Response of a Global Ocean Circulation Model to Real-Time Forcing and Implications to Earth’s Rotation

Abstract

The ocean?s contribution to interannual variations in length of day (lod) is investigated by means of the global Hamburg large-scale ocean circulation model (LSG) forced with observed wind stress and air temperature fields. The horizontal resolution of the model is 3.5° in latitude and longitude, and eleven layers exist in the vertical;the timestep used is one month. The atmospheric forcing is obtained by adding ECMWF 1000-mb monthly anomalies of wind stress and air temperature to climatological values of Hellerman and Rosenstein and COADS, respectively. The data extend from November 1979 to November 1993. Within this period three El Niño events (1982?83, 1986?87, and 1991?92) and two La Niña events (1984, 1988) were observed. Variations in the pressure 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 angular momentum from the atmosphere to the solid earth and that contributions to interannual variations of lod from the mass distribution term amount to four times the effect of the motion term. Contributions to interannual variability of lod can mainly be attributed to the pressure torque and the matter term, whereas the contribution by the ocean currents varies on shorter timescales up to one year. The calculated total changes in lod are in the order of 0.1 ms. This is the right order of magnitude to close the imbalance between observed changes and results from atmospheric circulation models, but the correlation between observed residuals and computed anomalies is still poor.