THE ROLE OF THE INTERFACE IN EXCHANGE THROUGH THE STRAIT OF GIBRALTAR

Five cross-strait hydrographic sections repeated several times during the Gibraltar Experiment in 1985-1986 are used to examine the structur e of the interface layer between the inflowing Atlantic waters and out flowing Mediterranean waters in the Strait of Gibraltar. The interface is 60-100 m thick, with a strong vertical salinity gradient identifie d 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 mini mum in thickness and a maximum in salinity gradient. Farther east, the interface thickens again and continues to get saltier, warmer, and sh allower. Property variations in ah three layers are also cast in terms of the three principal water types involved in the exchange. The trad itional Knudsen model of exchange is extended to three layers, assumin g that the interface is a transport-carrying third layer with uniform vertical shear. As much as half of the inflowing or outflowing transpo rt occurs in the interface layer. Transport converges in both the uppe r and lower layers, implying, over the length of the strait, vertical exchange between layers that is comparable to about half the horizonta l exchange. The richness of structure and complexity of interaction be tween the interface and the upper and lower layers argues against the use of two-layer models to characterize exchange through the Strait of Gibraltar.