Why do Intrathermocline Eddies Form in the Japan/East Sea? A Modeling Perspective

Abstract

Abstract : Intrathermocline eddies (ITEs) are characterized by a subsurface lens of relatively homogeneous water. By definition, they are situated within the thermocline and therefore split the stratified water column, taking the form of a dome in the upper part of the thermocline and a bowl in the lower part. Observations of ITEs in diverse regions of the world ocean indicate typical spatial scales of 10-100 km horizontally and 100 m vertically. In the Japan/East Sea (JES) there are at least three mechanisms for the formation of ITEs from pre-existing non-ITE eddies based on results from the HYbrid Coordinate Ocean Model (HYCOM). Those mechanisms include advection of the stratified seasonal variations of temperature and salinity through the Tsushima Strait, restratification of the upper water column due to seasonal heating and cooling of the upper ocean, and subduction of ITE water originating from the Tsushima Strait beneath the wintertime Subpolar Front. The formation mechanisms are not mutually exclusive. Indeed, all three are shown to be interactively affecting the formation of an ITE in at least one case. Gordon et al. (2002) reported the existence of ITEs in the JES based on observations from SeaSoar instrumentation, conductivity-temperature depth (CTD) sensors, and airborne expendable bathythermographs (AXBTs). Their paper contains extensive analysis of ITEs in the JES and observational evidence of formation mechanisms based on cruise data collected during 1999-2000 as part of the Office of Naval Research (ONR) JES Department Research Initiative as well as results from earlier studies. The Gordon et al. work inspired a numerical modeling study to examine whether or not similar features could be simulated. If they could be simulated, could the ocean model be used as a tool to elucidate the formation mechanisms of the ITEs? This study uses HYCOM to simulate JES ITEs that have domed stratification at the top, forming a lens-shaped interior of nearly unstratified water.