STABILITY OF A FRONT SEPARATING WATER MASSES WITH DIFFERENT STRATIFICATIONS

The linear instability of a narrow current flowing eastward between tw o semi-infinite water masses of different mean stratification is studi ed with a multi-mode quasi-geostrophic model. The mean stratification in each domain has an exponential profile. The current is symmetric ab out the vertical plane separating the two water masses and its vertica l profile is determined by the difference between the mean stratificat ion of the two domains, as imposed by the thermal-wind equation. The c urrent so defined is unstable over the entire parameter range we studi ed. As the ratio between the asymptotic depth of the thermocline in th e two domains tends to unity, the current weakens and becomes eventual ly stable, while the length of the fastest-growing wave increases towa rd a finite upper limit. Increasing the depth or gradient of the therm ocline in both domains by the same ratio also stabilizes the jet. Decr easing the width of the jet destabilizes it, but the length of the mos t unstable wave is almost independent of current width. The stability results are applied to the Gulf Stream after it has separated from Cap e Hatteras, using a realistic stratification. The most unstable wave h as a length of 200 km, which is comparable to twice the diameter of wa rm-core rings. The maximum amplitude of the unstable waves in the cros s-stream direction is shifted towards the northern domain. We interpre t this shift as inducing formation of warm-core eddies largely to the north of the current axis, while cold-core eddies capture water origin ally lying almost as much north of the axis as south of it. Anticyclon ic eddies in the model would thus tend to exhibit a weaker temperature contrast with the surrounding water mass than the cyclonic ones, as o bserved in the Gulf-Stream system. The effect of adding a barotropic c omponent to the basic jet depends on the sign and magnitude of this co mponent. For a small westward component or large components of either sign the jet is less stable, while an intermediate range of east- or w estward barotropic components reduces the instability. Both the Gulf S tream and Kuroshio are in this latter range, near their respective sep aration points, so their meandering is mainly induced by baroclinic in stability.