Bottom Topography Effect on the Seasonal Mode Generation of the Tsugaru Warm Current

Abstract Several regions in the World Ocean are known to exhibit eddies with characteristic structures controlled by bottom topography and surrounding currents. Here, we analyzed the generation of one such eddy, an anticyclonic gyre that seasonally forms the gyre mode (GM) of the Tsugaru Warm Current (TWC) along its path downstream of the Tsugaru Strait. This eddy greatly influences surface temperature and water mass distributions in this region of high fishery productivity. We constructed composite fields based on relative vorticity for both the GM and the coastal mode (CM) of the TWC, when it flows directly along the meridional Sanriku coast downstream of the strait. Within the strait, the CM-composite path is known to be located south of the GM-composite path. We found that clockwise turning of the barotropic CM-composite flow in shallow regions due to potential vorticity conservation generates the southward components of the outflow from the strait, connecting it smoothly to the meridional coastal current. In contrast, the rather baroclinic GM-composite flow is affected by the topography only near the bottom; as a result, it flows eastward from the strait, where the gyre is generated as has previously been investigated analytically and in laboratory experiments. We also found that the gyre includes both an isolated circulation surrounded by the TWC path and a lower layer with an isopycnal surface depression down to about 1000-m depth. This deep structure is supported by observation data.