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Circulation of the Mediterranean Sea and its Variability
Katrin Schroedera, Jesus Garcìa-Lafuenteb, Simon Joseyc, Vincenzo Artaled, Bruno Buongiorno-Nardellie, Adriana Carrillod, Miroslav Gacicf, Gian Pietro Gasparinia, Marine Herrmanng, Piero Lionelloh, Wolfgang Ludwigi, Claude Millotj, Emin Ozsoyk, Giovanna Pisacaned, Jose Carlos Sanchez-Garridob, Gianmaria Sanninod, Rosalia Santolerie, Samuel Somotg, Mariavittoria Strugliad, Emil Stanevl, Isabelle Taupier-Letagej, Mikis Tsimplisc, Manuel Vargas-Yanezm, Vassilis Zervakisn, George Zodiatiso
The Mediterranean Sea occupies an elongated area of about 2.5 million km2 between Europe and Africa, and has only a restricted communication with the world ocean, through the narrow and shallow Strait of Gibraltar. It is further subdivided into two main basins, the Eastern Mediterranean (EMED) and the Western Mediterranean (WMED), communicating through the Sicily Channel. Due to its relatively small size, its geographical location, and its semi-land locked nature, the Mediterranean Sea is very sensitive and responds quickly to atmospheric forcings and/or anthropogenic influences. Demographic growth, climate change and overexploitation are exerting exceptional pressure on the Mediterranean environment, its ecosystems, services and resources. Further it is a region where major oceanic processes occur, though on smaller scales than those occurring in the world ocean, such as deep water formation (DWF) that contributes to sustain a basin-wide thermohaline circulation cell, a reduced version of the large scale oceanic conveyor belt.
In addition to the obvious length-scale, the Mediterranean thermohaline circulation (MTHC) also differs from that of the world ocean in that it is an open cell: the MTHC starts in the Strait of Gibraltar with an inflow of Atlantic Water (AW) composed by Atlantic Surface Water (ASW) and North Atlantic Central Water (NACW), ends at the same site with an undercurrent of intermediate and deep Mediterranean Waters (MWs) and is driven by the net buoyancy flux toward the atmosphere that takes place on average in the Mediterranean Sea. The flux is mainly due to the freshwater deficit of the sea and transforms continuously the raw material of AW into the final product of MWs, which are 0.2% denser due basically to an increase of salinity. The volume of inflowing AW exceeds the one of outflowing MWs (both in the order of 0.8 Sv, Baschek et al., 2001; Sánchez-Román et al., 2009) by the needed amount to balance the freshwater deficit.
The upper branch of the MTHC that carries the relatively fresh AW towards the interior of the sea extends over the WMED and EMED and displays a rather complex surface circulation that could be considered as a superposition of interacting large-scale and mesoscale patterns, each of them showing their own variability (circulation schemes are given in Fig. 4.1). A debate on the surface circulation in the EMED is still open (as described more in detail in paragraphs 2.1. and 2.2). Thus for the AW path, the two schemes are shown in Fig. 4.1a and 4.1b. The lower branch of the MTHC is affected by the sea topography differently. While Levantine Intermediate water (LIW), the most important intermediate water, resides in the EMED at depths from which it can flow without major topographic constrictions through the Sicily Channel into the WMED, depicting a rather continuous return flow, the deep MWs circulation cells are separated by the topography of the channel and driven by specific DWF processes in the Adriatic/Aegean (for EMED) and the Provençal (for WMED) subbasins. Even when intermediate and deep circulation forming the lower branch of the MTHC are partially coupled to each other, they also have their own scales of variability that do not coincide necessarily. Sections 2.1 and 2.2 deal with the time variability of large scale and mesoscale circulation, respectively, issues that are obviously interconnected since the differentiation large-scale mesoscale is somewhat artificial. On the other hand, the expected quick response of a water body of reduced dimensions as the Mediterranean sea to the forcing variability through its surface has propitiated a considerable amount of literature about changes in MWs properties inside the sea and in the rates of DWF (Béthoux et al., 2002; Rixen et al., 2005, López-Jurado et al., 2005; Schroeder et al., 2006, 2008b; Smith et al., 2008) that are revised and discussed. Recently a new possibility that changes in MWs properties are originated by changes in the properties of the inflowing AW (Millot, 2007) has opened new perspectives to the analysis of the observed variability of MWs. All these issues are addressed in Sections 2.3 and 2.4.
The Mediterranean Sea is of major interest for air-sea interaction research because it provides the opportunity to study heat budget closure (Bunker et al., 1982; Garrett et al., 1993; Gilman and Garrett, 1994), the impacts of large scale modes of atmospheric variability and the influence of extreme heat loss on DWF (Josey, 2003; Herrmann and Somot, 2008). The availability of estimates of the heat transport at the Strait of Gibraltar (MacDonald et al., 1994), and the semi-enclosed nature of the basin, place constraints on the Mediterranean Sea heat and freshwater budgets and allow it to be used as a test bed for evaluation of the accuracy of air-sea flux datasets for this region. These budgets are analyzed in detail in Section 3.1, which is completed with a comprehensive description of continental freshwater inputs (river runoff) in Section 3.2, both within Section 3 that deals with the forcing of the Mediterranean Sea. The straits play a fundamental role in this forcing, particularly the Gibraltar strait whose topography is a key point in the functioning of the Mediterranean, since it limits the exchanged flows by establishing hydraulic controls that contribute to determine the final properties of the MWs (Bryden and Kinder, 1991; García Lafuente and Criado Aldeanueva, 2001). Another archetypical example of hydraulically controlled exchange is provided by the long and shallow system of Turkish straits whose influence in the nearby Aegean subbasin is obvious as the inflow of Black Sea Water (BSW) entering the Mediterranean through the Dardanelles strait forms a thin layer of relatively fresh and buoyant water that hinders DWF processes (Plakhin, 1971, 1972; Zodiatis, 1994). The DWF in the North Aegean would require the erosion of this layer, a fact that led Zervakis et al. (2000) to propose a mechanism connecting the rate of the Dardanelles outflow to the formation of North Aegean Deep Water (NAeDW) and, hence, to the triggering of the Eastern Mediterranean Transient (EMT, i.e. a shift of the formation site of deep waters, from the Adriatic to the Aegean, an event that has been extensively described in the first Medclivar book, Lionello et al., 2006; see also Sections 2.1, 2.3, 2.4, 3.5). The Sicily Channel differs from the former ones in the sense that internal hydraulics is not important; its importance stems from its connecting the WMED and the EMED, which makes it a very suitable place to monitor signals of the MTHC return flow from the latter to the former. Recent findings about the exchange through all these straits are presented in Sections 3.3, 3.4 and 3.5, respectively. Finally, a discussion about whether the Mediterranean is in a steady state is provided in Section 4, while some outlooks and future research priorities are proposed in the concluding Section 5.
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