Exchange flow through the strait of gibraltar as simulated by a s-coordinate hydrostatic model and a z-coordinate nonhydrostatic model

Abstract

The Mediterranean Sea is a semi-enclosed basin displaying an active thermohaline circulation (MTHC) that is sustained by the atmospheric forcing and controlled by the narrow and shallow Strait of Gibraltar (hereinafter SoG). The atmospheric forcing drives the Mediterranean basin toward a negative budget of water and heat. Over the basin, evaporation exceeds the sum of precipitation and river discharge, while a net heat flux is transferred to the overlying atmosphere through the sea surface. These fluxes are balanced by the exchange flow that takes place in Gibraltar. Within the SoG, the MTHC takes the form of a two-way exchange: an upper layer of fresh and relatively warm Atlantic water spreads in the Mediterranean basin, and a lower layer of colder and saltier Mediterranean water sinks as a tongue in the North Atlantic at intermediate depths. The interaction between the intense tidal forcing [Candela et al., 1990] and the complex geometry of the SoG (Figure 3.1a) influences the two-way exchange via hydraulic control [Bryden and Stommel, 1984]. The exchange is subject to vigorous mixing and entrainment [Wesson and Gregg, 1994] as well as intermittent hydraulic controls over the main sills and in its narrowest sections [Sannino et al., 2007; Sannino et al., 2009a]. The simultaneous presence in the SoG of at least two cross sections in which the exchange is controlled drives the strait dynamics toward the so-called maximal regime [Bryden and Stommel, 1984; Armi and Farmer, 1988]. If the exchange is subject to only one hydraulic control, the regime is called submaximal. The two regimes have different implications for property fluxes, response time, and other physical characteristics of the coupled circulation in the SoG and Mediterranean Sea. The maximal regime can be expected to have larger heat, salt, and mass fluxes and to respond more slowly to changes in stratification and thermohaline forcing within the Mediterranean Sea and the North Atlantic Ocean [Sannino et al., 2009a]. As first recognized by Bray et al. [1995], the strong entrainment and mixing present in the Strait of Gibraltar lead to the formation of a thick interfacial layer where density and velocity change gradually in the vertical direction. They also argued that the classical two-layer approach used to describe the two-way exchange was insufficient to account for the flow regime in the SoG. They found that a three-layer system, which includes an active interface layer, best represents the exchange through the SoG. The presence of a thick interfacial layer complicates the estimation of the hydraulic state of the flow exchange using the two-layer hydraulic theory. Such difficulty has been recently overcome by Sannino et al. [2007] who analyzed for the fist time the hydraulic regime of the exchange flow applying a three-layer hydraulic theory. Doing so they considered the thick interfacial layer as an active participant of the hydraulic regime. The hydraulic studies conducted by Sannino et al. [2007] were based on the analysis of numerical simulations. The simulations were carried out using a σ-coordinate model 3