The role of the interface in exchange through the Strait of Gibraltar

Abstract

Five cross‐strait hydrographic sections repeated several times during the Gibraltar Experiment in 1985–1986 are used to examine the structure of the interface layer between the inflowing Atlantic waters and outflowing Mediterranean waters in the Strait of Gibraltar. The interface is 60–100 m thick, with a strong vertical salinity gradient identified by fitting individual salinity profiles to a piecewise‐linear, three‐layer model. The interface is deeper, thicker, fresher, and colder on the west end of the strait than in the Narrows, where there is a minimum in thickness and a maximum in salinity gradient. Farther east, the interface thickens again and continues to get saltier, warmer, and shallower. Property variations in all three layers are also cast in terms of the three principal water types involved in the exchange. The traditional Knudsen model of exchange is extended to three layers, assuming that the interface is a transport‐carrying third layer with uniform vertical shear. As much as half of the inflowing or outflowing transport occurs in the interface layer. Transport converges in both the upper and lower layers, implying, over the length of the strait, vertical exchange between layers that is comparable to about half the horizontal exchange. The richness of structure and complexity of interaction between the interface and the upper and lower layers argues against the use of two‐layer models to characterize exchange through the Strait of Gibraltar.

The N:Si:P molar ratio in the Strait of Gibraltar

“…In order to estimate the shear from the mass conservation equation applied to the two layers system in the Strait, we used the water transport values obtained during the Gibraltar [Bryden et al, 1994;Bray et al, 1995] and CANIGO [Tsimplis and Bryden, 2000] experiments. There are several different water transport estimates in the Strait of Gibraltar in the literature, however we believe that the estimates from the aforementioned experiments are the most accurate and precise because they are based on a large yearlong data set.…”

Long-term monitoring programme of the hydrological variability in the Mediterranean Sea: a first overview of the HYDROCHANGES network

“…2), which have advantages for the monitoring purpose and display some similarities (García-Lafuente et al, 2007). Bray et al (1995) showed that the greatest AW vs. MOWs contrast is at CAS, which points at this sill as the best place for monitoring the twolayer exchange. On the other hand, experimental (Farmer and Armi, 1988;Sánchez-Román et al, 2009) and numerical (Castro et al, 2004;Sannino et al, 2004Sannino et al, , 2009Sánchez-Román et al, 2009) studies suggest that hydraulic control on MOW is a rather permanent feature at ES, which is the last topographic constriction before they sink in the ocean down to ∼ 1000 m (Baringer and Price, 1997;Ambar et al, 2002), while at CAS the hydraulic control is lost almost every tidal cycle.…”

Intensive shipboard observations of the flow through the Strait of Gibraltar

“…The highway-like shipping through the strait makes oceanographic work difficult as well. In spite of this, there have been a number of studies in the past, some using shipboard measurements Farmer and Armi, 1988;Bray et al, 1995] and several using moored equipment [Candela, 1991;Bryden et al, 1994]. The moorings generally only sample at a few points across the strait, while ship observations lack the temporal information on how observed structures may change with time (especially in this tidally dominated regime).…”