Mesoscale eddies in the South Atlantic Bight and the Gulf Stream Recirculation region: Vertical structure

Abstract

Sea level anomalies from altimeters are combined with decade-long potential temperature and salinity profiles from Argo floats to investigate the vertical structure of mesoscale eddies in the South Atlantic Bight (SAB) and the Gulf Stream Recirculation region. Eddy detection and eddy tracking algorithms are applied to the satellite observations, and hydrography profiles from floats that surfaced inside eddies are used to construct three-dimensional composites of cyclones and anticyclones. Eddies are characterized by large temperature and salinity anomalies at 500-1000 m depth and near the surface, and by small anomalies at 200-400 m below the surface at the depth of the North Atlantic Subtropical Mode Water. Anomalies associated with anticyclones are generally larger and found deeper in the water column compared to those due to the presence of cyclones. Geostrophic velocities around eddies generally exceed their translation speed in the top 1000 m of the water column. As such, these eddies can trap water in their interior as they propagate westward. Combining the volume of water inside eddies above their trapping depths with the number of eddies that propagate into the SAB each year, it is estimated that cyclones and anticyclones transport 3.5 6 0.9 Sv and 4.1 6 1.7 Sv onshore toward the Gulf Stream, respectively. The total volume transport of 7.6 6 2.2 Sv represents an important fraction of previous estimates of the onshore transport in the Gulf Stream Recirculation gyre. Since eddies are characterized by large temperature and salinity anomalies, they also contribute significantly to the onshore transport of heat and salt.

Benchmarking the mesoscale variability in global ocean eddy-permitting numerical systems

“…In South Atlantic Bight, the direct analysis of Argo profiles from Castelao (2014), suggest a "trapping depth" that can extend down to 1000 m. Their analysis does not take into account Argo profiles that extends less than 1000 m. Despite the difference in the number of eddies considered, the result is in fair agreement with the value extracted from our reanalysis dataset, where all the 3D eddies, lasting more than 4 weeks, are analyzed. As we approach the equator, the eddy thickness reduces in both CTRL and reanalysis, although the poor statistics in the region prevents us from trying any further analysis on the effective eddy extension.…”

“…Argo profiles have been recently combined with altimetric data to improve vertical and horizontal analysis (Chaigneau et al 2011) or heat transport (Souza et al 2011b). In Castelao (2014), gridded SLA data are firstly analysed through an SSH surface eddydetection system (Chelton et al 2011). Then eddies are combined with Argo floats to investigate their vertical structure in the South Atlantic Bight .…”

Main eddy vertical structures observed in the four major Eastern Boundary Upwelling Systems

“…As in Castelao [2014], these local mean profiles were obtained by averaging all the available h and S Argo casts acquired within a radius of 200 km and separated by less than 6 30 days from the month date of the considered profile, but including profiles from any year. Similarly, a local mean dynamic height (DH) is computed and removed from the DH of the considered profile, to obtain a dynamic height anomaly (DH 0 ) profile associated with the eddy.…”

“…Although CEs and AEs can be described by their mean vertical structure [Chaigneau et al, 2011;Kurian et al, 2011;Yang et al, 2013;Castelao, 2014], different eddy types can coexist in each region (e.g., surface or subsurface-intensified, multicores eddies, etc) and thus influence the shape of the mean vertical structure. In order to determine the main eddy-types observed in each EBUS, we applied the following procedure.…”

“…Given the opportunity to investigate the subsurface physical characteristics of the eddies from Argo float data [Chaigneau et al, 2011;Liu et al, 2012;Yang et al, 2013;Castelao, 2014], we analyze, classify and compare the eddy vertical structures in the 4 EBUS from altimetry maps and Argo profiles. In particular, as the 4 EBUS share similar large-scale dynamics, we aim to examine whether the mean eddy vertical thermohaline structures are also similar in the 4 EBUS.…”