Determining the applicability of the barotropic approximation to the mean seasonal flow through the Tsushima/Korean Strait using variational assimilation

Abstract

Abstract Variational assimilation is used to combine velocity and sea-surface height anomaly (SSHA) measurements with a system of dynamics to estimate the seasonal flow through the Tsushima/Korean Strait for the summer, autumn, and winter seasons of 1999–2000. The velocity measurements are from two lines of moored acoustic Doppler current profilers (ADCPs) spanning the Tsushima/Korean Strait just north and south of Tsushima Island and the SSHA measurements are from the TOPEX altimeter. The dynamics are the linear, time-independent, shallow-water equations and are forced by winds from the Navy Global Ocean and Atmospheric Prediction System. A weighted least-squares technique is used to determine the seasonal flow fields that simultaneously minimize the weighted residuals of the two data sets and the system of dynamics. The weights are based on expected errors, allowing the assimilation system to put more emphasis on the components of data and dynamics that are known more accurately. Earlier studies show that the flow through the Tsushima Strait is barotropic throughout most of the year. The region just offshore of the southeast coast of South Korea, however, has been identified as being subject to strong baroclinic processes during autumn. In an attempt to verify this observation, best estimates of the seasonal flow fields are computed by assimilating the two data sets with both a single-layered barotropic system of equations and a 2.5-layered baroclinic system of equations. The minimized residuals of the barotropic system of equations reveal that the solutions satisfy these equations within expected errors throughout most of the domain. Just offshore of the southeast coast of South Korea, however, the barotropic momentum equation residuals exceed 3 times the expected error during autumn, therefore implying that the barotropic approximation is not valid at this location. The 2.5-layered baroclinic momentum equation residuals within this region are smaller than the barotropic residuals, suggesting the existence of baroclinic processes.