Julio Candela

5.7.1 Marginal seas

Marginal seas bordering the world's oceans make important contributions to the global thermohaline ocean circulation through their exchange of water with the major oceanic basins. Their principal effect is related to processes of water mass formation that occur within these bordering seas where heat and/or net freshwater losses are intensified due to their confined nature. It is well known that some of the intermediate and all of the deep water masses of the world's oceans are produced as the result of intense air-sea exchange in marginal seas (Warren, 1981a). The water masses formed in the marginal sea typically enter the open ocean as a dense outflow through a restricted channel or strait. One of the best known outflows is that from the Mediterranean Sea, which enters the North Atlantic through the Strait of Gibraltar as the lower layer of a two-way exchange flow (Bryden et al., 1994; Bower et al., 1997). After rounding Cape St Vincent, at the southwestern corner of the Iberian Peninsula, the outflow water begins to spread into the eastern North Atlantic to form a warm and salty tongue that extends westward from Portugal across the eastern Atlantic (Wüst and Defant, 1936; Worthington, 1976; Lozier et al., 1995). This tongue is one of the most prominent features of the North Atlantic hydrography at intermediate depths, and its high-salinity water has been implicated in the preconditioning of the North Atlantic Deep Water formation (Reid, 1978).

This chapter deals with the Mediterranean Sea and its influence on the world's ocean thermohaline circulation and therefore on climate. It describes water formation processes within the Mediterranean Sea, outlining the characteristics of well-known water formation sites and the evidence of recent shifts in water formation locations within the sea. Then it focuses on the characteristics of the exchange flows between the Mediterranean and the North Atlantic Ocean through the Strait of Gibraltar based on historical as well as recent (1994-96) flow measurements that reveal significant seasonal and interannual variability of the exchange flows, and in particular of the Mediterranean outflow into the North Atlantic. This outflow is then the subject of a detailed description since it determines the way the Mediterranean Water mixes with the surrounding waters in the North Atlantic, creating a warm and saline tongue of water that can be identified throughout the whole North Atlantic at a depth of about 1100m (Fig. 5.7.1). It is fundamental for numerical models investigating the influence of the Mediterranean Water on the thermohaline circulation of the World Ocean to reproduce or parameterize the mixing processes in the outflow accurately, since they determine the properties and the depth of penetration of the Mediterranean Water in the North Atlantic. The chapter ends with a discussion of observational hydrographic evidence, mainly put forth in Reid's (1994) study, which supports the hypothesis that the Mediterranean contribution to the Nordic and Labrador Seas allows the formation of deep waters in the North Atlantic, or at least that without this contribution the waters formed would not be dense enough to penetrate to the depths they presently do.

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