INSTABILITY OF THE THERMOHALINE CIRCULATION WITH RESPECT TO MIXED BOUNDARY-CONDITIONS - IS IT REALLY A PROBLEM FOR REALISTIC MODELS

A global primitive equations oceanic GCM and a simple four-box model o f the meridional circulation are used to examine and analyze the insta bility of the thermohaline circulation in an ocean model with realisti c geometry and forcing conditions under mixed boundary conditions. The purpose is to determine whether this instability should occur in such realistic GCMs. It is found that the realistic GCM ablution is near t he stability transition point with respect to mixed boundary condition s. This proximity to the transition point allows the model to make a t ransition between the unstable and stable regimes induced by a relativ ely minor change in the surface freshwater flux and in the interior so lution. Such a change in the surface flux may be induced, for example, by changing the salinity restoring time used to obtain the steady mod el solution under restoring conditions. Thus, the steady solution of t he global GCM under restoring conditions may be either stable or unsta ble upon transition to mixed boundary conditions, depending on the mag nitude of the salinity restoring time used to obtain this steady solut ion. The mechanism by which the salinity restoring time affects the mo del stability is further confirmed by carefully analyzing the stabilit y regimes of a simple four-box model. The proximity of the realistic o cean model solution to the stability transition point is used to deduc e that the real ocean may also be near the stability transition point with respect to the strength of the freshwater forcing. Finally, it is argued that the use of too short restoring times in realistic models is inconsistent with the level of errors in the data and in the model dynamics, and that this inconsistency is a possible reason for the exi stence of the thermohaline instability in GCMs of realistic geometry a nd forcing. A consistency criterion for the magnitude of the restoring times in realistic models is formulated, that should result in steady states that are also stable under mixed boundary conditions. The resu lts presented here may be relevant to climate studies that run an ocea n model under restoring conditions in order to initialize a coupled oc ean-atmosphere model.