequently verify that the foehn clearance effect plays a crucial role in the formation of the SWP. Key Points: The foehn clearance effect is proposed to enhance the current explanation for the spring warm pool (SWP) formation mainly through the wake effectThe zonal difference of surface heat flux is driven largely by the short‐wave radiation difference caused by the foehn clearance effectThe SWP would not emerge if the foehn clearance effect was eliminated during the winter monsoon season [ABSTRACT FROM AUTHOR]