Global and regional axial ocean angular momentum signals and length-of-day variations (1985-1996)

Changes in ocean angular momentum nl about the polar axis are related to fluctuations in zonal currents (relative component M-r) and latitudinal shifts in mass (planetary component M-Omega). Output from a 10 ocean model is used to calculate global M-r, M-Omega, and M time series at 5 day intervals for the period January 1985 to April 1996. The annual cycle in M-r, M-Omega, and M is larger than the semiannual cycle, and M-Omega amplitudes are nearly twice those of M-r. Year-to-year modulation of the seasonal cycle is present, but interannual variability is weak. The spectrum of M is red (background slope between omega(-1) and omega(-2)) at subseasonal periods, implying a white or blue spectrum for the external torque on the ocean. Comparisons with previous studies indicate the importance of direct atmospheric forcing in inducing subseasonal M signals, relative to instabilities and other internal sources of rapid oceanic signals. Regional angular momentum estimates show that seasonal variability tends to be larger at low latitudes, but many local maxima exist because of the spatial structure of zonal current and mass variability. At seasonal timescales, latitudes similar to 20 degrees S-10 degrees N contribute substantial variability to M-Omega, while signals in M-r can be traced to Antarctic Circumpolar Current transports and associated circulation. Variability in M is found to be small when compared with similar time series for the atmosphere and the solid Earth, but ocean signals are significantly coherent with atmosphere-solid Earth residuals, implying a measurable oceanic impact on length-of-day variations.