GRACE mass variations and ocean bottom pressure fluctuations in the South Atlantic

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Böning, C. , Timmermann, R. , Macrander, A. and Schröter, J. (2007): GRACE mass variations and ocean bottom pressure fluctuations in the South Atlantic , XXIV IUGG General Assembly, 02--13 July 2007, Perugia, Italy. .
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The GRACE satellite mission provides time-variable gravity field solutions on a monthly time scale. In contrast to earlier systems, the accuracy of these data is expected to be good enough to derive ocean mass variations from the GRACE gravity anomalies. Since ocean bottom pressure (OBP) fluctuations are an excellent proxy for ocean mass variations an analysis of OBP anomalies in the South Atlantic has been performed. For this purpose we utilize a global ocean circulation model and compare the simulated monthly mean OBP anomalies with in situ data measured by bottom pressure recorders (e.g. PIES) and fields derived from GRACE data. The ocean model is a 1.5° global version of the 3D Finite Element Sea Ice--Ocean Model (FESOM) which has been developed at the Alfred Wegener Institute for Polar and Marine Research (AWI). In situ data from two AWI PIES in the South Atlantic are available for the period 2002-2005. First results indicate a strong correlation between FESOM and GRACE OBP anomalies on a global and regional scale. On smaller, synoptic scales the correlation weakens. Due to spurious elongated meridional patterns in the monthly gravity field anomalies, gravity fluctuations on scales less than 1000 km are difficult to detect. Since the in situ measurements are point measurements by nature, it is essential to identify the area for which their time series are representative. A cross-correlation analysis of FESOM data indicates that regions of spatially coherent patterns of OBP are separated by major features of bottom topography.Averaging GRACE data over these patterns improves agreement with the in situ data. Simulated and observed pressure gradient time series between the two PIES positions are compared to each other, to the simulated volume transports across the line between them, and to total ACC transport variations.

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