Long‐Term Decreasing of Sea Level Along Latitude of the Luzon Strait During 1993–2020: Surface Versus Subsurface Perspectives.

The sea level in the east of Luzon Strait along the latitudinal band (18°–22°N, 125°–170°E) demonstrates a unique decreasing trend during 1993–2020, with weakening of eddy kinetic energy (EKE), weaker Kuroshio transport and stronger looping pathway in the Luzon Strait. Neither the surface wind forcing (remote Rossby wave and local Ekman convergence/divergence) nor surface buoyancy forcing (heat flux and Ekman thermal advection) could explain the observed sea level and EKE decreasing trend. From the subsurface perspective, by using the observational data analysis and ventilated thermocline theory, we propose that the reduction of mode water in the subsurface ocean contributes to the above changes during the past ∼30 years. The mode water forms primarily in the mid‐latitude and then is carried by the thermocline circulation southwestward after it subducts into the subsurface ocean. It takes ∼5 years for the mode water to arrive at the latitudinal band of Luzon Strait and affect the vertical stratification by changing the slope of thermocline. When the volume of mode water shrinks, the slope of thermocline would flatten, making the upper thermocline lift along the Luzon Strait latitudinal band. Whereafter, the subtropical front weakens, reducing the vertical velocity shear between the surface eastward Subtropical Counter Current and subsurface westward North Equatorial Current and sea level due to the baroclinic adjustment. Besides, the flattened thermocline and weakened vertical velocity shear results in the EKE reduction, which will contribute to the decreasing of sea level due to the weakened eddy‐forcing. Plain Language Summary: Sea level change is an important and hot issue for the ocean and climate. In the past ∼30 years, the sea level in the east of Luzon Strait exhibits a unique band of decreasing trend, which affects the Kuroshio, one of the strongest ocean western boundary currents, and water exchange between the Western Pacific Ocean and the South China Sea. The decreasing of seal level can neither be explained by the surface wind forcing nor by the buoyancy forcing. Here we show that during 1993–2020, the reduction of mode water, a subsurface water mass between the upper and lower thermocline, contributes to the sea level decreasing along the latitudinal band of Luzon Strait. On the one hand, the volume shrinkage of mode water will reduce the tilt of thermocline and then the sea level. On the other hand, the decreasing of mode water and flattening of thermocline result in eddy activity reduction, which further lowers the sea level. Our study highlights the importance of subsurface forcing (The "subsurface forcing" means that we consider the forcing induced by mode water from a subsurface perspective in this study) induced by the mode water, in longer time scale. Key Points: The sea level along the latitude of Luzon Strait shows significant fine‐band structure of decrease during 1993–2020Neither the surface wind nor buoyancy forcing could explain the observed decreasing trend of the sea levelThe decreasing of subsurface mode water plays a key role for the decrease of sea level by changing the tilt of thermocline [ABSTRACT FROM AUTHOR]