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 (Kostianoy and Belkin, 1989) indicate typical spatial scales of 10–100 km horizontally and 100 m vertically. In the Japan/East Sea (JES) (Figure 1) 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.