Rapid changes of the oceanic circulation in a hierarchy of ocean models

Abstract

The response time of the large-scale oceanic circulation due to freshwater perturbations isinvestigated with models of different complexity. A three-dimensional ocean general circulationmodel (OGCM) and a zonally averaged ocean model are employed. In order to distinguishadvection and diffusion from fast baroclinic processes (e.g. waves in the OGCM) a color traceris injected at the same time and location as the freshwater discharges. In spite of the inabilityof the zonally averaged model to represent wave processes in a realistic way similarities withthe OGCM are found for the propagation patterns of density anomalies and of color tracer.In the OGCM as well as in the zonally averaged model, density anomalies propagate fasterthan anomalies of a passive color tracer in the case of vertical density stratification. The progressionof density anomalies leads to changes of the oceanic circulation, and both oceanic modelsexhibit circulation changes in areas distant from the discharge places long before the passivelyentrained color tracer has reached these regions. The fact that a zonally averaged model simulatesbaroclinic processes faster than advection even if internal gravity waves are not representeddue to neglected acceleration terms, is clarified with a conceptual box model.