Reconstruction of transports through the Strait of Gibraltar from limited observations

• Published in: Climate dynamics 2017-02, Vol.48 (3-4), p.851-865
• Main Authors: Jordà, G., Sánchez-Román, A., Gomis, D.
• Format: Article
• Language: English
• Subjects: Analysis; Climatology; Earth and Environmental Science; Earth Sciences; Gauges; Geophysics/Geodesy; Mathematical models; Methods; Numerical analysis; Observations; Oceanography; Radar systems; Simulation; Surface velocity; Surface water; Water circulation; Water transportation
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-016-3113-8

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.