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 fe atures simulated with a 0.5-degrees model. These experiments use a mod el spin-up methodology whereby the ocean model can subsequently be cou pled to an atmospheric model and used for order 100-year coupled model integrations. With present-day computers, 1-degrees is a reasonable c ompromise in resolution that allows for century-long coupled experimen ts. The 1-degrees ocean model is derived from a 0.5-degrees-resolution model developed by A. Semtner (Naval Postgraduate School) and R. Cher vin (National Center for Atmospheric Research) for studies of the glob al eddy-resolving world ocean circulation. The 0.5-degrees bottom topo graphy and continental outlines have been altered to be compatible wit h the 1-degrees resolution, and the Arctic Ocean has been added. We de scribe the ocean simulation characteristics of the 1-degrees version a nd compare the result of weakly constraining (three-year time scale) t he three-dimensional temperature and salinity fields to the observatio ns below the thermocline (710 m) with the model forced only at the top of the ocean by observed annual mean wind stress, temperature, and sa linity. The 1-degrees simulations indicate that major ocean circulatio n 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 obs erved temperature and salinity structure with most climate drift occur ring mainly beneath the thermocline in the first 50-75 years. Because the thermohaline circulation in the 1-degrees version is relatively we ak with annual mean forcing, we demonstrate the importance of the seas onal cycle by performing two sensitivity experiments. Results show a d ramatic intensification of the meridional overturning circulation (ord er of magnitude) with perpetual winter surface temperature forcing in the North Atlantic and strong intensification (factor of three) with p erpetual early winter temperatures in that region. These effects are f elt throughout the Atlantic (particularly an intensified and northward -shifted Gulf Stream outflow). In the Pacific, the temperature gradien t strengthens in the thermocline, thus helping counter the systematic error of a thermocline that is too diffuse.