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.