Frontal subduction of a cool surface water mass in the Gulf of Tehuantepec, Mexico

Abstract

The southward wind events in the Gulf of Tehuantepec generate large warm eddies and strong offshore current jets that produce entrainment of subsurface waters into the upper ocean resulting in cool (dense) water masses. Strong frontal features occur at the boundary between warm eddies and cool patches. An important fraction of the cool water is subducted beneath these eddies as intrapycnocline ‘lenses’ (eddies) within the boundary of a larger eddy. These small eddies are stable and interact with the larger one. Observational evidence of two such lenses is presented. Qualitative arguments based on observed flow and density fields, as well as vorticity arguments, confirm the existence of subduction processes. The magnitude of the subduction rate is estimated as large as 80 m d-1. A revised conceptual scheme of the eddy generating process is summarized as: a) during the event the wind-induced offshore current entrains subsurface water in the central gulf, thus establishing the initial density gradient; b) also during the event, the horizontal density gradient is maintained by horizontal warm water advection; c) after the event a coastal jet separates from the west coast reaches the central gulf and spins up to form an anticyclone; d) subduction due to intense convergence occurs where the warm coastal jet meets the cool water; e) the ‘mature’ warm eddy propagates offshore carrying a lens of cool subducted water within its boundary.