Further evidence for oceanic excitation of polar motion

While the role of the atmosphere in driving variations in polar motion is w ell established, the importance of the oceans has been recognized only rece ntly. Further evidence for the role of the oceans in the excitation of pola r motion is presented. To estimate the equatorial excitation functions, chi (1) and chi(2), for the ocean, we use velocity and mass fields from a const ant-density ocean model, driven by observed surface wind stresses and atmos pheric pressure, for the period 1993-1995; comparison with similar function s derived from a more complex density-stratified ocean model indicates the effectiveness of the simple constant-density modelling approach. Correspond ing atmospheric excitation functions are computed from NCEP/NCAR re-analyse s. Results indicate significant improvements in the agreement with the obse rved polar motion excitation when the simulated oceanic effects are added t o atmospheric excitation. Correlations between the polar motion and the geo physical signals at periods of 15-150 days increase from 0.53 to 0.80 and f rom 0.75 to 0.88 for chi(1) and chi(2), respectively. The oceanic signals a re particularly important for seasonal Variations in chi(1) (correlation in creases from 0.28 to 0.85 when oceanic excitation is included). A positive impact of the oceans on more rapid polar motion is also observed, up to per iods as short as 5 days. The sensitivity of the results to different forcin g fields and different amounts of friction in the oceans is also discussed.