Seasonal variability and geostrophic circulation in the eastern Mediterranean as revealed through a repeated XBT transect

Abstract. The evolution of the upper thermocline on a section across the eastern Mediterranean was recorded bi-weekly through a series of XBT transects from Piraeus, Greece to Alexandria, Egypt, extending from October 1999 to October 2000 on board Voluntary Observing Ships in the framework of the Mediterranean Forecasting System Pilot Project. The data acquired provided valuable information on the seasonal variability of the upper ocean thermal structure at three different regions of the eastern Mediterranean: the Myrtoan, Cretan and Levantine Seas. Furthermore, the horizontal distance (~12 miles) between successive profiles provides enough spatial resolution to analyze mesoscale features, while the temporal distance between successive expeditions (2–4 weeks) allows us to study their evolution. Sub-basin scale features are identified using contemporaneous sea surface temperature satellite images. The cross-transect geostrophic velocity field and corresponding volume fluxes for several sub-basin scale features of the Levantine Sea are estimated by exploiting monthly q / S diagrams from operational runs of the Princeton Ocean Model in use at NCMR. A southwestward transport in the proximity of the southeast tip of Crete was estimated between 1–3 Sv. The transport increases after the winter formation of dense intermediate water in the Cretan Sea strengthens the pressure gradient across the Cretan Straits. The Mersah-Matruh anticyclone was identified as a closed gyre carrying about 2–6 Sv. This feature was stable throughout the stratified period and disappeared from our records in March 2000. Finally, our data reveal the existence of an eastward-flowing coastal current along the North African coast, transporting a minimum of 1–2 Sv.

Key words. Oceanography: physical (eddies and mesoscale processes; currents; marginal and semi-closed seas)