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The controversial path of Atlantic Water in the Eastern Mediterranean

March 4, 2014

The Atlantic Water (AW) path suggested by two decades of thermal satellite data observations in the Eastern Mediterranean is similar to the path followed by the AW in the Western Mediterranean but is in contrast with the scheme proposed after the Physical Oceanography of the Eastern Mediterranean (POEM) experiment during the 1990s. Evidence of the AW path along the Libyo-Egyptian coast (Libyo-Egyptian Current; LEC) and around anti-cyclonic eddies of variable size and position (Libyo-Egyptian Eddies; LEEs) inferred from satellite data is reported in XBT transects, high resolution models and drifter trajectories. In the POEM scheme, the AW was carried eastwards by the Mid-Mediterranean Jet (MMJ) flowing in the middle of the Levantine basin. The MMJ is still noted in recent drifter trajectory analyses, and seasonal surveys and glider missions report that the AW is transferred towards the Levantine basin by the MMJ in subsurface layers.

The issue is investigated in this study by considering whether the determination of the AW path and the resolution of the surface circulation are, unlike in the Western Mediterranean, two distinct problems. Historical hydrographic datasets and numerical experiments illustrated in this study demonstrate that satellite observations in the Eastern Mediterranean reveal the path of the surface water of Atlantic origin, which is modified by heating and evaporation into the saltier and warmer Levantine Surface Water (LSW), but do not reveal the path of the submerged AW that maintains the distinctive character of fresh AW in the Levantine basin. Differences between the surface and the submerged circulation in the Mersa-Matruh area explain the reason for the discrepancy.

In summer the AW departs from the Sicily Channel at the sea surface and crosses the Ionian Sea in a surface layer 30 m deep. At the entrance to the Levantine basin, part of the AW recirculates westwards in two separate branches, one directed to the Southern Ionian and the other towards the Central Ionian, causing meanders in the incoming AW. In the Levantine basin, before the Cretan Passage, the AW is close to the Libyan coast with surface layers already modified into LSW and a subsurface fresh AW core centered at 50 m depth. Surface and subsurface flows diverge within the Cretan Passage as they approach the Mersa-Matruh area. According to satellite observations, the LSW flows along the Libyo-Egyptian coast (LEC) and around anti-cyclonic LEEs of variable size and position. In the subsurface layers, the blend and fragmented anti-cyclonic circulation found at the surface is augmented by the AW itself, which becomes saltier and negatively buoyant, and veers offshore joining the MMJ. In summer, the jet assumes the aspect of a submerged injection of fresh AW into the Levantine basin below a depth of 60-70 m. From the Ionian eastwards, the depth of the fresh core of AW sinks by tens of meters from the beginning to the end of summer. In the Sicily Channel, the AW in winter is saltier than in other periods of the year due to evaporation and mixing occurring between the fresh AW and the saltier Ionian Surface Water intruding into the southern side of the Channel in summer.

Ciappa AC, in press: The controversial path of Atlantic Water in the Eastern Mediterranean, Progress in Oceanography, doi: 10.1016/j.pocean.2014.02.001. Article.


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