Subarctic Bottom Pressure Oscillation observed by GRACE, linked to ENSO

The Gravity Recovery and Climate Experiment (GRACE) satellites have observed strong signals during 2003-04 in the meridional differences in ocean bottom pressure (OBP) in the high-latitude N. Pacific (50N minus 25N) and in the Indian Ocean sector of the Antarctic Circumpolar Current (55S minus 25 S). `Strong' means peak-to-peak amplitudes in the annual variability of 6 mbars, when averaged zonally over about 4,000 km. Reasonableness of the signal is determined by comparison with outputs from a non-Boussinesq model that does not assimilate data, as well as with a version of the ECCO model with altimetric data assimilation. Focussing on the N. Pacific signal, there is a strong hint of interannual trend in both data and model. The non-Boussinesq model is then used to analyze a 50 year long time series of model output. We find that the N. Pacific signal leads the Niño3.4 index by more than 12 months with a correlation of 0.52 in the 99% significant interval. The model also shows a moderate east-west OBP oscillation in the tropical Pacific. Unlike the subarctic Pacific oscillation, the bottom pressure difference between the eastern (Niño3.4 region) and the western tropical Pacific (warm-pool region) is in phase with the Niño3.4 index with a correlation 0.82 and has a characteristics of eastward mass-shift during El Niño events, indicating their direct effect on deep oceans, however, the GRACE satellites cannot `see' this much smaller E-W signal. Lower latitude applications of GRACE data, with signals of ~1-2 mbar, need to wait for expected improvements in GRACE fields after one or more reprocessing cycles. The E-W difference also amplifies the zonal errors in GRACE data, while the N-S differences then to cancel them.