The Creation of Reveresed Baroclinicity and Subsurfaces Jets in Oceanic Eddies

Abstract

Reversals of baroclinicity and associated subsurface jets have often been observed in oceanic eddies, but without adequate explanation. In this study, a two-dimensional adiabatic frontogenesis model is applied and several features of this phenomenon are elucidated. Observations show that reversals of baroclinicity and associated with maxima of jet velocities are found primarily between the seasonal and main thermoclines at depths of about 100 m. This is accounted for in the model's initial conditions by superimposing a shallow seasonal thermocline with small isopycnal slopes over a more strongly baroclinic main thermocline. The initial model domain comprises a 1600-km wide strip which is then compressed by an external deformation field to simulate the formation of fronts by gyre-scale confluence. After integrating over 30 model days, a front has formed and the sign of isopyncal slope has changed in the seasonal thermocline, leading to maximum velocity in the frontal jet near 100-m depth. The reversal of slope is caused by a difference in direction of vertical motion between the cyclonic and anticylonic sides of the jet due to stretching and compression of vortex tubes. The subsurface jet maximum is due simply to a sign change in the thermal wind. These model results show that for realistic conditions, the reversals of baroclinicity and associated subsurface jet maxima in oceanic eddies can already have been produced during frontal formation and retained by the eddies after they become detached by hydrodynamic instabilities.