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

Abstract

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)

The surface circulation in the eastern basin of the Mediterranean Sea

“…From at least October the images display the continuous signature of the general circulation alongslope. The XBT sections in Figure 3b,c of Zervakis et al (2002) crossed both T+S and I98/9.…”

“…Consistency between IR images of Algerian eddies and drifting buoy trajectories was also established by Salas et al (2002) and Font et al (2004). We also compared all XBT transects collected in 1999-2000 during the Mediterranean Forecasting System Pilot Project (MFSPP) to IR images, both in the western basin (Fuda et al, 2000) and the eastern one: all mesoscale anticyclonic features that were crossed have a clear in situ signature, at least down to 400-500 m (Manzella et al, 2001;Zervakis et al, 2002;Fusco et al, 2003;Hamad, 2003). Analysing anticyclonic eddies propagating north off Crete and tracked by their IR signatures has also been backed by moored ADCP time series (Cardin and Hamad, 2003).…”

“…"Mersa-Matruh" is also described as a complex system composed of several mesoscale eddies (Özsoy et al, 1989;Horton et al, 1994;Ayoub, 1997) or as a meander of the "MMJ" (Larnicol et al, 2002). Zervakis et al (2002) consider that "Mersa-Matruh" is a stable anticyclone that progressively disappears along a specific transect, probably having disintegrated into smaller scales, and suggest that it is more a closed eddy (transport >4 Sv) than a meander of the "NAC". Horton et al (1994) and Ayoub (1997) mention that the most convenient period to observe an intense "Mersa-Matruh" would be during the weakening / absence of Ierapetra while the analysis of the POEM-1991 data (Malanotte-Rizzoli et al, 1999) shows the simultaneous presence of a strong Ierapetra and a strong "Mersa-Matruh".…”

“…14g), the warm water is distributed over both eddies and separating them becomes harder in October ( Fig. 14h-j; called MM in the XBT section in Figure 3a of Zervakis et al, 2002 section), Q has decreased, and U or a new coastal eddy grows and propagates downstream. From at least October the images display the continuous signature of the general circulation alongslope.…”

Dynamical Evolution of Intense Ierapetra Eddies on a 22 Year Long Period

“…However, the technological development of remote sensing measurements had opened a new window for the detection of surface eddies from 1980s. The satellite sea surface temperature (SST) images first (Matteoda & Glenn, 1996;Millot, 1985;Puillat et al, 2002;Taupier-Letage et al, 2003;Zervakis et al, 2003) and later the sea surface height (SSH) maps (Amitai et al, 2010;Isern-Fontanet et al, 2006;Mkhinini et al, 2014;Pascual et al, 2007;Rio et al, 2007) provide an efficient way to identify the long-lived eddies and follow their trajectories. The analysis of Lagrangian drifters were also used to quantify the intensity of the surface eddies, for instance the typical angular velocities within the eddy core (Gerin et al, 2009;Matteoda & Glenn, 1996;Menna et al, 2012;Mkhinini et al, 2014;Sutyrin et al, 2009).…”