Description: A long-term mean flow field for the subpolar North Atlantic region with a horizontal resolution of approximately 25km is created by gridding Argo-derived velocity vectors using two different topography-following interpolation schemes. The 10-day float displacements in the typical drift depths of 1000 to 1500m represent the flow in the Labrador Sea Water density range. Both mapping algorithms separate the flow field into potential vorticity (PV) conserving, i.e., topography-following contribution and a deviating part, which we define as the eddy contribution. To verify the significance of the separation, we compare the mean flow and the eddy kinetic energy (EKE), derived from both mapping algorithms, with those obtained from multiyear mooring observations. The PV-conserving mean flow is characterized by stable boundary currents along all major topographic features including shelf breaks and basin-interior topographic ridges such as the Reykjanes Ridge or the Rockall Plateau. Mid-basin northward advection pathways from the northeastern Labrador Sea into the Irminger Sea and from the Mid-Atlantic Ridge region into the Iceland Basin are well-resolved. An eastward flow is present across the southern boundary of the subpolar gyre near 52°N, the latitude of the Charlie Gibbs Fracture Zone (CGFZ). The mid-depth EKE field resembles most of the satellite-derived surface EKE field. However, noticeable differences exist along the northward advection pathways in the Irminger Sea and the Iceland Basin, where the deep EKE exceeds the surface EKE field. Further, the ratio between mean flow and the square root of the EKE, the Peclet number, reveals distinct advection-dominated regions as well as basin-interior regimes in which mixing is prevailing.

Description: Highlights: • A joint analysis of deep current meter records in the western North Atlantic. • Intra-seasonal variability dominates the deep boundary current. • Topographic waves near 10d periods trapped over steep topography. • Basin centers are showing longer periods (50d) caused by the eddy field. • Observed variability characteristics compared to high resolution model simulation. Abstract The Deep Western Boundary Current (DWBC) along the western margin of the subpolar North Atlantic is an important component of the deep limb of the Meridional Overturning near its northern origins. A network of moored arrays from Denmark Strait to the tail of the Grand Banks has been installed for almost two decades to observe the boundary currents and transports of North Atlantic Deep Water as part of an internationally coordinated observatory for the Atlantic Meridional Overturning Circulation. The dominant variability in all of the moored velocity time series is in the week-to-month period range. While the temporal characteristics of this variability change only gradually between Denmark Strait and Flemish Cap, a broad band of longer term variability is present farther along the path of the DWBC at the Grand Banks and in the interior basins (Labrador and Irminger Seas). The vigorous intra-seasonal variability may well mask possible interannual to decadal variability that is typically an order of magnitude smaller than the high-frequency fluctuations. Here, the intra-seasonal variability is quantified at key positions along the DWBC path using both, observations and high resolution model data. The results are used to evaluate the model circulation, and in turn the model is used to relate the discrete measurements to the overall pattern of the subpolar circulation. Topographic waves are found to be trapped by the steep topography all around the western basins, the Labrador and Irminger Seas. In the Labrador Sea, the high intra-seasonal variability of the boundary current regime is separated by a region of extremely low variability in narrow recirculation cells from the basin interior. There, the variability is also on intra-seasonal timescales, but at much longer periods around 50 days.

Description: Two consecutive summer upwelling events, each lasting for less than 24 h, were surveyed in high temporal and vertical resolution close to the Boknis Eck time-series station (BE) in the western Belt Sea (Baltic Sea) in summer 2010 with an autonomous glider. Driven only by moderate offshore winds both events resulted in more than 5 K cooling of surface waters, while only for the second event were significant irreversible changes in the vertical stratification observed. Generalizing the glider survey observations with hourly wind data from nearby meteorological stations, it is found that upwelling in the BE area occurs for wind directions between 190 to 260° and wind speed exceeding 4 m s−1. Based on these thresholds the wind-induced summer (June to September) upwelling conditions in the BE area for the period 1982 to 2012 are reconstructed. On average about 18 days of upwelling favourable wind conditions are found for the four summer months, with significant interannual variability ranging from 7.7 days (2006) to more than 28 days (1985). By aligning upwelling favourable wind conditions with the monthly BE surveys it is found that extreme anomalies in BE surveys follow extended periods of upwelling favourable winds.

