Time-variable gravity signal of an anomalous redistribution of water mass in the extratropic Pacific during 1998-2002

Cox and Chao [2002] reported the detection of a large anomaly in the time series of Earth's dynamic oblateness J2, the lowest‐degree gravity spatial harmonic, in the form of a positive jump since 1998 overshadowing the decreasing secular trend in J2 caused primarily by the postglacial rebound (PGR). Here we report that recent data show that J2 has been rapidly returning toward “normal” (with PGR considered) since early 2001. In search of the geophysical and climatic causes for this “1998–2002 J2 anomaly,” we report an oceanographic event that took place in the extratropic north and south Pacific basins that was found to match remarkably well with the time evolution of the anomaly. We examine the leading (nonseasonal, extratropic Pacific) Empirical Orthogonal Function/Principal Component modes in the sea‐surface height (SSH) data from TOPEX/Poseidon, sea surface temperature (SST) data from the National Center for Environmental Predictions, and output fields of the Estimating the Circulation and the Climate of the Ocean (ECCO) ocean general circulation model (OGCM), including ocean bottom pressure (OBP) and temperature and salinity profiles. The phenomenon appears to be part of the Pacific Decadal Oscillation, and temporal correlations are made. However, quantitatively, the OBP field of the ECCO model predicts a J2 anomaly that is smaller in magnitude than the observed by a factor of about 3. We discuss various possibilities for reconciling this discrepancy in terms of inadequacies of present OGCMs and considering other geophysical contributions; a complete resolution of the J2 enigma awaits further studies.