Thermohaline intrusions created isopycnically at oceanic fronts are inclined to isopycnals

Abstract

Thermohaline intrusions have often been observed extending several kilometres from ocean fronts¹. They have a central role in models of mixing^2,3; however, their origin has been the subject of controversy. Laboratory studies led to the idea that intrusions might be formed by double diffusion^4–6, in which case they would slope across isopycnals (surface of constant density). That signature has been identified in ocean fine-structure sections⁷. Observations of the three-dimensional structure of meandering fronts led to a different conjecture: that intrusions might be formed isopycnically by the ageostrophic circulation within unstable meanders on the frontal jet^1,8. In that conceptual model, double diffusion does not initiate intrusions, but may extend and dissipate those created by frontal circulation⁹. Here we present first the observed structure of such an intrusion and then the results of a dynamical model simulating the isopycnic generation mechanism at an unstable meandering front. We show that thermohaline intrusions formed isopycnically also slope across density surfaces, so there is no need to invoke double diffusion to explain such structures.