Denmark Strait Overflow Water (DSOW) supplies the densest contribution to North Atlantic Deep Water and is monitored at several locations in the subpolar North Atlantic. Hydrographic (temperature and salinity) and velocity time series from three multiple-mooring arrays at the Denmark Strait sill, at 180 km downstream (south of Dohrn Bank) and at a further 320 km downstream on the east Greenland continental slope near Tasiilaq (formerly Angmagssalik), were analyzed to quantify the variability and track anomalies in DSOW in the period [2007][2008][2009][2010][2011][2012]. No long-term trends were detected in the time series, while variability on time scales from interannual to weekly was present at all moorings. The hydrographic time series from different moorings within each mooring array showed coherent signals, while the velocity fluctuations were only weakly correlated. Lagged correlations of anomalies between the arrays revealed a propagation from the sill of Denmark Strait to the Angmagssalik array in potential temperature with an average propagation time of 13 days, while the correlations in salinity were low. Entrainment of warm and saline Atlantic Water and fresher water from the East Greenland Current (via the East Greenland Spill Jet) can explain the whole range of hydrographic changes in the DSOW measured downstream of the sill. Changes in the entrained water masses and in the mixing ratio can thus strongly influence the salinity variability of DSOW. Fresh anomalies found in downstream measurements of DSOW within the Deep Western Boundary Current can therefore not be attributed to Arctic climate variability in a straightforward way.

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 multi-year 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 intra-to interannual timescales.From the moored observations, we derive a mean pole-ward along-slope volume flux of 3.7±0.7 Sv for the period 2017-2019. Using a multi-linear 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 (4500 m depth), but confirm a persistent along-slope flow at the shallower slope (1500 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.

MotivationDiapycnal diffusivity plays an important role in the ventilation of the Eastern Tropical North Atlantic (ETNA) Oxygen Minimum Zone (OMZ). Studies by Fischer et al. (2013), Banyte et al. (2012) and the synthesis by Brandt et al. (2015) found that diapycnal mixing contributes up to 20%, locally up to 30%, to the oxygen supply in the OMZ. Diapycnal diffusivity in the core and oxycline of the tropical North Atlantic Oxygen Minimum Zone SFB 754This comparatively high contribution to the oxygen supply for the ETNA OMZ is the consequence of the weak horizontal circulation within the so-called shadow zone of the subtropical gyre and possibly enhanced mixing over rough topography in the seamount area south of the Guinea Dome (Brandt et al., 2015). Diapycnal diffusivityThe mean diapycnal diffusivity was found to be (1.06 ± 0.24) х 10 -5 m 2 s -1 in the OMZ core during OSTRE.The result is consistent with the diapycnal diffusivity estimates from GUTRE in the oxycline, as well as with microstructure measurements in the OMZ region. This demonstrate that the diapycnal diffusivity does not vary significantly in the OMZ within the depth range of 200-600 m and does not change in time.For both experiments no significant vertical displacements of the tracer peak larger than 5 m per year were observed over the entire time period of both TREs.Oxygen concentration map (a) at 400 m depth in the ETNA OMZ from MIMOC climatology (Sunke Schmidtko, pers. Comm.) and section (b) along 23°W from meridional ship sections (Brandt et al., 2015 • GUTRE tracer was detected during two of the OSTRE surveys which allowed to estimate diapycnal diffusivity from GUTRE over a time period of seven years. Normalized mean tracer concentration profiles for GUTRE (a) and OSTRE (b). Diapycnal diffusivity coefficients for GUTRE (white diamonds) and OSTRE (black dots). The mean coefficients are marked with horizontal lines. Mean diapycnal diffusivity coefficients for OSTRE (blue), GUTRE (magenta), and estimates from microstructure measurements (black). Boxes are giving the uncertainty ranges. Mean tracer profiles during OSTRE-III 2014 (left) and OSTRE-IV 2015 (right) from raw data (dots) for OSTRE (solid line) and GUTRE (dashed line).Power spectral densities of vertical shear, normalized by buoyancy frequency, horizontally averaged over GUTRE (black) [kg m -3 ] from meridional ship sections (Brandt et al., 2015) along 23°W.. Ellipses mark the tracer dispersion of both TREs. Enhanced mixingThe diapycnal diffusivity estimates from both TREs are about five times higher than expected from the Garrett and Munk reference (background) internal wave field. Vertical shear spectra from ship ADCP data showed elevated internal wave energy level in the seamount vicinity. Both tracer patches covered increasingly overlapping areas with time and thus spatially integrated increasingly similar fields of local diffusivity, as well as a different local stratification counteracted the influence of roughness on diffusivity.injection density mean GUTRE-II ( 7 months...