Description: The flow field in the area of what was thought to be the source region of the North Brazil Current (NBC) off the northeast coast of Brazil between 5 degrees 30'S and 10 degrees S was investigated in austral spring during November 1992 and compared with observations in October 1990. The data were taken with several different instruments, including vessel-mounted ADCP, lowered-ADCP, Pegasus, CTD and XBTs. The flow was found off the coast at 5 degrees 30'S as well as at 10 degrees S as an undercurrent, the North Brazil Undercurrent (NBUC). The NBUC shows a subsurface core at about 200 m depth with velocities of up to 90.0 cm s(-1), resulting in large northward transports of more than 22 Sv in the upper 1000 m. The transport is about the same at 5 degrees 30'S and 10 degrees S, hence no net inflow from the east is required to feed the NBUC. The climatological Ekman transport is to the south between 5 degrees 30'S and 10 degrees S, and in consequence the northward flow near the surface was reduced and might be one reason for the existence of the undercurrent. The flow near the coast was to the north at 10 degrees S, therefore the Brazil Current had to start as a coastal current south of 10 degrees S. For the zonal sections at 5 degrees 30'S and 10 degrees S the geostrophic computations relative to the density surface sigma(1) = 32.15 kg m(-3) (about 1150 m depth) resulted in transports comparable to those obtained from direct measurements. The results further show that the choice of a correct level of no motion can be supported by the direct observations. A shallower reference based on water mass boundaries alone would reduce the NBUC transport to almost zero. Computations with data from the historical data base for austral fall resulted in a weaker NBUC of less than 20 Sv near 10 degrees S, indicating a possible seasonal signal in the NBUC with a stronger NBUC in austral spring.

Description: Observations in the central tropical Atlantic are used to investigate the circulation, the variability, and the near-equatorial meridional flow in this oceanic region. Meridional sections confirm that the southern band of the South Equatorial Current is a broad sluggish flow transporting subtropical water northwestward toward the western boundary. Variability in the South Equatorial Current is weak with an annual signal of about 2 cm/s. Recent equatorial flow observations agree with the previously proposed mean flow field, indicating that a permanent tropical circulation exists that is composed of several zonal current and countercurrent bands of small vertical and meridional extent compared to the subtropical gyres. However, wave phenomena superimpose on the mean flow field. On seasonal time scales the variability in the zonal flow field near the equator is dominated by the semiannual cycle in the central and eastern part while the annual cycle dominates in the western part. This seasonal variability is caused by the propagation of equatorial Rossby and Kelvin waves generated mainly by the zonal wind anomaly at the equator. Despite the observations of instantaneous cross-equatorial velocities and of floats crossing the equator it remains unclear whether there is a net cross-equatorial flow in the central tropical Atlantic in addition to cross-equatorial exchanges via thermocline convergence, upwelling and Ekman divergence. Three floats deployed at 200 m and 400 m depth either leave their deployment region at the equator to join the North Equatorial Undercurrent and progress further northward or in two cases have been deployed in the southern hemisphere and drift towards the equator.

