A WORLD OCEAN MODEL FOR GREENHOUSE SENSITIVITY STUDIES - RESOLUTION INTERCOMPARISON AND THE ROLE OF DIAGNOSTIC FORCING

We have developed an improved version of a world ocean model with the intention of coupling to an atmospheric model. This article documents the simulation capability of this 1-degrees global ocean model, shows improvements over our earlier 5-degrees version, and compares it to features simulated with a 0.5-degrees model. These experiments use a model spin-up methodology whereby the ocean model can subsequently be coupled to an atmospheric model and used for order 100-year coupled model integrations. With present-day computers, 1-degrees is a reasonable compromise in resolution that allows for century-long coupled experiments. The 1-degrees ocean model is derived from a 0.5-degrees-resolution model developed by A. Semtner (Naval Postgraduate School) and R. Chervin (National Center for Atmospheric Research) for studies of the global eddy-resolving world ocean circulation. The 0.5-degrees bottom topography and continental outlines have been altered to be compatible with the 1-degrees resolution, and the Arctic Ocean has been added. We describe the ocean simulation characteristics of the 1-degrees version and compare the result of weakly constraining (three-year time scale) the three-dimensional temperature and salinity fields to the observations below the thermocline (710 m) with the model forced only at the top of the ocean by observed annual mean wind stress, temperature, and salinity. The 1-degrees simulations indicate that major ocean circulation patterns are greatly improved compared to the 5-degrees version and are qualitatively reproduced in comparison to the 0.5-degrees version. Using the annual mean top forcing alone in a 100-year simulation with the 1-degrees version preserves the general features of the major observed temperature and salinity structure with most climate drift occurring mainly beneath the thermocline in the first 50-75 years. Because the thermohaline circulation in the 1-degrees version is relatively weak with annual mean forcing, we demonstrate the importance of the seasonal cycle by performing two sensitivity experiments. Results show a dramatic intensification of the meridional overturning circulation (order of magnitude) with perpetual winter surface temperature forcing in the North Atlantic and strong intensification (factor of three) with perpetual early winter temperatures in that region. These effects are felt throughout the Atlantic (particularly an intensified and northward-shifted Gulf Stream outflow). In the Pacific, the temperature gradient strengthens in the thermocline, thus helping counter the systematic error of a thermocline that is too diffuse.