BIFURCATION OF THE KUROSHIO EXTENSION AT THE SHATSKY RISE

A 1/16 degrees six-layer Pacific Ocean model north of 20 degrees S is used to investigate the bifurcation of the Kuroshio Extension at the m ain Shatsky Rise and the pathway of the northern branch from the bifur cation to the subarctic front. Upper ocean-topographic coupling via a mixed barotropic-baroclinic instability is essential to this bifurcati on and to the formation and mean pathway of the northern branch as are several aspects of the Shatsky Rise complex of topography and the lat itude of the Kuroshio Extension in relation to the topography. The flo w instabilities transfer energy to the abyssal layer where it is const rained by geostrophic contours of the bottom topography. The topograph ically constrained abyssal currents in turn steer upper ocean currents , which do not directly impinge on the bottom topography. This include s steering of mean pathways. Obtaining sufficient coupling requires ve ry fine resolution of mesoscale variability and sufficient eastward pe netration of the Kuroshio as an unstable inertial jet. Resolution of 1 /8 degrees for each variable was not sufficient in this case. The lati tudinal extent of the main Shatsky Rise (31 degrees N-36 degrees N) an d the shape of the downward slope on the north side are crucial to the bifurcation at the main Shatsky Rise, with both branches passing nort h of the peak. The well-defined, relatively steep and straight eastern edge of the Shatsky Rise topographic complex (30 degrees N-42 degrees N) and the southwestward abyssal flow along it play a critical role i n forming the rest of the Kuroshio northern branch which flows in the opposite direction. A deep pass between the main Shatsky Rise and the rest of the ridge to the northeast helps to link the northern fork of the bifurcation at the main rise to the rest of the northern branch. T wo 1/16 degrees ''identical twin'' interannual simulations forced by d aily winds 1981-1995 show that the variability in this region is mostl y nondeterministic on all timescales that could be examined (up to 7 y ears in these 15-year simulations). A comparison of climatologically f orced and interannual simulations over the region 150 degrees E-180 de grees E, 29 degrees N-47 degrees N showed greatly enhanced abyssal and upper ocean eddy kinetic energy and much stronger mean abyssal curren ts east of the Emperor Seamount Chain (about 170 degrees E) in the int erannual simulations but little difference west of 170 degrees E. This greatly enhanced the upper ocean-topographic coupling in the interann ual simulations east of 170 degrees E. This coupling affected the lati tudinal positioning of the eastward branches of the Kuroshio Extension and tended to reduce latitudinal movement compared to the climatologi cally forced simulation, including a particularly noticeable impact fr om the Hess Rise. Especially in the interannual simulations, effects o f almost all topographic features in the region could be seen in the m ean upper ocean currents (more so than in instantaneous currents), inc luding meanders and bifurcations of major and minor currents, closed c irculations, and impacts from depressions and rises of large and small amplitudes.