Global eddy-resolving ocean simulations driven by 1985-1995 atmospheric winds

Results are presented from a high-resolution global ocean model that is dri ven through three decadal cycles of increasingly realistic prescribed atmos pheric forcing from the period 1985-1995. The model used (the Parallel Ocea n Program) is a z level primitive equation model with active thermohaline d ynamics based on the formulation of Bryan [1969] rewritten for massively pa rallel computers. Improvements to the model include an implicit free-surfac e formulation of the barotropic mode [Dukowicz and Smith, 1994] and the use of pressure averaging for increasing the numerical time step. This study e xtends earlier 0.5 degrees simulations of Semtner and Chervin [1992] to hig her horizontal resolution with improved treatments of ocean geometry and su rface forcing. The computational grid is a Mercator projection covering the global ocean from 77 degrees N to 77 degrees S and has 20 vertical levels. Three successive simulations have been performed on the CM-5 Connection Ma chine system at Los Alamos using forcing fields from the European Centre fo r Medium-Range Weather Forecasts (ECMWF). The first run uses monthly wind s tresses for 1985-1995 and restoring of surface temperature and salinity to the Levitus [1982] seasonal climatology. The second run is the same but wit h 3 day-averaged rather than monthly averaged wind stress fields, and the t hird is the same as the second but uses the monthly climatological ECMWF he at fluxes of Barnier et al. [1995] instead of restoring to climatological s ea surface temperatures. Many features of the wind-driven circulation are w ell represented in the model solutions, such as the overall current pattern s, the numerous regions of hydrodynamic instability which correspond to the se observed by satellite altimetry, and the filamented structure of the Ant arctic Circumpolar Current. However, some features such as the separation p oints of the Gulf Stream and Kuroshio and the transport through narrow pass ages such as the Florida Straits are clearly inaccurate and indicate that s till higher resolution may be required to correct these deficiencies. Water mass properties and some aspects of the thermohaline circulation are also not always well reproduced, which is partly due to the relatively short len gth of the integrations. The use of the ECMWF heat fluxes, rather than rest oring to climatological surface temperatures, leads to stronger and more re alistic surface and deep western boundary currents (primarily in the Atlant ic) as well as more realistic meridional heat transport; this is primarily because the equilibrium meridional heat transport implied by the ECMWF surf ace fluxes is quite large. The ECMWF heat fluxes also produce improved seas onal cycles of sea surface temperature and height in both the northern and southern hemispheres. The 3-day wind forcing gives rise to modes of model v ariability that are clearly seen in synoptic observations, such as the larg e-scale 20-100-day oscillations seen in the TOPEX/POSEIDON data, which are barotropic oscillations induced by the high-frequency wind forcing. Additio nal studies on other aspects of the simulations described here are being co nducted by a variety of investigators, and some of these are briefly descri bed.