More Like this

1. Lagrangian coherence and source of water of Loop Current Frontal Eddies in the Gulf of Mexico. Progress in Oceanography.

   Hiron, L. (August 2022, Progress in Oceanography)

   Elsevier Publishing Company (Ed.)

   Loop Current Frontal Eddies (LCFEs) are known to intensify and assist in the Loop Current (LC) eddy shedding. In addition to interacting with the LC, these eddies also modify the circulation in the eastern Gulf of Mexico by attracting water and passive tracers such as chlorophyll, Mississippi freshwater, and pollutants to the LC-LCFE front. During the 2010 Deepwater Horizon oil spill, part of the oil was entrained not only in the LC-LCFE front but also inside an LCFE, where it remained for weeks. This study assesses the ability of the LCFEs to transport water and passive tracers without exchange with the exterior (i.e., Lagrangian coherence) using altimetry and a high-resolution model. The following open questions are answered: (1) How long can the LCFEs remain Lagrangian coherent at and below the surface? (2) What is the source of water for the formation of LCFEs? (3) Can the formation of Lagrangian coherent LCFEs attract shelf water? Strong frontal eddies leading to LC eddy shedding are investigated using a 1-km resolution model for the Gulf of Mexico and altimetry. The results show that LCFEs are composed of waters originating from the outer band of the LC front, the region north of the LC, and the western West Florida Shelf and Mississippi/Alabama/Florida shelf, and potentially drive cross-shelf exchange of particles, water properties, and nutrients. At depth (≈180 m), most LCFE water comes from the outer band of the LC front in the form of smaller frontal eddies. Once formed, LCFEs can transport water and passive tracers in their interior without exchange with the exterior for weeks: these eddies remained Lagrangian coherent for up to 25 days in the altimetry dataset and 18 days at the surface and 29 days at depth (≈180 m) in the simulation. LCFE can remain Lagrangian coherent up to a depth of ≈ 560 m. Additional analyses show that the LCFE involved in the Deepwater Horizon oil spill formed from water near the oil rig location, in agreement with previous studies. Temperature-salinity diagrams from a high-resolution model and aircraft expendable profilers show that LCFEs are composed of Gulf of Mexico water as opposed to LC water. Therefore, LCFE formation and propagation actively modify the surrounding circulation and affect the evolution of the flow and the transport of oil and other passive tracers in the Eastern Gulf of Mexico. 

   more » « less
2. Wind- and Wave-driven Ocean Surface Boundary Layer in a Frontal Zone: Roles of Submesoscale Eddies and Ekman-Stokes Transport

   https://doi.org/10.1175/JPO-D-20-0270.1  

   Yuan, Jianguo; Liang, Jun-Hong (June 2021, Journal of Physical Oceanography)

   null (Ed.)

   Abstract Large-eddy simulations are used to investigate the influence of a horizontal frontal zone, represented by a stationary uniform background horizontal temperature gradient, on the wind- and wave-driven ocean surface boundary layers. In a frontal zone, the temperature structure, the ageostrophic mean horizontal current, and the turbulence in the ocean surface boundary layer all change with the relative angle among the wind and the front. The net heating and cooling of the boundary layer could be explained by the depth-integrated horizontal advective buoyancy flux, called the Ekman Buoyancy Flux (or the Ekman-Stokes Buoyancy Flux if wave effects are included). However, the detailed temperature profiles are also modulated by the depth-dependent advective buoyancy flux and submesoscale eddies. The surface current is deflected less (more) to the right of the wind and wave when the depth-integrated advective buoyancy flux cools (warms) the ocean surface boundary layer. Horizontal mixing is greatly enhanced by submesoscale eddies. The eddy-induced horizontal mixing is anisotropic and is stronger to the right of the wind direction. Vertical turbulent mixing depends on the superposition of the geostrophic and ageostrophic current, the depth-dependent advective buoyancy flux, and submesoscale eddies. 

   more » « less
3. Observational Evidence of Ventilation Hotspots in the Southern Ocean

   https://doi.org/10.1029/2021JC017178  

   Dove, Lilian A.; Thompson, Andrew F.; Balwada, Dhruv; Gray, Alison R. (July 2021, Journal of Geophysical Research: Oceans)

