Combining ocean and earth models, we show that there is a region in the central Pacific ocean where ocean bottom pressure is a direct measure of interannual changes in ocean
mass, with a noise level for annual means below 3 mm water equivalent, and a trend error below 1 mm/yr. We demonstrate this concept using existing ocean bottom pressure
measurements from the region, from which we extract the annual cycle of ocean mass (amplitude 8.5 mm, peaking in late September), which is in agreement with previous
determinations based on complex combinations of global data sets. This method sidesteps a number of limitations in satellite gravity-based calculations, but its direct implementation is currently limited by the precision of pressure sensors, which suffer from significant drift. Development of a low-drift method to measure ocean bottom pressure at a few sites could provide an important geodetic constraint on the earth system.
Citation: Hughes, C. W., M. E. Tamisiea, R. J. Bingham, and J. Williams (2012), Weighing the ocean: Using a single mooring to measure changes in the mass of the ocean, Geophys. Res. Lett., 39, L17602, doi:10.1029/2012GL052935.
The GRACE satellite gravity mission has revolutionized our ability to monitor regional mass redistribution in the earth system, and hence monitor changes in ocean mass and the source of those changes. However, GRACE does not monitor the degree 1 terms in mass movement, associated with geocenter motion, and is weak for the C2,0 harmonic [Chen et al., 2006; Swenson et al., 2008; Leuliette and Miller, 2009]. It also suffers from limited spatial resolution, making it hard to distinguish the much larger land signals from ocean signals near the ocean boundaries [Chambers et al., 2007], and secular trends include a contribution from glacial isostatic adjustment (GIA), the solid earth’s ongoing response to the change in load since the last glaciations [Tamisiea, 2011]. Together, these difficulties lead to an uncertainty approaching 1 mm/yr in the measured mass component of global sea level trend.
If sea level changes were spatially uniform, then variation of the volume of the ocean could be monitored using a single tide gauge. Similarly, spatially uniform changes in ocean bottom pressure (OBP) would mean ocean mass changes could be monitored with a single OBP Recorder. However, spatial variations mean that sea level measurements must be made over the entire ocean (by satellite altimetry), or statistical extrapolation must be used to mitigate the sampling problems of tide gauge data [Hughes and Williams, 2010; Church and White, 2006; Jevrejeva et al., 2006]. Fortunately, as we will show, OBP observations in one specific region do allow us to measure ocean mass changes with a single station.
h/t to Dr. Leif Svalgaard who has the full paper here