On the observed annual gravity variation and the effect of sea surface height variations

Abstract

Based on two datasets of sea surface height (SSH) variations, the Parallel Ocean Climate Model (POCM) [J. Geophys. Res. 101 (C10) (1996) 25779] and the TOPEX/POSEIDON (T/P) altimeter, we have analyzed the effect of SSH variations on gravity observations. For this purpose, we first estimated the steric component of SSH variations which we characterized by a thermal steric coefficient relating the sea surface temperature (SST) to SSH. The steric coefficient was estimated on each ocean mesh by assuming a simple linear relation ship between the time variations in the SSH and SST fields. We obtained a value of 0.60×10 −2 m/°C averaged over the central parts of the Pacific and Atlantic Oceans. We then estimated the annual gravity change by calculating the effects of the solid tide, ocean tide, polar motion and SSH variations. The predicted values at the three observation sites (i.e. Esashi in Japan, Canberra in Australia and Syowa Station in Antarctica) were compared with the actual data obtained from the superconducting gravimeters installed at these three sites. The results of the comparison indicate that the predictions agree with the observations within 20% in amplitude (i.e. within 0.2 μGal, where 1 μGal=1×10 −8 ms −2) and 20° in phase at each observation site for both SSH datasets of the POCM and T/P. We have also tested other values for the steric coefficient, i.e. 0.0×10 −2 and 1.0×10 −2 m/° C , but find that the fit-to-gravity observations made at the mid-latitudes is clearly better at 0.60×10 −2 m/°C. It is noted that our gravity observations point to a value of steric coefficient similar to that independently determined from the relationship between the SSH and SST data. We have also tried to investigate the effects of SSH on the gravity observations in other frequency bands. Among these effects, one of the interesting results is the gravity changes induced by ENSO-like ocean oscillations. Our computations suggest that the oscillations contribute 2–3 μGal in peak-to-peak amplitude to gravity variations in the equatorial Pacific at the maximum.