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The Denmark Strait overflow is the major export route of dense water from the Arctic Mediterranean into the North Atlantic. At the Strait's shallow sill, the overflow is a bottom‐intensified cold and dense plume, bound to the east by a thermal front formed with the warmer, northward flowing North Icelandic Irminger Current. More than two decades of observations at the sill show strong fluctuations of volume flux on daily time scales. To better understand the source of this variability, a five‐mooring array was installed at the sill, capturing nearly 1 year of velocity and bottom temperature measurements at a high temporal and spatial resolution. Bottom temperature fluctuations that exceed 4 °C indicate a meandering of the front between the plume and the North Icelandic Irminger Current. Current vector rotation shows trains of alternating cyclones and anticyclones at the sill. An eddy crosses the sill every 3 to 6 days with a mean velocity of 0.4 m/s and a typical diameter of 30 to 40 km. The results suggest that anticyclones, with centers passing through the deepest part of the sill, may be responsible for periods of increased volume flux—also referred to as boluses and pulses in previous studies. Although the relationship between eddies, pulses, and boluses is still unclear, the results show that eddies are directly linked to fluctuations in the strength, thickness, and position of the overflow plume. The southward flow of dense water from the Arctic ocean plays a crucial role in global ocean circulation but is almost immediately constrained on its way south by a submarine ridge that connects Greenland, Iceland, the Faroe Islands, and Scotland. The southward flow is therefore forced to pass through several straits and up and over relatively shallow sills. Most of the flow passes over a sill in the Denmark Strait, located between Greenland and Iceland. In this study, we present observations from an array of instruments, which measure the southward flow as it passes over the Denmark Strait sill. The flow is characterised by trains of eddies (vortices), with an alternating sense of rotation; meaning a counterclockwise eddy is usually followed by a clockwise eddy. The eddies are 30 to 40 km wide and need about 1 day to pass over the sill. These eddies help to explain pronounced changes in the flow across the sill, as they can help either to speed up the flow or slow it down. The results of this study contribute to understanding mesoscale fluctuations, which influence local mixing processes and water mass transports. Mesoscale flow variability is observed at the Denmark Strait Sill with a five‐mooring arrayTrains of alternating eddies are passing the sillAnticyclones are found to be related to the occurence of boluses and pulses

The descent of the Denmark Strait overflow plume is an important process in the Atlantic Meridional Overturning Circulation. Downstream of the sill, the plume entrains ambient water, increasing its volume transport. The entrainment and related transfer of energy can be driven by vertical or horizontal turbulent mixing, and varies spatially, as the plume descends, and temporally, as the volume transport at the sill changes. Using over 30 years of profile data, this spatial and temporal variability in the first 200 km downstream of the sill was investigated. Dissipation and entrainment rates were derived from Thorpe scales, and each profile was identified as either a low‐ or high‐transport flow, defined as below or above‐average volume transport at the sill. In the first 175 km flow type explains most of the variability in entrainment and dissipation rates, with high‐transport flow producing order of magnitude higher rates. Sections crossing the plume and yo‐yo casts (continuous profiling) indicate that dissipation and entrainment are likely driven by the formation of shear instabilities in the interfacial layer, when the vertical velocity shear overcomes the stratification. This vertical turbulent mixing explains most of the variability within the first 175 km, suggesting horizontal turbulent mixing processes may not play as important a role in this region. The importance of temporal flow variability means that further improvements to our understanding of plume dynamics in the Denmark Strait will require a novel observational approach to fully account for spatial and temporal contributions. Analysis of over 30 years of CTD and LADCP profiles along the descent of the Denmark Strait overflow plumeDownstream dissipation and entrainment rates are higher for flows with increased volume transport at the sillVertical turbulent processes drive dissipation and entrainment in the first 175 km of the plume's descent

