Ageostrophic secondary circulation in a subtropical intrathermocline eddy

Abstract

Vertical motions play a key role in the enhancement of primary production within mesoscale eddies through the introduction of nutrients into the euphotic layer. However, the details of the vertical velocity field w driving these enhancements remain under discussion. For the first time the mesoscale w associated with an intrathermocline eddy is computed and analyzed using in situ high-resolution three-dimensional (3D) fields of density and horizontal velocity by resolving a generalized omega equation valid for high Rossby numbers. In the seasonal pycnocline the diagnosed w reveals a multipolar structure with upwelling and downwelling cells located at the eddy periphery. In the main pycnocline w is characterized by a dipolar structure with downwelling velocities upstream of the propagation path and upwelling velocities downstream. Maximum values of w reach 6.4 m day(-1). An observed enhancement of chlorophyll-a at the eddy periphery coincides with the location of the upwelling and downwelling cells. Analysis of the forcing terms of the generalized omega equation indicates that the mechanisms behind the dipolar structure of the w field are a combination of horizontal deformation and advection of vertical relative vorticity by ageostrophic vertical shear. The wind during the eddy sampling was rather constant and uniform with a speed of 5 m s(-1). Diagnosed nonlinear Ekman pumping leads to a dipolar pattern that mirrors the inferred w. Horizontal ageostrophic secondary circulation is dominated by centripetal acceleration and closes the dipole w structure. Vertical fluxes act to maintain the intrathermocline eddy structure.