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 degrees-22 degrees N, 125 degrees-170 degrees 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 Ross by 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 similar to 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 similar to 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. Where after, 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.