Zapiola Gyre, velocities and mixing, new Argo insights

The Zapiola Gyre is a large, full-depth, bottom-intensified, anticyclonic recirculation in the Argentine Basin. It rotates around the Zapiola Drift, a sedimentary rise standing a few hundred meters above the abyssal plain, tall enough to create closed contours of planetary potential vorticity. Hence the gyre has been posited to be an eddy-driven free mode that is damped primarily by the bottom Ekman layer. It describes approximately a zonally elongated ellipse with a zonal semi-major axis of ∼440 km and a meridional semi-minor axis of ∼125 km. Its volume transport is estimated here at ∼110 (±25) × 106 m3 s−1. It has peak depth-averaged meridional velocities of ∼0.06 and ∼0.08 m s−1, and peak depth-averaged zonal velocities of ∼0.11 and ∼0.12 m s−1. Peak surface velocities are ∼57% of peak bottom velocities, consistent with the dynamics of a bottom-intensified Taylor column in a stratified flow. Deep and Core Argo float trajectories follow the gyre, with a couple of Core Argo floats with 1000-dbar parking depths executing more than three anticyclonic rotations around it, and Core Argo floats that approach within ∼75 km of the gyre center executing on average ∼1.7 circumnavigations. Deep Argo temperature-salinity profiles combined with historical shipboard CTD profile data afford maps newly illuminating the advective swirling of water-mass signatures around the gyre at the density of the North Atlantic Deep Water salinity maximum. Their patterns are consistent with a Peclet number of ∼30 estimated here using previously published lateral eddy diffusivities in the region.