The major export route of dense water from the Nordic Seas into the North Atlantic is in the deep channel in Denmark Strait. Here currents have been monitored with one or two moored Acoustic Doppler Current Profilers (ADCPs) since 1996. Volume transport estimates of the Denmark Strait Overflow Water (DSOW) so far were based on these data, which were regressed to the total transport of dense water in a numerical model. The resulting transport has been used in many publications. Here we present results from an extended five‐mooring array deployed in 2014/2015, which included measurements outside the swift overflow core. This array provided the basis for new calculations to estimate the DSOW transports. Furthermore, a correction is proposed for biases detected on some ADCPs, which led to earlier underestimation of the flow in the lower part of the plume. Using the new method, the mean DSOW transport is estimated to be 3.2 Sv in the period 1996–2016, without a significant trend. Uncertainties are typically ±0.5 Sv. Beyond variations on the eddy scale, an empirical orthogonal functions (EOF) analysis of the velocity field reveals three dominant modes of variability: the first mode is roughly barotropic and corresponds to pulsations of the plume, the second mode represents the laterally shifting component of the plume's core position, and the third mode indicates the impact of the varying overflow thickness. Finally, DSOW transports are compared to the Faroe Bank Channel overflow transports, but no clear relationship is found. A new calculation method for the DSO based on observations is introduced, which provides estimates from 1996 to 2016Velocity measurements from near‐bottom moorings using ADCPs of the Workhorse Longranger type (75 kHz) show a low‐velocity biasOverflow variability beyond the eddy scale can be explained to 90% by three modes. No long‐term trend was found

The Eastern Boundary Current is an essential part of the water mass exchange between the subtropical and subpolar North Atlantic. Here, we study the offshore branch of the European Shelf Current (ESC) over the Goban Spur slope area off Ireland. Our analysis is based on current measurements obtained from a multiyear mooring effort, complemented by ship‐board observations along a hydrographic section, satellite‐derived estimates of absolute dynamic topography with geostrophic currents, and float trajectories. These data serve to quantify the offshore branch of the ESC on intraannual to interannual timescales. From the moored observations, we derive a mean poleward along‐slope volume flux of 3.7 ±0.7 Sv for the period 2017–2019. Using a multilinear regression model and geostrophic surface velocities, we extend the time series to the period 1993–2019 and obtain a long‐term mean transport of 3.2 ±0.4 Sv. Both time series show strong variability ranging from −7.5 to 15.7 Sv. The variability is linked to a dynamic eddy field, especially a stationary cyclonic circulation pattern near the mooring array, and meandering of current branches originating from the North Atlantic Current. We find no evidence of a consistent deep boundary current extending from the shelf break to the position of the offshore mooring (4,500 m depth), but confirm a persistent along‐slope flow at the shallower slope (1,500 m depth). Geostrophic surface velocities and float trajectories reveal that the offshore branch of the ESC does not follow a clear northward path from the eastern subtropical regions but rather indicate the intermittent character of the flow. In this study, we describe the European Shelf Current (ESC) in the eastern North Atlantic. The study area is the deeper part of the Goban Spur area off Ireland. We use current measurements of instruments deployed in the water column from 2016 to 2019 combined with satellite observations of surface circulation and drifting float paths. We aim to quantify the contribution of the offshore branch to the ESC system. We calculate a poleward volume transport following the slope that is on average 3.7 ± 0.7 Sverdrup (Sv; 1 Sv = 1 million cubic meter per second) for the period 2017–2019. We use a mathematical relation between mooring data and satellite observations to extend the time series to the period 1993–2019 and obtain an average transport of 3.2 ± 0.4 Sv. There occur strong changes over time ranging from −5 to 15 Sv. These changes are caused by rotating vortices (eddies), especially a counter‐clockwise rotating circulation pattern near the moorings, and changing positions of currents branching from the North Atlantic Current. We find that the offshore branch of the ESC does not follow a clear northward path from the eastern subtropical regions of the North Atlantic. Instead, the flow is of rather unsteady character. The analysis of multiplatform observational efforts is used to study the circulation at the North Atlantic Eastern BoundaryThe volume transport of the offshore branch of the European Shelf Current is on average 3.2 ± 0.4 Sv for the period 1993–2019Transport variability is in the range of −7.5–15.7 Sv and induced by eddies and meandering North Atlantic Current branches The analysis of multiplatform observational efforts is used to study the circulation at the North Atlantic Eastern Boundary The volume transport of the offshore branch of the European Shelf Current is on average 3.2 ± 0.4 Sv for the period 1993–2019 Transport variability is in the range of −7.5–15.7 Sv and induced by eddies and meandering North Atlantic Current branches