RECENT ADVANCES IN MODELING THE OCEAN CIRCULATION AND ITS EFFECTS ON CLIMATE

The ocean plays a central role for the dynamics of the climate system. It can simultaneously act to damp certain components of the man-made climate changes, while amplifying others. At the same time, it is resp onsible for many aspects of climate variability which occur naturally (i.e. independent of man's activities). In the last decade there has b een a considerable improvement in the skill with which the ocean can b e modelled. There also have been advances in observing and monitoring certain aspects of the ocean circulation and thermal structure, althou gh we are far short of a global ocean observing system. The physical b asis for ocean modelling is outlined. While three-dimensional models b ased on the Navier-Stokes equations are now well developed, the parame trization of unresolved motions and the specification of appropriate b oundary conditions still cause many difficulties. The wind-driven circ ulation is now reasonably well understood. Increasing computer power h as permitted an explicit inclusion of mesoscale eddies which are parti cularly crucial for the dynamics of western boundary currents such as the Gulf Stream. An area of considerable recent progress is the intera ction of the tropical oceans with the atmosphere which leads to short- term climate fluctuations known as ENSO (El Nino-Southern Oscillation) . On time scales of several months some rudimentary forecasts have bee n achieved although it is not yet clear how precisely the climate fluc tuations associated with ENSO may be predicted. Considerable uncertain ties remain about climate variations on decadal and longer time scales . Due to differences in the way heat and fresh water are exchanged wit h the atmosphere, the thermohaline circulation can have more than one stable equilibrium state. Small changes in surface salinity can disrup t the thermohaline circulation and cause transitions to different clim atic states within a few decades (so-called halocline catastrophe).