The role of upper ocean - topographical coupling and isopycnal outcropping in simulations of the Japan/East Sea circulation

Patrick Joseph Hogan

Abstract

A regional primitive equation ocean model is used to investigate the impact of grid resolution, baroclinic instability, bottom topography, and isopycnal outcropping on the dynamics of the wind and throughflow forced surface circulation in the Japan/East Sea. The results demonstrate that at least 1/32° (3.5 km) horizontal grid resolution is necessary to generate sufficient baroclinic instability to produce eddy-driven cyclonic deep mean flows. These abyssal currents follow the f/h contours of the bottom topography and allow the bottom topography to strongly influence mean pathways of the upper ocean currents in the Japan/East Sea. This upper ocean--topographical coupling via baroclinic instability (actually a mixed baroclinic-barotropic instability) requires that mesoscale variability be very well resolved to obtain sufficient coupling. For example, 1/32° resolution is required to obtain a realistic separation latitude of the East Korean Warm Current (EKWC) from the Korean coast when Hellerman-Rosenstein monthly climatological wind stress forcing is used. Separation of the EKWC is more realistic at 1/8° resolution when the model is forced with climatological winds formed from the ECMWF 10 meter reanalyses due to strong positive wind stress curl north of the separation latitude, but at 1/8° the level of baroclinic instability is insufficient to initiate upper ocean--topographical coupling. Hence, this major topographical effect is largely missed at coarser resolution and leads to erroneous conclusions about the role of bottom topography and unexplained errors in the pathways of current systems. Results from a 1/64° simulation are quite similar to those at 1/32°, particularly where the EKWC separates from the Korean coast, suggesting statistical simulation convergence for mesoscale variability has been nearly achieved at 1/32° resolution. Isopycnal out-cropping and associated vertical mixing yield a realistic residence time for abyssal water in the model (∼130 years) and they provide an alternate mechanism to topographical control in developing and maintaining a boundary current along the west coast of Japan. They are also the mechanism for transferring energy to the abyssal layer, but are less important than baroclinic instability in driving deep mean flows.