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