Tidal dissipation and stratification in the Gulf of California

Abstract

The effect of tidal mixing upon the distribution of stratification and sea surface temperature (SST) in the Gulf of California is studied by direct observation and by numerical modeling. Data from conductivity‐temperature‐depth surveys made in September and December of 1986 are used to describe the distribution of stratification and SST. A vertically integrated finite differences numerical model was used to obtain the geographical distribution of available turbulent kinetic energy (TKE) from the M2 tide. The following three areas were found to be strongly affected by tidal mixing: the shallow coastal area at the head of the gulf, the shelf south of Tiburón island, and the area of the midgulf archipelago. The first two are vertically well mixed and are therefore similar to other shelf sea frontal areas. Dissipation occurs all around the islands of the archipelago, and the sills are foci of the most intense tidal dissipation. Stratification in this area was reduced with respect to the background, but complete vertical mixing does not occur. Therefore the strong SST fronts that occur in this area do not mark the boundary between well‐mixed and stratified water. The Simpson‐Hujiter criterion (Simpson and Hunter, 1974) for stratification and frontal position produced 2.75 ≤ λcrit ≤ 3.0 for the frontal position in the shallow northern gulf. This value of λcrit is larger than in other shelf sea fronts, which may be due to horizontal advection, which is not included in the criterion. We found that the effect of advection may be of the same order of magnitude as that of buoyancy flux. In the area over the sills, λ > λcriti the available TKE is not enough to produce complete mixing over the ∼ 400 m of water over the sills.