Role of ocean currents and bottom pressure variability on seasonal polar motion

Abstract

Changes in the ocean angular momentum (OAM) components about the equatorial axes, either due to fluctuations in currents or bottom pressure (mass redistribution), can induce movements of the Earth's pole of rotation, commonly referred to as polar motion or wobble. Output from a 1° resolution ocean model is used to calculate the effective equatorial OAM functions χ1O and χ2O, corresponding to polar motion excitation about the equatorial axis pointing to the Greenwich and 90°E meridians, respectively. Time series of χO are combined with similar atmospheric series χA, computed from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalyses, to interpret the observed low‐frequency polar motion excitation for the period 1985–1996. Results indicate that the oceans are a very important excitation source for the Chandler (∼433 days), annual, and semiannual wobbles, providing for much better amplitude and phase agreement with the observed excitation at these periods, in comparison with what is obtained when only the atmosphere is considered. Both oceanic mass and motion terms are found to be important but with mass signals having somewhat larger amplitudes. The role of regional variability in ocean currents and bottom pressure in contributing to χO signals is quantified. Midlatitude regions (∼30°–70°) figure prominently as places of strong local oceanic excitation signals. The North Pacific basin is found to be generally important for χ1O excitation, while the Southern Ocean is important for both χ1O and χ2O. The largest positive covariances of local with global χ1O signals occur in the Kuroshio region near the western boundary of the North Pacific for χ1O and southwest of Australia for χ2O.