Interannual Variations of Wintertime Mixed Layer Depth in the Northern South China Sea

Abstract The interannual variations of wintertime mixed layer depth (MLD) in the northern South China Sea (SCS), a region characterized by the deepest MLD within the basin and subduction process, are elucidated based on the Simple Ocean Data Assimilation (SODA, version 2.2.4) reanalysis data between 1950 and 2010. Our results reveal that the wintertime MLD possesses two predominant modes of 2–4‐year and 7–8‐year, involving different dynamical processes. Responses of thermal structure to local air‐sea interface factors primarily explain the 2–4‐year period MLD variability, with sea surface net heat flux playing a more crucial role than wind stress curl. Whereas for the 7–8‐year period MLD variability, Luzon Strait Transport (LST), high‐dense inflow from the Pacific Ocean into the northern SCS, is the dominant driver, while local air‐sea interface factors play a minor role. The difference between surface and subsurface layers inflow (), which is appropriate to represent the LST impact on vertical density gradient, can effectively modulate the upper layer stratification intensity in the northern SCS. Thus, a stronger (weaker) conduces to establish a more unstable (stable) state therein, favorable to a vigorous (stagnant) vertical mixing and MLD deepening. Further analyses demonstrate the importance of both horizontal heat and salt advections related to LST in the northern SCS. That is to say, different from the 2–4‐year MLD variability, the salinity effect is of importance in driving this 7–8‐year MLD variability.