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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. 

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2. Frontal Subduction of the Mid-Atlantic Bight Shelf Water at the Onshore Edge of a Warm-Core Ring

   https://doi.org/10.1029/2018JC013794  

   Zhang, Weifeng G.; Partida, Jacob (October 2018, Journal of Geophysical Research: Oceans)

   This work studies the subduction of the shelf water along the onshore edge of a warm-core ring (WCR) that impinges on the edge of the Mid-Atlantic Bight continental shelf. The dynamical analysis is based on observations by satellites and from the Ocean Observatories Initiative Pioneer Array observatory as well as idealized numerical model simulations. They together show that frontogenesis-induced submesoscale frontal subduction with order-one Rossby and Froude numbers occurs on the onshore edge of the ring. The subduction flow results from the onshore migration of the WCR that intensifies the density front on the interface of the ring and shelf waters. The subduction is a part of the cross-front secondary circulation trying to relax the intensifying density front. The dramatically different physical and biogeochemical properties of the ring and shelf waters provide a great opportunity to visualize the subduction phenomenon. Entrained by the ring-edge current, the subducted shelf water is subsequently transported offshore below a surface layer of ring water and alongside of the surface-visible shelf-water streamer. It explains the historical observations of isolated subsurface packets of shelf water along the ring periphery in the slope sea. Model-based estimate suggests that this type of subduction-associated subsurface cross-shelfbreak transport of the shelf water could be substantial relative to other major forms of shelfbreak water exchange. This study also proposes that outward spreading of the ring-edge front by the frontal subduction may facilitate entrainment of the shelf water by the ring-edge current and enhances the shelf-water streamer transport at the shelf edge. 

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3. (2014) Coastal Pioneer New England Shelf Mobile Assets (CP05MOAS)

   https://doi.org/10.58046/ooi-cp05moas  

   NSF_Ocean_Observatories_Initiative (January 2014, US NSF Ocean Observatories Initiative)

   Two kinds of Mobile Assets survey the area in and around the array of moorings at the Coastal Pioneer Array – Coastal Gliders and Coastal Autonomous Underwater Vehicles (AUVs).\n\nAn array of 6 Coastal Gliders (Teledyne-Webb Slocum Gliders) sample large, mesoscale features through a broad region (130 x 185 km) of the outer continental shelf between the shelf break and the Gulf Stream. The role of these gliders in monitoring this broader area is to resolve rings, eddies and meanders from the Gulf Stream as they impinge on the shelf break front. These Teledyne-Webb Slocum Gliders fly through the water column along saw-tooth paths, penetrating the sea surface and diving down to a maximum depth of 1000 meters.\n\nAn array of two Coastal AUVs (REMUS-600 AUVs) travel along transects across the shelf-break frontal system extending beyond the mooring array, covering an area approximately 80 x 100 km in size centered on the array of moorings. The primary role of the AUVs is to resolve cross- and along-front “eddy fluxes” due to frontal instabilities, wind forcing, and mesoscale variability. These AUVs travel along saw-toothed transects, penetrating the sea surface and diving down to a maximum depth of 600 meters. 

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4. Bloom compression alongside marine heatwaves contemporary with the Oregon upwelling season

   https://doi.org/10.1002/lno.12757  

   Black, Ian T; Kavanaugh, Maria T; Reimers, Clare E (December 2024, Limnology and Oceanography)

   Abstract Marine heatwave (MHW) events have led to acute decreases in primary production and phytoplankton biomass in the surface ocean, particularly at the mid latitudes. In the Northeast Pacific, these anomalous events have occasionally encroached onto the Oregon shelf during the ecologically important summer upwelling season. Increased temperatures reduce the density of offshore waters, and as a MHW is present offshore, coincident downwelling or relaxation may transport warmer waters inshore. As an event persists, new upwelling‐driven blooms may be prevented from extending further offshore. This work focuses on MHWs and coincident events that occurred off Oregon during the summers of 2015–2023. In late summer 2015 and 2019, both documented MHW years, coastal phytoplankton biomass extended on average 6 and 9 km offshore of the shelf break along the Newport Hydrographic Line, respectively. During years not influenced by anomalous warming, coastal biomass extended over 34 km offshore of the shelf break. Reduced biomass also occurs with reduced upwelling transport and nutrient flux during these anomalous warm periods. However, the enhanced front associated with a MHW aids in the compression of phytoplankton closer to shore. Over shorter events, heatwaves propagating far inshore also coincide with reduced chlorophyllaand sea‐surface density at select cross‐shelf locations, further supporting a physical displacement mechanism. Paired with the physiological impacts on communities, heatwave‐reinforced physical confinement of blooms over the inner‐shelf may have a measurable effect on the gravitational flux and alongshore transport of particulate organic carbon. 

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5. A Gulf Stream frontal eddy harbors a distinct microbiome compared to adjacent waters

   https://doi.org/10.1371/journal.pone.0293334  

   Gronniger, Jessica L; Gray, Patrick C; Niebergall, Alexandria K; Johnson, Zackary I; Hunt, Dana E (November 2023, PLOS ONE)

   Casotti, Raffaella (Ed.)

   Mesoscale oceanographic features, including eddies, have the potential to alter productivity and other biogeochemical rates in the ocean. Here, we examine the microbiome of a cyclonic, Gulf Stream frontal eddy, with a distinct origin and environmental parameters compared to surrounding waters, in order to better understand the processes dominating microbial community assembly in the dynamic coastal ocean. Our microbiome-based approach identified the eddy as distinct from the surround Gulf Stream waters. The eddy-associated microbial community occupied a larger area than identified by temperature and salinity alone, increasing the predicted extent of eddy-associated biogeochemical processes. While the eddy formed on the continental shelf, after two weeks both environmental parameters and microbiome composition of the eddy were most similar to the Gulf Stream, suggesting the effect of environmental filtering on community assembly or physical mixing with adjacent Gulf Stream waters. In spite of the potential for eddy-driven upwelling to introduce nutrients and stimulate primary production, eddy surface waters exhibit lower chlorophyllaalong with a distinct and less even microbial community, compared to the Gulf Stream. At the population level, the eddy microbiome exhibited differences among the cyanobacteria (e.g. lowerTrichodesmiumand higherProchlorococcus) and in the heterotrophic alpha Proteobacteria (e.g. lower relative abundances of specific SAR11 phylotypes) versus the Gulf Stream. However, better delineation of the relative roles of processes driving eddy community assembly will likely require following the eddy and surrounding waters since inception. Additionally, sampling throughout the water column could better clarify the contribution of these mesoscale features to primary production and carbon export in the oceans. 

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