Mean sea level and surface circulation variability of the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetry

Abstract

We describe the circulation and mean sea level variations of the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetric data. It is first shown that the response of the Mediterranean Sea to atmospheric pressure forcing is close to an inverse barometer (except at high frequencies) which means that the adjustment is accompanied by a flow through the Straits of Sicily and Gibraltar. We then use TOPEX/POSEIDON to study the mean sea level variations, representing steric effects and integrated large‐scale changes of the mass of the Mediterranean Sea. We observe an annual cycle with a fast drop during winter. Steric effects account for about half of the observed variations. The remaining signal is believed to be driven by evaporation minus precipitation (E ‐ P) forcing and internal hydraulic control in the Straits of Gibraltar. Using suboptimal space‐time objective analysis, the classic components of the Mediterranean surface circulation are recovered, despite low signal‐to‐noise ratio (the rms of sea level variability is less than 10 cm). The variable Mediterranean circulation is seen as a complex combination of mesoscale and large‐scale variations. The surface circulation is more complex in the eastern basin than in the western basin. In the east it is composed of subbasin‐scale gyres, such as the so‐called Mersa‐Matruh and Shikmona gyres, which do not have an obvious recurrence period. We also observe an intensification of the large‐scale cyclonic winter circulation in the western and in the Ionian basins. Several mesoscale structures, such as the Alboran gyres and the Ierepetra gyre, show a clear seasonal cycle, with a maximum in summer. The good qualitative and quantitative agreement of the results with previous data from the Mediterranean illustrates the improved accurary of TOPEX/POSEIDON over its predecessors.

Tracking a big anticyclonic eddy in the western Mediterranean Sea

“…SLA maps of the Mediterranean Sea are regularly produced by Collecte Localisation Satellites (CLS) in Toulouse, France, from the combination of data of the ERS and TOPEX/Poseidon altimeters, by subtracting a four-year mean value from the actual measurements. Besides the usual corrections, data are low-pass filtered to reduce altimetric noise and SLA maps are built on a regular grid of 0.2 x 0.2 degrees every 10 days using an improved space/time objective analysis method (Larnicol et al, 1995;LeTraon et al, 1998).…”

Comparison between XBT data and TOPEX/Poseidon satellite altimetry in the Ligurian-Tyrrhenian area

“…Pioneering studies (Larnicol et al, 1995;Iudicone et al, 1998;Ayoub et al, 1998) have demonstrated the usefulness of satellite altimetry for monitoring oceanographic conditions in marginal seas like the Mediterranean Sea. However, it is clear that corroborative information and any available in situ data from historical archives are often indispensable to validate satellite-derived results and to give them the correct interpretation on the basis of the regional phenomena.…”

A comparative study of marine zooplankton communities in the Tangier and M’Diq (Gibraltar strait) regions

“…This seasonal cycle of the zooplankton activity along the Moroccan coast is related to the upwelling cycle which develops in spring, is reinforced in summer, attenuates in autumn and decreases in winter (Furnestin, 1959;Thiriot, 1978;Belvèze, 1991;Orbi & Nemmaoui, 1992 ). Thus, the zooplankton activity in M'Diq can be related to the seasonal cycle of the Mediterranean water mass which follows change of flow entering and outgoing through the Gibraltar Strait (Larnicol et al, 1995;Traon et al, 2007). Moreover, Fernandez de Puelles et al, (2003) showed a seasonal cycle related to an oscillation of the zooplankton abundance with maximum values recorded during the first six months of the year.…”