Water mass variation in the Mediterranean and Black Seas
Introduction
Being an almost closed sub-system, the Mediterranean-Black Sea region provides an interesting setting to study regional mass transports and redistribution.
The two semi-enclosed seas and the Atlantic Ocean are connected by the Bosphorus Strait and by the Gibraltar Strait respectively. Considering its size, the Mediterranean Sea is five times larger than the Black Sea (2.5 × 1012 m2 versus 0.42 × 1012 m2). The Mediterranean Sea can be classified as a lagoon-type basin, whereas the Black Sea is an estuarine-type basin. Although the region is densely populated, many components of the water cycle are still poorly quantified. For example, for the river-runoff R and the strait flows of Gibraltar (FG) and Bosphorus (FB) only climatological estimates are available (Mariotti et al., 2002, Grayek et al., 2010).
Starting from 2002, the Gravity Recovery and Climate Experiment (GRACE) mission has been providing observations of water mass change, by measuring small variations of the Earth's gravity field that predominantly originate from mass redistributions in the Earth's system (Tapley et al., 2004). Generally, in order to cope with increasing noise and artefacts present in the high resolution components of the GRACE models, the GRACE models are smoothed by convolution with a kernel of gradually decreasing power. Isotropic (Wahr et al., 1998) and non-isotropic (Han et al., 2005) smoothing as well as empirical de-correlation (Swenson and Wahr, 2006) and regularization (Kusche, 2007, Kusche et al., 2009) have been applied. The smoothing procedure reduces the correlated noise at the cost of signal attenuation and a decreased spatial resolution.
For basin averages, these side effects depend on (1) the type of the signal, (2) the smoothing applied and (3) the dimension and shape of the region (Klees et al., 2007, Kusche, 2007). In regional studies on small ocean basins, filtering causes significant leakage of terrestrial hydrology in the oceanic mass estimated from GRACE, as the land signal is typically much larger. Retrieval of GRACE derived ocean mass variations in small ocean basins presents additional difficulties, as the dimension of the regions are small compared to the resolution of filtered GRACE estimates (Chambers, 2006). Furthermore, the oceanic background models used for de-aliasing the measurements have marginal performance in semi-enclosed basins, which increases the noise in the estimated GRACE residuals (Flechtner, 2007a, Flechtner, 2007b).
Alternatively, steric-corrected altimetry also observes water mass changes. The satellite radar altimetry provides total (steric plus non-steric) sea level heights with an accuracy close to 3 cm (Beckley et al., 2007). In order to correct the total sea level for its steric component, this last is derived from either observed or modeled temperature and salinity. Several studies have compared steric corrected altimetry with estimates from GRACE at both global (Chambers, 2006, Leuliette and Miller, 2009, Willis et al., 2008) and regional scales (Swenson and Wahr, 2007, Fenoglio-Marc et al., 2006, Fenoglio-Marc, 2007, Garcia et al., 2006, Garcia et al., 2010, Calafat et al., 2010) and have shown that the two methods yield mass change estimates which are consistent at both seasonal and inter-annual time-scales. This paper is an extension of our previous analysis in the Mediterranean Sea (Fenoglio-Marc et al., 2006, Fenoglio-Marc, 2007) to a larger region including the Black Sea.
The main objectives of this paper are to assess in semi-enclosed basins the ability of GRACE to recover (1) the seawater mass variations at both seasonal and inter-annual time-scales and (2) the total water budget and its various components. The effect of filtering the basin averages as well as the magnitude and consistency of the corrections applied are investigated.
Section snippets
GRACE gravimetry
We use global GRACE gravity field monthly (GSM) solutions provided by the GeoForschungsZentrum (GFZ) (level-2 products, release 4) between August 2002 and July 2008, which contain atmosphere- and ocean-corrected gravity field solutions expressed in Stokes coefficients from degree 2 to degree 120. Since we consider the complete oceanographic signal, we restore the background models, subtracted at an earlier stage during the GRACE processing. We restore here the signal over the ocean areas using
Error estimation in terms of the Basin averages
Estimated errors of the monthly values and annual amplitudes are tabulated in Table 7 for various measured and inferred quantities: total sea level (Stot), steric sea level (Sster), hydrological leakage (Shyd), mass-induced sea level (Smass) and its rate of change , river runoff (R), E − P and strait flows in terms of uniform basin changes.
The errors are based on either error propagation of the various components which flow into the estimated quantity, or they are based on the RMS
Conclusions and discussion
We have investigated the mass-induced sea level Smass in the Mediterranean and Black Sea basins over an interval of six years, from August 2002 to July 2008, at both seasonal and inter-annual time scales. In addition to Fenoglio-Marc et al. (2006) and Fenoglio-Marc (2007), we have studied the closure of the water budget in both the Mediterranean and Black Sea deriving the strait flows at Gibraltar and through the Bosphorus from the water budget.
The comparison required a variety of auxiliary
Acknowledgements
The authors acknowledge G. Spada for the GIA corrections, A. Güntner for the WaterGAP2 data, B. Barnier and R. Dussin for support on DFS4 and ERA-Interim airflux data, M. Rixen and A. Shaw for helpful discussions on the MEDAR/Medatlas data. Comments by W. Bosch and by two anonymous reviewers helped to improve the manuscript. This study has been performed within the STREMP project funded by the Deutsche Forschungsgemeinschaft (SPP1257, FE-534/3-2).
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