Elsevier

Journal of Geodynamics

Volumes 59–60, September 2012, Pages 168-182
Journal of Geodynamics

Water mass variation in the Mediterranean and Black Seas

https://doi.org/10.1016/j.jog.2012.04.001Get rights and content

Abstract

The mass-induced sea level variability and the net mass transport between Mediterranean Sea and Black Sea are derived for the interval between August 2002 and July 2008 from satellite-based observations and from model data. We construct in each basin two time series representing the basin mean mass signal in terms of equivalent water height. The first series is obtained from steric-corrected altimetry while the other is deduced from GRACE data corrected for the contamination by continental hydrology. The series show a good agreement in terms of annual and inter-annual signals, which is in line with earlier works, although different model corrections influence the consistency in terms of seasonal signal and trend.

In the Mediterranean Sea, we obtain the best agreement using a steric correction from the regional oceanographic model MFSTEP and a continental hydrological leakage correction derived from the global continental hydrological model WaterGAP2. The inter-annual time series show a correlation of 0.85 and a root mean square (RMS) difference of 15 mm. The two estimates have similar accuracy and their annual amplitude and phase agree within 3 mm and 23 days respectively. The GRACE-derived mass-induced sea level variability yields an annual amplitude of 27 ± 5 mm peaking in December and a trend of 5.3 ± 1.9 mm/yr, which deviates within 3 mm/yr from the altimetry-derived estimate.

In the Black Sea, the series are less consistent, with lower accuracy of the GRACE-derived estimate, but still show a promising agreement considering the smaller size of the basin. The best agreement is realized choosing the corrections from WaterGAP2 and from the regional oceanographic model NEMO. The inter-annual time series have a correlation and RMS differences of 0.68 and 55 mm, their annual amplitude and phase agree within 4 mm and 6 days respectively. The GRACE-derived seawater mass signal has an annual amplitude of 32 ± 4 mm peaking in April. On inter-annual time scales, the mass-induced sea level variability is stronger than in the Mediterranean Sea, with an increase from 2003 to 2005 followed by a decrease from 2006 to 2008.

Based on mass conservation, the mass-induced sea level variations, river runoff and precipitation minus evaporation are combined to derive the strait flows between the basins and with the Atlantic Ocean. At the Gibraltar strait, the net inflow varies annually with an amplitude of 52 ± 10 × 10−3 Sv peaking end of September (1 Sv = 106 m3 s−1). The inflow through the Bosphorus strait displays an annual amplitude of 13 ± 3 ×10−3 Sv peaking in the middle of March. Additionally, an increase of the Gibraltar net inflow (3.4 ± 0.8 × 10−3 Sv/yr) is detected.

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 S˙mass, 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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