FY 2023

Zapiola Gyre, velocities and mixing, new Argo insights

Johnson, G.C., and B.A. King

J. Geophys. Res., 128(6), e2023JC019893, doi: 10.1029/2023JC019893, View article online at AGU/Wiley (external link) (2023)

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.

Plain Language Summary.. Deep Argo floats profile from the sea surface to its floor every 10 days, spending most of their time parked at mid-depth, swept along by ocean currents. Eleven of these floats were deployed in the Argentine Basin, starting in January 2021. Some of them sampled within the Zapiola Gyre, a large counter-clockwise swirling ocean current with volume transport (∼110 million cubic meters per second) comparable to the largest ocean currents, like the Gulf Stream and Antarctic Circumpolar Current. This gyre is hypothesized to owe its large size to flowing freely around a large rise in the ocean seafloor, energized by eddies and damped only by bottom friction. Some of the floats drifted completely around the gyre over the span of about a year, and the full-depth temperature and salinity profiles they collected, together with historical profiles collected from ships, allowed construction of a map of the swirl of relatively warm salty waters from the north and cold fresh waters from the south around the gyre. Analyses of data from these new floats and other sources allow insights into structure and roles of velocity and mixing in transporting heat and salt around this prominent and unusual feature of the ocean circulation.