Interannual Migration of the Summertime Eastward Jet in the South China Sea Associated With the Upper Layer Thickness Distribution.

The summertime eastward jet (SEJ) in the South China Sea (SCS) can be regarded as the boundary between the southern and northern upper circulation of the SCS in climatology. The dynamic process of SEJ migration in response to the variation of SCS summer monsoon (SCSSM) strength on the interannual scale is investigated, based on the Simple Ocean Data Assimilation (SODA) dataset and by a 1.5‐layer reduced gravity model. Analysis shows that SEJ position is closely related to SCSSM strength, with southward/northward migration of SEJ coinciding with stronger/weaker SCSSM. Besides migration of wind stress curl (WSC) zero‐line and north‐south difference of WSC variation, the north‐south difference of the SCS upper layer thickness (ULT) in summer, that is, thick in the southern SCS and thin in the northern SCS, also plays a significant role in the dynamic process of SEJ migration. It is the joint effect of wind strength and ULT distribution. With the variation of SCSSM strength, the thinner ULT in the northern SCS corresponds to the larger magnitude of potential vorticity (PV) variation inputted by wind stress, which leads to more pronounced strengthening/weakening of the western boundary current (WBC) in the northern SCS than that in the southern SCS. Then, the north and south WBCs compete with each other until the upper layer circulation reaches a new equilibrium state, and SEJ migrates southward/northward accordingly. Plain Language Summary: The summertime eastward jet (SEJ) in the South China Sea (SCS) plays a significant role in the regional climate and ecosystem by transporting the coastal cold water with high chlorophyll‐a concentration into the interior sea, and thus its north‐south migration from year to year is an important issue. Analysis shows that with enhanced/weakened summer monsoon in the SCS, the SEJ migrates southward/northward. Our study aims to understand their relationship better and highlight the role of the summer upper layer thickness difference between the southern and northern basins. With the variation of summer monsoon strength, the circulation of the thinner water column responds more significantly than that of the thicker water column, and then SEJ migrates southward/northward. This is another important perspective to understand the migration of the summertime eastward jet, besides the migration of zero‐line of wind stress curl and the difference of monsoon variation over the southern and northern basins. Key Points: The summertime eastward jet migrates southward/northward with enhanced/weakened summer monsoon in the South China SeaIts migration is caused by different variations of potential vorticity input between southern and northern basinsBesides the effect of wind stress curl, the joint effect of monsoon strength and upper layer thickness distribution also plays a role [ABSTRACT FROM AUTHOR]