A persistent deep anticyclonic vortex in the Rockall Trough sustained by Anticyclonic Vortices Shed From the slope current and wintertime convection

The presence of a persistent surface anticyclone centered at approximately 55°N, 12°W in the Rockall Trough, northeast North Atlantic, has been previously noted in satellite altimetry data. Here, we show that this surface anticyclone is the imprint of a deep, persistent, non‐stationary anticyclonic vortex. Using wintertime 2007 and 2011 ship‐board data, we describe the anticyclone's vertical structure for the first time and find that the anticyclone core is partly made of warm and salty Mediterranean Overflow Water. The anticyclone has a radius of ~40 km, it stretches down to 2,000 m, with a velocity maximum around 500 m. To analyze the anticyclone's generating mechanism, we use a mesoscale‐resolving (~2 km) simulation, which produces a realistic pattern of the Rockall Trough anticyclone. The simulation indicates that the anticyclone is locally formed and sustained by two types of processes: wintertime convection and merger with anticyclonic vortices shed from the slope current flowing poleward along the eastern Rockall Trough slope. Intense negative vorticity filaments are generated along the Rockall Trough south‐eastern slope, and they encapsulate Mediterranean Overflow Water as they detach and grow into anticyclonic vortices. These Mediterranean Overflow Water‐rich vortices are advected into the trough, consequently merging with the Rockall Trough anticyclone and sustaining it. We suggest that the Rockall Trough anticyclone impacts regional intermediate water masses modifications, heat and salt budgets locally, and further afield into the neighboring subpolar northeast North Atlantic

Mechanisms for a nutrient-conserving carbon pump in a seasonally stratified, temperate continental shelf sea

Continental shelf seas may have a significant role in oceanic uptake and storage of carbon dioxide (CO2) from the atmosphere, through a ‘continental shelf pump’ mechanism. The northwest European continental shelf, in particular the Celtic Sea (50°N 8°W), was the target of extensive biogeochemical sampling from March 2014 to September 2015, as part of the UK Shelf Sea Biogeochemistry research programme (UK-SSB). Here, we use the UK-SSB carbonate chemistry and macronutrient measurements to investigate the biogeochemical seasonality in this temperate, seasonally stratified system. Following the onset of stratification, near-surface biological primary production during spring and summer removed dissolved inorganic carbon and nutrients, and a fraction of the sinking particulate organic matter was subsequently remineralised beneath the thermocline. Water column inventories of these variables throughout 1.5 seasonal cycles, corrected for air-sea CO2 exchange and sedimentary denitrification and anammox, isolated the combined effect of net community production (NCP) and remineralisation on the inorganic macronutrient inventories. Overall inorganic inventory changes suggested that a significant fraction (>50%) of the annual NCP of around 3 mol-C m–2 yr–1 appeared to be stored within a long-lived organic matter (OM) pool with a lifetime of several months or more. Moreover, transfers into and out of this pool appeared not to be in steady state over the one full seasonal cycle sampled. Accumulation of such a long-lived and potentially C-rich OM pool is suggested to be at least partially responsible for the estimated net air-to-sea CO2 flux of ∼1.3 mol-C m–2 yr–1 at our study site, while providing a mechanism through which a nutrient-conserving continental shelf pump for CO2 could potentially operate in this and other similar regions

Perspectives on driving mechanisms affecting intermediate water masses presence in the Rockall Trough

The Rockall Trough (RT), a deep channel in the northeast North Atlantic (NA), hosts water masses of subpolar and subtropical origins. Large-scale atmospheric (North Atlantic oscillation, Eastern Atlantic pattern) and oceanic (NA subpolar gyre) settings have been noted as the major drivers of water masses presence in the region, their properties, thus impacting heat and salinity inputs into the RT and higher northern latitudes. Intermediate water masses are known to retain their characteristics long distance away from their places of origin, thus their presence and impact on water properties further afield notable. To detect/discern large-scale driver(s) of intermediate water masses presence in the RT, empirical orthogonal function (EOF) analysis was used. Water masses metrics, used in the EOF analysis, are fractions based on a mixing triangle approach and derived from high-resolution ship-board conductivity-temperature-depth (CTD) and delayed mode processed Argo (ISAS15) in-situ datasets. The large-scale atmospheric and oceanic signals did not emerge as the main drivers. The EOF analysis pointed to intermediate water masses presence within the RT, southern and central domains in particular, to be most likely influenced by locally induced interior (sub)mesoscale processes and features, and possible consequent mixing. These results brought forward the role of interior water masses pathways, i.e., intermediate water currents, notably the deep, Mediterranean Overflow Water (MOW)-rich slope current, and interior (sub)mesoscale dynamics. The use of ship-board in-situ CTD, Coastal and Regional Ocean COmmunity (CROCO) model output and altimetry absolute dynamic topography datasets permitted the identification of a deep, recurrent, non-stationary anticyclone, centred at ~12 °W, 55 °N, named here the RT anticyclone. The above datasets were further used to perform analysis of RT anticyclone generating mechanism and core water masses origin. The analysis shows that the RT anticyclone is the result of the merging of, and sustained by, smaller anticyclones, generated by bottom topography-slope current interactions at intermediate depths along the southeast banks of the trough. High ship-board in-situ-derived salinity and temperature anomalies, found within the anticyclone deep core, fall within MOW upper and conservative lower ~750-1100 m regional depth bounds and inner 27.41-27.60 kg m-3 density ranges. The in-situ analysis supported the model-based deductions that the RT anticyclone is a locally generated deep vortex, stretching throughout the water column and imprinting on the ocean surface between ~11-13 °W, 54-56 °N. The findings are the first insight on the generation and water masses origin of the RT anticyclone. To further check and extend previous analyses of MOW presence regionally and within the larger scale northeast Atlantic, GLORYS12v1 (Global Reanalysis-PHY-001-030), eddy-resolving (1/12 °) reanalysis data, and Ariane, a Lagrangian particle tracking tool, were used. The GLORYS12v1 reanalysis product, jointly with the Ariane particle tracking tool, allowed for investigations into MOW pathways, and further, the origin of water masses, encapsulated in the RT anticyclone core. The particle tracking within the MOW upper ~700-950 m and lower 1000-1300 m veins’ depth ranges complemented the findings of the RT anticyclone generation and core water masses origins. The depth-restricted particle tracking does not permit for tracing RT (modified) MOW beyond the Wyville-Thomson Ridge. However, the particle tracking analysis showed that MOW reaches the RT, propagates deep into the trough (≥60 °N), and further westward and northward (≥65 °N) towards Iceland and Irminger basin, and beyond. The particles also spread westward of the trough’s southern approach, into the NA subpolar gyre, encapsulated in (sub)mesoscale eddies, i.e., of both submesoscale (up to 50 km) and mesoscale (>50 km) dimensions. The presence of MOW within the RT and MOW extension throughout the length of the trough were confirmed and supported by Argo floats trajectories and Argo-based (ISAS15) water masses metrics. The presented findings highlight the role of interior water masses pathways, i.e., intermediate water currents, notably the deep MOW-rich slope current. Interior (sub)mesoscale dynamics in the southern and central RT domains emerged as the predominant mechanism influencing intermediate water masses presence in the RT. The results further suggest that the deep MOW-rich slope current and the regional interior (sub)mesoscale processes may play a role not only in the local heat, salt, biogeochemical (re)distribution, but also in the neighbouring northeast NA subpolar gyre and higher northern latitudes