RESPONSE OF A GLOBAL OCEAN CIRCULATION MODEL TO REAL-TIME FORCING ANDIMPLICATIONS TO EARTHS ROTATION

The ocean's contribution to interannual variations in length of day (l od) is investigated by means of the global Hamburg large-scale ocean c irculation model (LSG) forced with observed wind stress and air temper ature fields. The horizontal resolution of the model is 3.5 degrees in latitude and longitude, and eleven layers exist in the vertical; the timestep used is one month. The atmospheric forcing is obtained by add ing ECMWF 1000-mb monthly anomalies of wind stress and air temperature to climatological values of Hellerman and Rosenstein and GOADS, respe ctively. The data extend from November 1979 to November 1993. Within t his period three El Nino events (1982-83, 1986-87, and 1991-92) and tw o La Nina events (1984, 1988) were observed. Variations in the pressur e torque, the inertia tensor of the ocean. and the momentum connected with the currents are calculated diagnostically from the OGCM output. Model results show that the ocean works mainly as a transmitter of ang ular momentum from the atmosphere to the solid earth and that contribu tions to interannual variations of Iod from the mass distribution term amount to four rimes the effect of the motion term. Contributions to interannual variability of lod can mainly be attributed to the pressur e torque and the matter term, whereas the contribution by the ocean cu rrents varies on shorter timestaIes up to one year. The calculated tot al changes in Iod are in the order of 0.1 ms. This is the right order of magnitude to close the imbalance between observed changes and resul ts from atmospheric circulation models, but the correlation between ob served residuals and computed anomalies is still poor.