Oceanic excitation of daily to seasonal signals in Earth rotation: results from a constant-density numerical model

Abstract

S U M M A R YVelocity and mass fields from a constant-density, near-global ocean model, driven with observed twice-daily surface wind stresses and atmospheric pressures for the period October 1992-September 1993, are used to calculate oceanic excitation functions for the length of day (LOD) and for polar motion (PM), and results are analysed as a function of the frequency band. Variable currents and mass redistributions are both important in determining oceanic excitation functions. For bands with periods longer than one month, wind-driven variability is the primary cause of oceanic excitation signals. At higher frequency bands, larger deviations from the inverted barometer response occur, and pressure-driven signals contribute more significantly to the variance in the excitation functions. Oceanic LOD excitation is generally small compared to that of the atmosphere, except for the 2-10 day band. At these scales, adding oceanic to atmospheric excitation series does not lead to better agreement with the observed LOD, although this result may be related to data quality issues. With regard to the excitation of PM, the ocean is in general as important as the atmosphere at most time scales. Combined oceanic and atmospheric excitation series compare visibly better with geodetic series than do atmospheric series alone, pointing to the ocean as a source of measurable signals in PM.

Regional atmospheric angular momentum contributions to polar motion excitation

“…center is at ---70 days. Generally, also, the prograde parts of the cross spectrum and coherences are stronger than the retrograde in the non-IB case but comparable in the IB case, which is more likely at timescales greater than several days [Ponte, 1997] as is the case here. Such close amplitudes of prograde and retrograde parts of coherences and cross spectra correspond to quasi-linear polarization of the EAAM function, and they exhibit large amplitudes in X2 and small ones in X•.…”

“…Evidence of the role of the ocean on polar motion has been noted using ocean modeling approaches [e.g., Ponte, 1997;Ponte et al, 1998], yielding a signal approaching in amplitude that of the atmosphere and reducing the discrepancies between polar motion and atmospheric excitation. Nevertheless, given the strong relationship between the atmosphere and solid Earth reconfirmed here, it seems worthwhile to identify the regional sources within the atmosphere for polar motion excitation.…”

Rapid ocean signals in polar motion and length of day

“…[4] The barotropic model used to calculate OAM series is that of Ponte [1993] and has been used in previous OAM studies [Ponte, 1997;Nastula and Ponte, 1999]. The present configuration has been optimized to explain sea level variance in the TOPEX/Poseidon altimeter data and includes an improved representation of topography and parameterization of frictional processes [Hirose et al, 2001].…”

“…Early estimates of OAM variance at periods <10 days [Ponte, 1997] pointed to the possible role of the oceans as a source of rapid LOD and PM signals. A clear connection was later found between OAM and PM [Ponte et al, 1998] and LOD [Marcus et al, 1998] at seasonal to $10-day periods, but the use of 5-day averaged OAM values prevented the study of shorter periods.…”

Polar motions with a half-Chandler period and less in their temporal variability

“…Salstein and Rosen (1989), Nastula and Salstein (1999). Based on a constant-density, nearly global numerical ocean model, the results of the oceanic excitation of signals in the Earth's rotation are presented in Ponte (1997) and Ponte et al (1998), indicating that the oceanic circulation and mass-field variability play important roles in the excitation of seasonal to fortnightly polar motion. Studies dealt with the hydrological contributions to polar motion excitation are made, for example, by Chao et al (1987), Chao and O'Connor (1988), Wilson and Kuehne (1990), and Wilson (1993).…”