   Abstract Standing meanders are a key component of the Antarctic Circumpolar Current (ACC) circulation system, and numerical studies have shown that these features may locally enhance subduction, upwelling, as well as lateral and vertical tracer transport. Yet, observational data from these regions remain sparse. Here, we present results based on measurements made by a group of autonomous platforms sampling an ACC standing meander formed due to the interaction of the Polar Front with the Southwest Indian Ridge. Two Seagliders were deployed alongside a Biogeochemical‐Argo float that was advected through the standing meander. In the high eddy kinetic energy region of the standing meander, the glider observations reveal enhanced submesoscale frontal gradients as well as heightened tracer variability at depth, as compared to the more quiescent region further downstream. Vertical gradients in spice and apparent oxygen utilization are reduced in the standing meander despite similarities in the large‐scale vertical stratification, suggesting greater ventilation of the surface ocean. These observations are consistent with numerical studies that highlight standing meanders as hotspots for ventilation and subduction due to enhanced mesoscale stirring and submesoscale vertical velocities. Our results emphasize the need to account for spatial heterogeneity in processes influencing air‐sea exchange, carbon export, and biogeochemical cycling in the Southern Ocean. 

   more » « less
4. Observations of Upwelling and Downwelling Around Antarctica Mediated by Sea Ice

   https://doi.org/10.3389/fmars.2022.864808  

   Ramadhan, Ali; Marshall, John; Meneghello, Gianluca; Illari, Lodovica; Speer, Kevin (June 2022, Frontiers in Marine Science)

   We infer circumpolar maps of stress imparted to the ocean by the wind, mediated by sea-ice, in and around the Seasonal Ice Zone (SIZ) of Antarctica. In the open ocean we compute the wind stress using surface winds from daily atmospheric reanalyses and applying bulk formulae. In the presence of sea ice, the stress imparted to the underlying ocean is computed from satellite observations of daily ice concentration and drift velocity assuming, first, that the ocean geostrophic currents beneath are negligible, and then including surface geostrophic ocean currents inferred from satellite altimetry. In this way maps of surface ocean stress in the SIZ are obtained. The maps are discussed and interpreted, and their importance in setting the circulation emphasised. Just as in parallel observational studies in the Arctic, we find that ocean currents significantly modify the stress field, the sense of the surface ageostrophic flow and thus pathways of exchange across the SIZ. Maps of Ekman pumping reveal broad patterns of upwelling within the SIZ enhanced near the sea ice edge, which are offset by strong narrow downwelling regions adjacent to the Antarctic continent. 

   more » « less
5. Satellite-Derived Lagrangian Transport Pathways in the Labrador Sea

   https://doi.org/10.3390/rs15235545  

   Castelao, Renato M; Oliver, Hilde; Medeiros, Patricia M (December 2023, Remote Sensing)

   The offshore transport of Greenland coastal waters influenced by freshwater input from ice sheet melting during summer plays an important role in ocean circulation and biological processes in the Labrador Sea. Many previous studies over the last decade have investigated shelfbreak transport processes in the region, primarily using ocean model simulations. Here, we use 27 years of surface geostrophic velocity observations from satellite altimetry, modified to include Ekman dynamics based on atmospheric reanalysis, and virtual particle releases to investigate seasonal and interannual variability in transport of coastal water in the Labrador Sea. Two sets of tracking experiments were pursued, one using geostrophic velocities only, and another using total velocities including the wind effect. Our analysis revealed substantial seasonal variability, even when only geostrophic velocities were considered. Water from coastal southwest Greenland is generally transported northward into Baffin Bay, although westward transport off the west Greenland shelf increases in fall and winter due to winds. Westward offshore transport is increased for water from southeast Greenland so that, in some years, water originating near the east Greenland coast during summer can be transported into the central Labrador Sea and the convection region. When wind forcing is considered, long-term trends suggest decreasing transport of Greenland coastal water during the melting season toward Baffin Bay, and increasing transport into the interior of the Labrador Sea for water originating from southeast Greenland during summer, where it could potentially influence water column stability. Future studies using higher-resolution velocity observations are needed to capture the role of submesoscale variability in transport pathways in the Labrador Sea. 

   more » « less