Methods
The DYNED-Atlas data base focuses on two areas of interest: the Mediterranean Sea and the Arabian Sea. We have plotted below the mean absolute dynamic topography (MADT) averaged over the 2000-2017 period in the Mediterranean Sea or 2000-2015 period for the Arabian Sea.
We first get, the geostrophic velocity fields, produced by Salto/Duacs, distributed by AVISO and derived from absolute dynamical topography (ADT). This “all-sat-merged” series combines the up-to-date datasets with up to four satellites at a given time, using all missions available (Topex/poseidon, ERS-2, GFO, Jason-1, Envisat, Jason-2, Cryosat-2, Altika et HY2A). Unlike the global altimetric product gridded at 1/4°, a specific product gridded at 1/8° is available for the Mediterranean Sea. A similar ADT product for the Arabian Sea was processed by CLS on a 1/8° Mercator grid, with time intervals of 24 h, for the 2000-2015 period.
An iterative method is then used on the surface geostrophic velocities in order to compute the cyclogeostrophic velocity components. The addition of these ageostrophic terms leads to a significant velocity increase for some mesoscale anticyclones. An exemple of the initial geostrophic velocities (black arrows) and the final cyclogeostrophic velocities (red arrows) is shown below for the intense Alboran anticyclone, located at the east of the Gibraltar strait the 7th of October 2000. For this case and few others, the cyclogeostrophic velocity correction could be significant.
We applied to the daily surface cyclogeostrophic velocity field, the Angular Momentum Eddy Detection Algorithm (AMEDA) in order to locate and track the surface signature of the mesoscale eddies (LeVu et al. 2017) during the 2000-2015 period. The eddy centers are first identified as an extremum of the local normalized angular momentum. All the closed streamlines surrounding this center are then computed to identify the characteristic eddy contour (solid contour/blue) which corresponds to the closed streamline of maximal speed. After this maxima, the azimuthal speed of the eddy decreases until the last closed contour (dashed contour/black) is reached. An exemple of these dynamical and geometrical eddy characteristics is shown in the figure below for an Ierapetra anticyclone (figure from Ioannou et al. 2017).
In order to estimate the three-dimensional structures of the detected eddies we use the Argo profile data-base available since 2000 in the Mediterranean and the Arabian Seas. Once all the detected eddies are identified during the 2000-2015 period, we can separate the Argo profiles in two groups: the ones that are located inside an eddy (i.e. inside the last closed streamline) and the one which are outside of all the detected eddies. With the second group we can build an unperturbed climatological T,S profiles around a given position and a given date. We consider here all the Argo profiles (out of eddies) located at less than 150km around the selected position and at +/- 30 days from the target day during the 15 years. Such climatological profiles (plotted in black) give a reference for the T,S profiles associated to an unperturbed ocean (i.e. without coherent eddies). Hence, the comparison of these climatological profiles with the Argo profiles taken inside the eddies allow us to quantify the hydrological anomalies associated to each individual eddy.
The upper panel of the above figure shows- the positions of all the Argo floats co-located inside a long-lived anticyclone (728 days) south to Cyprus. The position of the Argo profile measured the 23 of October is indicated with a large circle while the positions of the other profiles located inside the anticyclone from 2006 to 2009 are plotted with small circles. The central panels show all the T, S, density profiles below the mixed layer (z<100m). The mean climatological profiles taken outside of all eddies are plotted with a solid black line (the gray area indicates their dispersion). The comparison with the profiles measured inside this specific eddy (magenta lines) provide quantitative estimates of the T,S, density anomalies induced by this anticyclone (lower panels).