Potential Vorticity Structure in the North Atlantic Western Boundary Current from Underwater Glider Observations

Abstract

Potential vorticity structure in two segments of the North Atlantic's western boundary current is examined using concurrent, high-resolution measurements of hydrography and velocity from gliders. Spray gliders occupied 40 transects across the Loop Current in the Gulf of Mexico and 11 transects across the Gulf Stream downstream of Cape Hatteras. Cross-stream distributions of the Ertel potential vorticity and its components are calculated for each transect under the assumptions that all flow is in the direction of measured vertically averaged currents and that the flow is geostrophic. Mean cross-stream distributions of hydrographic properties, potential vorticity, and alongstream velocity are calculated for both the Loop Current and the detached Gulf Stream in both depth and density coordinates. Differences between these mean transects highlight the downstream changes in western boundary current structure. As the current increases its transport downstream, upper-layer potential vorticity is generally reduced because of the combined effects of increased anticyclonic relative vorticity, reduced stratification, and increased cross-stream density gradients. The only exception is within the 20-km-wide cyclonic flank of the Gulf Stream, where intense cyclonic relative vorticity results in more positive potential vorticity than in the Loop Current. Cross-stream gradients of mean potential vorticity satisfy necessary conditions for both barotropic and baroclinic instability within the western boundary current. Instances of very low or negative potential vorticity, which predispose the flow to various overturning instabilities, are observed in individual transects across both the Loop Current and the Gulf Stream.

Mechanisms for enhanced turbulence in the Drake Passage region of the Southern Ocean

“…Building on experiments in the Kuroshio (D'Asaro et al (2011)) and the Gulf Stream (Thomas et al (2013), Todd et al (2016)), two boundary currents with many similar properties to the ACC, the rich submesoscale frontal field provides a link between mesoscale and dissipation scale motions, and can cascade energy to smaller scales. These O(1-10km) fronts provide an energy source for enhanced turbulent dissipation in the upper ocean when released by downfront winds.…”

Absolute Velocity Estimates from Autonomous Underwater Gliders Equipped with Doppler Current Profilers

“…These newer instruments are now replacing the 750-kHz Sontek acoustic Doppler profilers (ADPs) that have been flown on Spray gliders for more than a decade (Davis 2010;Todd et al 2011b). The AD2CP is a broadband instrument with four transducers mounted in a Janus configuration with two beams oriented 258 from vertical toward port and starboard (Fig.…”

“…4a) from 14 July to 30 October 2015 as part of the ongoing California Underwater Glider Network (Davis et al 2008;Todd et al 2011bTodd et al ,a, 2012Ohman et al 2013;Johnston and Rudnick 2015;Zaba and Rudnick 2016;Rudnick et al 2016). The glider completed four cross-shore transects from the coast to approximately 525 km offshore, sampling from the surface to 500 m approximately every 3 h. The glider had little growth on it upon recovery.…”

“…Estimates of absolute currents averaged (spatially and temporally) between surfacings of a glider are obtained by comparing the predicted displacement using a model of glider flight and measurements of pressure, pitch, and heading to actual displacement measured by GPS; the resulting velocity estimates, which we refer to as vertically averaged currents given the near-constant vertical speed of a glider during a dive (Rudnick and Cole 2011), are typically accurate to within 0.01 m s 21 (Eriksen et al 2001;Todd et al 2011b). Estimates of vertical shear are obtained from either thermal wind calculations using measured temperature and salinity profiles or using small high-frequency acoustic Doppler current profilers (ADCPs; Davis 2010;Todd et al 2011b). These shear profiles are referenced to vertically averaged current estimates to produce absolute velocity profiles.…”

“…Geostrophic shear estimates, on the other hand, are limited to the cross-track component of the velocity and must be smoothed horizontally and temporally to remove the effects of unbalanced high-frequency motions (e.g., internal waves; Todd et al 2011b) that are aliased with horizontal variability due to the relatively slow movement of gliders (Rudnick and Cole 2011). However, velocity profiling with ADCPs requires sufficient acoustic scatterers throughout the sampled portion of the water column.…”