The Vertical Structure of a Loop Current Eddy

Abstract

The vertical structure of a recently detached Loop Current Eddy (LCE) is studied using in situ data collected with an underwater glider from August to November 2016. Altimetry and Argo data are analyzed to discuss the context of the eddy shedding and evolution as well as the origin and transformation of its thermohaline properties. The LCE appeared as a large body of nearly homogeneous water between 50 and 250 m confined between the seasonal and main thermoclines. A temperature anomaly relative to surrounding Gulf's water of up to 9.7 ∘ C was observed within the eddy core. The salinity structure had a double core pattern. The subsurface fresh core had a negative anomaly of 0.27 practical salinity unit, while the deeper saline core's positive anomaly reached 1.22 practical salinity unit. Both temperature and salinity maxima were stronger than previously reported. The saline core, of Caribbean origin, was well conserved during its journey from the Yucatan Basin to the Loop Current and at least 7 months after eddy detachment. The fresher homogeneous core resulted from surface diabatic transformations including surface heat fluxes and mixing within the top 200 m during the winter preceding eddy detachment. Heat and salt excess carried by the LCE were large and require important negative heat fluxes and positive fresh water input to be balanced. The geostrophic velocity structure had the form of a subsurface intensified vortex ring.

Picoplankton Carbon Biomass Assessments and Distribution of Prochlorococcus Ecotypes Linked to Loop Current Eddies During Summer in the Southern Gulf of Mexico

“…This suggests that spatial scales, westward subsurface advection of cells within anticyclonic structures (LCEs), and a preference to be located close to the nutricline are the key mechanisms for LL‐PRO ecotypes distribution in the GoM. Westward advection has been suggested by the observations of Meunier et al (), who found the core of North Atlantic Subtropical Underwater mostly unperturbed after several months of drifting. Locally, inside the LCE, eddy‐induced pycnocline displacements regulate the depth of the nutricline, defined here by the depth of [NN] = 0.5 μmol/kg (Cianca et al, ).…”

“…They drifted westward following approximately central (XIXIMI‐4 and XIXIMI‐6) and southern (XIXIMI‐5) paths (as described in Vukovich, ), before colliding with the continental slope or interacting with other mesoscale structures, as has been described for these anticyclonic rings (Hamilton et al, ; Sturges & Leben, ; Vidal et al, ). Given that detached eddy orbital speeds are much larger than translation ones (e.g., Meunier et al, ), LCEs are nonlinear structures capable of trapping fluid and planktonic ecosystems inside them (McGillicuddy, ). Thus, a solid body swirl along LCE trajectories suggests that LL‐PRO ecotypes are kept trapped within these anticyclonic structures (with continuing growth) while they travel to the west.…”

“…This suggests that spatial scales, westward subsurface advection of cells within anticyclonic structures (LCEs), and a preference to be located close to the nutricline are the key mechanisms for LL-PRO ecotypes distribution in the GoM. Westward advection has been suggested by the observations of Meunier et al (2018), who found the core of North Atlantic Subtropical Underwater mostly unperturbed after several months of (Cianca et al, 2007). The nutricline is deeper in the northern region (Figures 7g-7i), at the same stations where relatively high deep PRO biomass is found (Figures 7d-7f), suggesting the uptake of new nitrogen by these deep subgroups.…”

Heat Content Anomaly and Decay of Warm‐Core Rings: the Case of the Gulf of Mexico

“…This criterion also ensures that the edge contour corresponds to the zone where thermohaline properties have the sharpest gradients, assuring the capture of most of the thermohaline anomalies. This is illustrated in Figure 1g (adapted from Meunier et al, 2018, Figure 6b), which shows a vertical section of temperature anomaly through a Loop Current Ring. The core of anomalously warm water is confined between the velocity maxima (thick black vertical lines).…”

Intrathermocline Eddies Embedded Within an Anticyclonic Vortex Ring

“…They drift westward through the GoM (Glenn & Ebbesmeyer, ; Johnson et al, ), carrying warm and salty subtropical underwater (SUW; (Hernández‐Guerra & Joyce, ; Wüst, )). LCE Poseidon (Meunier et al, ), in which the ITEs were observed, was a large (300 km in diameter) structure with particularly large temperature and salinity anomalies (up to 9.7° and 1.22 psu) resulting in an almost homogeneous core of warm and salty water surrounded above and below by well stratified thermocline water. The maximum velocity ranged from 0.8 to 1.5 m/s in subsurface at the eddy's periphery, at about 150 km from its center.…”