February 1, 2007: Bottom Pressure Measurements and Indications of Change in the Arctic Ocean
Jamie Morison, Applied Physics Laboratory-University of Washington
In the late 1980s and through the 1990s we saw major shifts in the Arctic Ocean. The influence of Atlantic Water in the Arctic Ocean became more widespread and intense and the pattern of water circulation and ice drift shifted, resulting in a more cyclonic circulation. These changes became manifest in the central Arctic near the North Pole as increases in upper ocean salinity and Atlantic Water temperature. They occurred in concert with a decrease in surface atmospheric pressure. With the aim of helping to track such changes, we have undertaken in situ ocean bottom pressure measurements and the analysis of Gravity Recovery and Climate Experiment (GRACE) data. For the in situ measurements we have developed, with the help of the NOAA tsunami early warning system group, an Arctic Bottom Pressure Recorder (ABPR), which is suitable for deployment through pack ice. The ABPRs are equipped with acoustic modems to allow annual data recovery while leaving the instruments undisturbed on the bottom for up to 3 years. Recovery of the first year of data from gauges installed near the North Pole was achieved in April 2006. The comparison between GRACE- derived bottom pressure at the North Pole and the ABPR data is quite good. The GRACE data are filtered with a 400 km radius Gaussian filter, so their footprint easily covers both ABPR locations. At this scale, the two ABPR records are highly correlated. Both GRACE and the ABPRs show a declining bottom pressure trend in 2005-2006. The complete GRACE record indicates this has been going on since the start of the GRACE record and amounts to about a 10 cm decrease in bottom pressure from 2002 to 2006. We believe this trend is largely associated with a steric change due to a drop in upper ocean salinity near in the central Arctic Ocean. The change in hydrography near the Pole has been tracked for the last 6 years by the North Pole Environmental Observatory (NPEO). The NPEO record shows a reduction in upper ocean salinity to near the pre-1990s climatology. This is arguably related to ocean circulation changes associated with the decline in the Arctic Oscillation (AO) index. We have averaged all the NPEO hydrographic casts taken each year since 2000 within 400 km of the Pole, and computed the average bottom pressure change associated with the average density changes in each year. The agreement with the GRACE and ABPR trends is good through 2005. This suggests sea level changes are not contributing greatly to the bottom pressure change at the Pole, a conclusion reinforced by preliminary analysis of sea surface height trends estimated from ICESat data. We have also investigated the effect on bottom pressure of a hypothetical return to pre-1990s hydrography over a larger area of the Arctic Ocean. We find reasonable agreement with the spatial distribution of bottom pressure trends from GRACE, especially a decrease in bottom pressure in the Makarov Basin associated presumably with the return of less saline, Pacific-derived upper ocean water to that region. Associated hypothetical trends in sea surface height (SSH) are generally smaller and in an opposite sense from the bottom pressure trends, and their spatial pattern is roughly consistent with an observed shift to a more anticyclonic ice drift.