Description: Moored Acoustic Doppler Current Profilers (ADCPs) were used to analyse the daily vertical zooplankton migration and its seasonality. One-year records of vertical velocity and acoustic backscatter were obtained at four stations in the Greenland Sea. Both parameters exhibited a diurnal cycle typical for vertically migrating zooplankton. Upward and downward migration occured in short periods approximately 5 h long, and peak migration velocities were around 1.5 cm s−1. Similar structures were observed at all four mooring sites in the 200–300 m depth range. Farther down, between 1000 and 1400 m, no daily migration was observed. Strong seasonal variations are evident, and both the phase and intensity of the migration pattern change with daylight as the season progresses. In summer and during the polar night the migration became very weak and was only detectable in the displacement of scattering layers. When the day/ night contrast was large, intense upward or downward motion was accompanied by sloping backscatter isopleths. We observed two main scattering layers, a deep layer that varies in depth with season and an almost invariable shallow scattering layer at about 150 m depth. The deep layer was interpreted as the “resting depth” of the migrating plankter, and the latter as their “feeding horizon”. Changes in the “resting depth” from about 400 m in autumn and spring to about 200 m in winter lead to seasonal variations in the migration distance. This behaviour is discussed with respect to environmental conditions.

Description: River water is the main source of dissolved organic carbon (DOC) in the Arctic Ocean. DOC plays an important role in the Arctic carbon cycle, and its export from land to sea is expected to increase as ongoing climate change accelerates permafrost thaw. However, transport pathways and transformation of DOC in the land-to-ocean transition are mostly unknown. We collected DOC and aCDOM(λ) samples from 11 expeditions to river, coastal and offshore waters and present a new DOC–aCDOM(λ) model for the fluvial–marine transition zone in the Laptev Sea. The aCDOM(λ) characteristics revealed that the dissolved organic matter (DOM) in samples of this dataset are primarily of terrigenous origin. Observed changes in aCDOM(443) and its spectral slopes indicate that DOM is modified by microbial and photo-degradation. Ocean colour remote sensing (OCRS) provides the absorption coefficient of coloured dissolved organic matter (aCDOM(λ)sat) at λ=440 or 443 nm, which can be used to estimate DOC concentration at high temporal and spatial resolution over large regions. We tested the statistical performance of five OCRS algorithms and evaluated the plausibility of the spatial distribution of derived aCDOM(λ)sat. The OLCI (Sentinel-3 Ocean and Land Colour Instrument) neural network swarm (ONNS) algorithm showed the best performance compared to in situ aCDOM(440) (r2=0.72). Additionally, we found ONNS-derived aCDOM(440), in contrast to other algorithms, to be partly independent of sediment concentration, making ONNS the most suitable aCDOM(λ)sat algorithm for the Laptev Sea region. The DOC–aCDOM(λ) model was applied to ONNS-derived aCDOM(440), and retrieved DOC concentration maps showed moderate agreement to in situ data (r2=0.53). The in situ and satellite-retrieved data were offset by up to several days, which may partly explain the weak correlation for this dynamic region. Satellite-derived surface water DOC concentration maps from Medium Resolution Imaging Spectrometer (MERIS) satellite data demonstrate rapid removal of DOC within short time periods in coastal waters of the Laptev Sea, which is likely caused by physical mixing and different types of degradation processes. Using samples from all occurring water types leads to a more robust DOC–aCDOM(λ) model for the retrievals of DOC in Arctic shelf and river waters.

Description: Recent results from hydrographic, chlorofluoromethane (CFM) and current measurements during an R.V. Meteor cruise in February/March 1994 underscore the importance of the Vema Fracture Zone (VFZ), located near 11°N on the Mid-Atlantic Ridge, for the transport of bottom water from the deep western basin of the equatorial Atlantic into the eastern abyss. The eastward transport in the bottom water range, of 1.8-2.0 Sv below 2.0°C, and of 2.1–2.4 Sv below the level of no motion at 3640 m, was determined by a combination of geostrophic calculations and direct current observations by a lowered ADCP. The comparison to former results indicates that the eastward flow in the VFZ is rather persistent. The water mass properties (Θ, S and CFMs) in the VFZ were compared to stations in the Guiana Basin, in the equatorial channel, and in the Brazil Basin at 10°S suggesting a significant contribution of North Atlantic Deep Water to the entire bottom water layer in the VFZ.