Reconstruction of transports through the Strait of Gibraltar from limited observations
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Observing the water transports through the Strait of Gibraltar is a difficult task. Here we present a methodology aimed to obtain the inflow, outflow and net transport of water from the limited set of available observations, currently consisting of an upward looking ADCP deployed at Espartel sill, two tide gauges located at each side of the Strait and radars monitoring the surface velocities. More precisely, we reconstruct the velocity field over a vertical section across the Strait using a reduced order optimal interpolation technique fed with the spatial covariance patterns deduced from high resolution numerical simulations. As a first step we carry out some sensitivity experiments with synthetic data that demonstrate the high potential of the approach. The reconstruction methodology can reproduce very satisfactorily the variability of the transports with estimated correlations for the inflow, outflow and net over 0.9 in all the cases and estimated RMS errors of 0.03, 0.08 and 0.05 Sv, respectively. However, we have also found that the reconstruction is sensible to bias problems, mostly due to the sensitivity of the method to the differences between the statistics of the actual and modeled velocity profiles. The sensitivity experiments have been used to tune the parameters of the method and a reconstruction of actual monthly transports has been performed for the period 2004–2010 along with an estimate of the associated uncertainty. This reconstruction provides for the first time a multiannual time series of the inflow and the net transports solely based on in situ observations. Therefore it can be used as an independent estimate for the validation of numerical models and surface freshwater fluxes in the Mediterranean.
KeywordsGibraltar Strait Mediterranean Sea Transports Reconstruction
G. Jordà acknowledges a Ramón y Cajal contract (RYC-2013-14714) funded by the Spanish Ministry of Economy and the Regional Government of the Balearic Islands; he also acknowledges a post-doctoral Grant Funded by the Regional Government of the Balearic Islands and the European Social Fund. A. Sánchez-Román acknowledges a Juan de la Cierva contract (JCI-2011-10196) funded by the Spanish Ministry of Economy and Competitiveness. The authors thank J. Polcher for the river runoff datasets, the Permanent Service for Mean Sea Level (www.psmsl.org) for making available the tide gauge data and J. García-Lafuente for the ADCP data of the Espartel monitoring station, which were collected in the frame of the Spanish Funded projects INGRES (REN2003-01608/MAR), INGRES2 (CTM2006-02326/MAR), INGRES3 (CTM2010-21229-C02-01/MAR) and CTM2009-05810-E/MAR. We also thank Diego Fernandez and the European Space Agency for partially funding this study under the project « ESA WACMOS Mediterranean » (No. 4000114770/15/I-SBo). The reconstructed transports and the indirect estimate of the net transport presented here are available at http://marine-climate.uib.es.
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