DYNAMICS OF THE KUROSHIO OYASHIO CURRENT SYSTEM USING EDDY-RESOLVING MODELS OF THE NORTH PACIFIC-OCEAN/

A set of numerical simulations is used to investigate the Pacific Ocea n circulation north of 20 degrees S, with emphasis on the Kuroshio/Oya shio current system. The primitive equation models used for these simu lations have a free surface and realistic geometry that includes the d eep marginal seas such as the Sea of Japan. Most of the simulations ha ve 1/8 degrees resolution for each variable but range from 1/2 degrees , 1.5-layer reduced gravity to 1/16 degrees, six layer with realistic bottom topography. These are used to investigate the dynamics of the K uroshio/Oyashio current system and to identify the processes that cont ribute most to the realism of the simulations. This is done by model-d ata comparisons, by using the modularity of layered ocean models to in clude/exclude certain dynamical processes, by varying the model geomet ry and bottom topography, and by varying model parameters such as hori zontal grid resolution, layer structure, and eddy viscosity. In compar ison with observational data the simulations show that the barotropic mode, at least one internal mode, nonlinearity, high ''horizontal'' re solution (1/8 degrees or finer), the regional bottom topography, and t he wind forcing are critical for realistic simulations. The first four are important for baroclinic instability (eddy-mean energetics actual ly show mixed barotropic-baroclinic instability), the wind curl patter n for the formation and basic placement of the current system, and the bottom topography for the distribution of the instability and for inf luences on the pathways of the mean how. Both the Hellerman and Rosens tein (1983) (HR) monthly wind stress climatology and 1000-mbar winds f rom the European Centre for Medium-Range Weather Forecasts (ECMWF) hav e been used to drive the model. East of about 150 degrees E, they give a mean latitude for the Kuroshio Extension that differs by about 3 de grees, approximately 34 degrees N for HR, 37 degrees N for ECMWF, and 35 degrees N observed. The subarctic front is the northern boundary of the subtropical gyre. It is associated with the annual and April-Sept ember mean zero wind stress curl lines (which are similar), while the Kuroshio Extension is associated with wintertime zero wind stress curl . This means that part of the flow from the Kuroshio must pass north o f the Kuroshio Extension and connect with the Oyashio and subarctic fr ont. Realistic routes for this connection are flow through the Sea of Japan, a nonlinear route separated fi om the east coast of Japan, and bifurcation of the Kuroshio at the Shatsky Rise. In addition, the six layer simulations show a 3-Sv meridional overturning cell with southwa rd surface flow and northward return flow centered near 400 m depth. B aroclinic instability plays a critical role in coupling the shallow an d abyssal layer circulations and in allowing the bottom topography to strongly influence the shallow circulation. By this means the Izu Ridg e and Trench and seamounts upstream and downstream of these have profo und influence on (1) the mean path of the Kuroshio and its mean meande rs south and east of Japan and (2) on separating the northward flow co nnecting the Kuroshio and the Oyashio/subarctic front from the east co ast of Japan. Without the topographic influence the models show an unr ealistic northward current along the east coast of Japan. In essence, the topography regulates the location and strength of the baroclinic i nstability. The baroclinic instability gives eddy-driven deep mean flo ws that follow the f/h contours (where f is the Coriolis parameter and h is the depth of the water column) of the bottom topography. These a byssal currents then strongly influence the pathway for subtropical gy re flow north of the Kuroshio Extension and steer the mean meanders in the Kuroshio south and east of Japan, This is corroborated by current meter data from the Kuroshio Extension Regional Experiment (World Oce an Circulation Experiment line PCM 7). The meander path south of Japan depends on the occurrence of baroclinic instability west of the Izu R idge; otherwise, a straight path occurs. The pathway shows little sens itivity to the Tokara Strait transport over the range simulated (36-72 Sv in yearly means). However, interannual increases in wind forcing o r Tokara Strait transport give rise to a predominant meander path, whi le decreases yield a predominant straight path. Resolution of 1/8 degr ees in an ocean model is comparable to the 2.5 degrees resolution used in atmospheric forecast models in the early 1980s based on the first internal mode Rossby radius of deformation. Model comparisons at 1/8 d egrees and 1/16 degrees resolution and comparisons with current meter data and Geosat altimeter data show that 1/16 degrees resolution is ne eded for adequate eastward penetration of the high eddy kinetic energy associated with the Kuroshio Extension.