THERMOHALINE STRUCTURE OF AN EDDY-RESOLVING NORTH-ATLANTIC MODEL - THE INFLUENCE OF BOUNDARY-CONDITIONS

A T-S volumetric census, with a resolution of 0.2 degrees C and 0.1 ps u, for years 20-25 of the World Ocean Circulation Experiment Community Modeling Effort eddy-resolving simulation of the equatorial and North Atlantic Ocean, reveals how the thermohaline character of the model h as changed from the initial conditions, which were taken from the Levi tus climatology. Any changes in the thermohaline structure, other than stirring, mixing, or geostrophic adjustment of smoothed climatology, must be due to the boundary conditions, which are imposed at the surfa ce and at four sponge layers (northern boundary, southern boundary, La brador Sea and Mediterranean Sea), where water temperature and salinit y are nudged toward climatological conditions. Several unrealistic the rmohaline features appear in the solution, which can be traced to thes e surface and lateral sponge boundary conditions. 1) Water masses from the Arctic Ocean are overrepresented in the model. The volume transpo rt across the northern sponge is twice the value estimated from observ ations. The heat flux is approximately correct, while the salt flux is large by a factor of 4. 2) Water masses from the South Atlantic are u nderrepresented. The transport of water across the southern sponge is about two-thirds of the observed value, but the salt flux is comparabl e with estimates. However, the heat flux is only 10% of measured value s due to a missing equatorward motion of warm surface waters. 3) Water masses from the Labrador Sea and Baffin Bay are overrepresented. The volume nux is twice that observed, while the heat flux from the sponge is realistic. The salt flux is about 20% of the observed value. 4) Fi nally, Mediterranean Water is underrepresented. Even though the volume transport across the sponge is eight times the observed value, the ne t salt flux is small by a factor of 400, leading to an insufficient pr oduction of salt. All of these difficulties with the model T-S structu re are traced to three general problems. First, the flow at the outer edge of the sponges is strongly barotropic in spite of the fact that t he temperature and salinity fields are from climatology. Part of the p roblem with the sponges may be the smoothed nature of the climatology, which has the effect of reducing density gradients, thereby reducing geostrophic shears. In all cases, except the southern sponge, the volu me transport across the sponge is two to eight times larger than the v alue expected from other analyses or observations. Since the vertical structure of the now is set by the climatology, the only way to create this additional transport is through barotropic now. The reason for t he additional transport is not entirely clear, but it may be due to th e excessive vertical velocities that are demanded by the conversion pr ocess in the sponges. These vertical motions create bound vortices in the sponge layers that drive recirculation in the vicinity of the spon ges;increasing the transport without changing the heat or salt flux. T he second problem is due to geometric effects within the sponges. One such problem is that Iceland blocks the exchange along the northern sp onge. Another problem is that the ocean bathymetry is specified in the sponge layer. For example, the inner Mediterranean sponge is so shall ow (around 100 m) that there is very little area in which to modify th e water. Similar conditions occur in the Labrador sponge where the wat er is also 100 m deep. The third general problem is the use of relaxat ion to climatology to represent surface freshwater fluxes, which leads to unrealistic surface forcing if the currents are displaced from cli matological locations. The combination of a displaced Gulf Stream and the relaxation of surface salinity to climatology produces mode waters that are unrealistically cool and fresh.