Baltic Sea Mass Variations from GRACE: Comparison with In Situ and Modelled Sea Level Heights
The monthly variation in the water mass of the semi-enclosed Baltic Sea is about 60 Gt RMS over an area of \(390,000 \textrm{km}^2\) . The Baltic has a dense network of tide gauges (TGs), and several high-resolution regional hydrodynamic models, making it
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Baltic Sea Mass Variations from GRACE: Comparison with In Situ and Modelled Sea Level Heights J. Virtanen, J. Mäkinen, M. Bilker-Koivula, H. Virtanen, M. Nordman, A. Kangas, M. Johansson, C.K. Shum, H. Lee, L. Wang, and M. Thomas
Abstract The monthly variation in the water mass of the semi-enclosed Baltic Sea is about 60 Gt RMS over an area of 390,000km2 . The Baltic has a dense network of tide gauges (TGs), and several high-resolution regional hydrodynamic models, making it one of the best-monitored seas for mass variations of this size in the world. We investigate the performance of different GRACE gravity field solutions to recover this oceanic mass variation using in situ measurements of sea-level heights. For GRACE, we use both the standard monthly solutions as well as regional solutions to estimate the total water storage in the Baltic Sea. For the “ground truth”, we use sea-level measurements in the network of tide gauges around the Baltic Sea. For comparison, we have access to data from two hydrodynamic models. The water mass estimated from the constructed sea surfaces is then compared with different GRACE estimates. At present, we ignore the steric anomalies, as they are small in the Baltic Sea. We also discuss the “leakage” between land hydrology and the Baltic Sea in the GRACE estimates of water storage.
76.1 Introduction The sea-level variations in semi-enclosed Baltic Sea have been monitored in several complementary ways for more than a century (Novotny et al., 2006). The Baltic has a dense network of tide gauges (TGs),
J. Virtanen () Finnish Geodetic Institute, Masala FI-02431, Finland e-mail: [email protected]
and several altimetry missions (like TOPEX/Poseidon, ERS-1/2, ENVISAT) have provided observations over the region. Moreover, several high-resolution oceanographic models have been developed for forecasting purposes as well as for studying different physical and biological phenemena. Covering an area of some 390,000km2 , the Baltic can be considered to be one of the best-monitored seas for mass variations of this size in the world. Now the satellite gravity mission GRACE (Tapley et al., 2004) provides a method to directly measure the total water mass variability in the Baltic. The GRACE mission has provided global solutions for Earth gravity field since mid-2002. The standard data products are monthly spherical harmonics (SH) coefficients that can be used to make global estimates of mass variations. GRACE has been used to study ocean mass variation from global (Chambers et al., 2004) to regional scales, e.g., by Kuo et al. (2008) in the Southern Ocean. Fenoglio-Marc et al. (2006) analyse the mass variation in the Mediterranean Sea, a semi-enclosed basin with an area of 2.5 million km2 , while Swenson and Wahr (2007) carry out a similar study for the smaller Caspian Sea (areal extent of 436,000km2 , comparable to the Baltic). Both use satellite altimetry as independent in situ measurements and find good agreements between the two data products. We investigate the capability of GRACE to r
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