Low-degree, non-seasonal time-variable gravity and oceanic contributions

We investigate the non-seasonal, low-degree time-variable gravity field induced by the oceanic mass circulation variations. We compute the ocean bottom pressure field (OBP) using the classical outputs (i.e. temperature and salinity at several depths and the sea surface height deviations) from two ocean general circulation models -- CLIO (Coupled Large-scale Ice Ocean model, 1997-2001 which does not assimilate observational data) and ECCO (Estimating the Circulation and Climate of the Ocean, 1993-2000, which assimilates sea surface height and temperature and monthly Levitus climatology) and validate them with comparison to several pressure recorders. We examine the non-seasonal OBP variations in terms of the empirical orthogonal function (EOF) decomposition in various ocean basins. We especially put in evidence in the two models a quick OBP change occurring in 1998 in the southern Pacific Ocean of about 10 millibars. The results show that the ocean mass redistribution explains most of the non-seasonal low-degree (i.e. large-scale) gravity field variations after removal of atmospheric and post-glacial rebound (PGR) contributions, in particular with respect to the observed J2 anomaly that happened during 1998 as reported by Cox and Chao (2002).