Frontal structure and thermohaline intrusions in the South Sea of Korea from observed data and a relocation method

Abstract

To investigate the spatial variation and temporal evolution of a shelf front within 50 km by 50 km area in the central south sea of Korea, shipboard acoustic Doppler current profiler current measurements and a hydrographic survey along closely spaced conductivity‐temperature‐depth (CTD) stations were conducted across the shelf front from 17 to 18 November 2003. The cross‐frontal structures drastically changed during the 25 hour CTD survey because cross‐frontal intrusions produced thermohaline inversions in the sloped pycnocline layer. Spatial fields of temperature and salinity are constructed here using a retrogressive vector diagram method, which relocate CTD stations using the observed currents data. In agreement with satellite‐observed sea surface temperature, the surface temperature distribution obtained from hydrographic data relocation reproduces the meandering surface front with an amplitude larger than 15 km and a wavelength of 30∼35 km. The constructed subsurface temperature and salinity fields on isopycnal surfaces reveal that the intrusions within the pycnocline layer are the crests and troughs of meandering internal fronts with vertically lagged phase. Advection of the internal fronts by strong mean flow and tidal currents is found to explain short‐period variations of frontal structure. Stability analysis of the intrusion slope and the vertical phase dependence of the internal fronts suggest that the multilayered intrusions are primarily generated by baroclinic instabilities caused by the strong vertical shear in the mean flow. Analysis of along‐intrusion density ratio suggests that double diffusion can occur in our baroclinic frontal zone but its contribution to the generation of intrusion be small.