A set of 1/8° and 1/16° 6-layer Pacific Ocean simulations north of 20°S is used to investigate the dynamics of the Kuroshio/Oyashio current system. The models include realistic coastline geometry, a free surface and, in most cases, realistic or deliberately altered bottom topography. Three sets of wind forcings are used in different simulations: the Hellerman and Rosenstein (1983; JPO) monthly wind stress climatology, a monthly climatology derived from 1981-1994 ECMWF 1000 mb winds and a 1981-1995 hybrid wind set consisting of daily ECMWF winds with the 1981-1994 mean replaced by the annual mean from Hellerman and Rosenstein. This includes two pairs of interannual identical twin simulations with 1/8 and 1/16° resolution for each variable. These were run 1981-1995 forced by the daily ECMWF hybrid winds.
The subarctic front is the northern boundary of the subtropical gyre. It is associated with the annual and Apr-Sept mean zero wind stress curl (which are similar) while the Kuroshio Extension is associated with winter time zero wind stress curl. This means that part of the flow from the Kuroshio must pass north of the Kuroshio Extension and connect with the Oyashio and subarctic front. The models show how the combined effects of baroclinic instability and specific topographic features have a profound influence on 1) the mean path of the Kuroshio just south and east of Japan and 2) on separating the northward flow connecting the Kuroshio and Oyashio/subarctic front from the east coast of Japan. Without the topographic influence the models show an unrealistic northward current along the east coast of Japan. In essence the topography regulates the location and strength of the baroclinic instability (actually a mixed barotropic/baroclinic instability in eddy-mean energetics). The baroclinic instability gives eddy driven deep mean flows that follow the f/h contours of the bottom topography. These abyssal currents then strongly influence the pathway for subtropical gyre flow north of the Kuroshio Extension and steer the mean meanders in the Kuroshio south and east of Japan (Hurlburt et al., 1996; JGR-O). This is corroborated by current meter data from the Kuroshio Extension Regional Experiment (WOCE line PCM-7).
Model comparisons at 1/8° and 1/16° resolution and comparisons with current meter data and Geosat altimeter data show that 1/16° resolution is needed for adequate eastward penetration of the high EKE associated with the Kuroshio Extension. It is also required to simulate the bifurcation of the Kuroshio near the Shatsky Rise, which again is associated with upper ocean - topographic coupling via baroclinic instability. At 1/16° resolution we are discovering the widespread importance of baroclinic instability in allowing bottom topography to steer upper ocean currents. This requires that mesoscale eddies be very well resolved in order to obtain sufficient coupling. With monthly climatological forcing, the EKE levels are 3 to 10 or more times higher in the 1/16° vs the 1/8° model outside the open ocean equatorial Pacific. In general, the 1/16° model gives widespread improvements over 1/8° resolution on the large scale as well as the mesoscale, including sharper definition and greater eastward penetration of the basin wide subarctic and Kuroshio Extension fronts.
by H. E. Hurlburt, A. J. Wallcraft, W. J. Schmitz, Jr., E. J. Metzger, and P. J. Hogan
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