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Ocean warm and saline Atlantic water (AW) flows northward towards the Arctic. This water crosses the Greenland‐Scotland Ridge in three inflow branches:  the Iceland branch,  the Faroe branch and  the Shetland branch. The first monitoring of these branches was obtained along standard hydrographic sections and in the 1990s these observations were complemented by – at that time the state‐of‐the‐art technology – Acoustic Doppler Current Profilers (ADCPs) that could measure ocean currents directly. For many years the ADCPs were the backbone in transport estimates of the inflowing AW, but in order to get reliable estimates, a high number of moorings were necessary which was costly both in consumables and man‐power. Alternative methods were therefore needed. The process to optimise the inflow arrays began several years ago by the integration of Satellite Altimetry data . Over the years, more data have been obtained at the inflow arrays, including new data types, and within Blue‐Action analyses have been performed utilizing the available data in order to optimise the monitoring of the inflow arrays both with respect to cost and in order to produce more accurate estimates of AW volume, heat and salt transports. Resulting from the work undertaken in Blue‐Action, the recommendations for future monitoring the three inflow branches are as follows: Iceland branch: Combined observations from one or two ADCP moorings (including hydrographicobservations at intermediate depth) and four annual hydrographic surveys. Faroe branch: Combined observations from satellite altimetry, one ADCP mooring, three PIES (Pressure Inverted Echo Sounders), one bottom temperature logger and at least three annual hydrographic surveys. Shetland branch: A combination of gridded geostrophic surface velocities from satellite altimetry, at least three annual hydrographic cruises along the section and continued ADCP deployments at key sites (such as in the Shetland slope current).

Current meter moorings in different numbers and with different instruments have been deployed on the Hornbanki section north of Iceland since 1994 or for 25 years. The water mass transformation along the path of the AW towards the mooring section is investigated. It shows that major changes in water mass properties are observed along the path, caused mainly by mixing with the adjacent Polar water in Denmark Strait.

Ocean bottom pressure (OBP) variability serves as a proxy of ocean mass variability. A question how well it can modeled by the present general ocean circulation models on time scales of 1 day and more is addressed. It is shown that the models simulate consistent patterns of bottom pressure variability on monthly and longer scales except for areas with high mesoscale eddy activity, where high resolution is needed. The simulated variability is compared to a new data set from an array of PIES (Pressure-Inverted Echo Sounder) gauges deployed along a transect in the Southern Ocean. We show that while the STD of monthly averaged variability agrees well with observations except for the locations with high eddy activity, models lose a significant part of variability on shorter time scales. Furthermore, despite good agreement in the amplitude of variability, the OBP from the PIES and simulation show almost no correlation. Our findings point to limitations in geophysical background models required for space geodetic applications. We argue that major improvements in OBP modelling require data assimilation in order to increase the coherence between modelled and observed signals.

We discuss time series of ocean bottom pressure (OBP) computed by the Finite Element Sea Ice-Ocean Model (FESOM) driven by realistic forcing. The influence of atmospheric pressure and mesoscale eddies on the OBP and surface height anomalies on time scales up to years was investigated. Also, we estimated space and time scales of mass variability simulated by both climate-type (resolution about 1 degree) and eddy resolving (down to about 10 km) versions of the model. We analyze the African sector of the Southern Ocean. A part of the OBP variance there is associated with eddy activity (especially in the Agulhas region) and explore its respective contribution. Assessment of averaging interval of simulated data for the purpose of minimizing aliasing in variability of OBP is additionally carried out. An important aspect of this study is the comparison of modeled and in situ OBP records. High frequency measurements of OBP with sub-daily resolution available from Pressure Inverted Echo Sounders (PIES) used to infer temporal co-spectra of OBP variability. The PIES are placed along the prime meridian south of Africa can be used to evaluate variations of both barotropic and baroclinic geostrophic transport fluctuations of the Antarctic Circumpolar Current and verify corresponding GRACE estimates. The distance between PIES stations is chosen to resolve the major oceanic fronts for this region, which allows us to compare co-spectra of observed and simulated OBP variability. Variability of the oceanic bottom pressure from sensor observations and ocean models. Available from: https://www.researchgate.net/publication/316789208_Variability_of_the_oceanic_bottom_pressure_from_sensor_observations_and_ocean_models [accessed May 10, 2017].

To study the Antarctic Circumpolar Current (ACC) volume transport several cruises have taken place. The results of these cruises show snapshots without information about the time variability. To investigate the time variability of the ACC the Alfred Wegener Institute operates an array of Pressure Inverted Echo Sounders (PIES) along a satellite altimeter ground track south of Africa. PIES monitor ocean bottom pressure and acoustic travel time across the water column. A Gravest Empirical Mode (GEM, Meinen and Watts 1998) was applied to determine the geostrophic transport between the PIES. These time series were used to compute a transfer function between satellite Sderived transport and geostrophic transport. Satellite altimetry offers the possibility to calculate ACC transport between 1992 and 2010. A mean transport of 115 Sv and a variability of 7 Sv were derived for the Topex/Poseidon, Jason 1 and Jason 2 time period. A wavelet analysis shows that the ACC transport highly correlates with the winter and spring SAM index, whereas a direct correlation on a monthly scale could not be shown.

The Atlantic Meridional Overturning Circulation (AMOC) is considered as an important part of the global climate system. The densest component of the AMOC is the Denmark Strait Overflow Water, entering the deep Atlantic across the sill between Greenland and Iceland. Here, four years of overflow measurements in Denmark Strait are analyzed. The data suggest, that the overflow consists of a density driven, hydraulically controlled part, and a barotropic, wind stress forced component. The observed overflow transport reduction of 20% from 1999 (3.7 Sv) to 2003 (3.1 Sv) is likely caused by both a dense water reservoir height decrease in the Iceland Sea, and a reduction of the local wind stress forcing. The interannual fluctuations are consistent with a reduction of the North Atlantic Oscillation (NAO). Interannual temperature variability of 0.5 °C is linked to variable upstream entrainment rates and/or variable percentages of different water masses rather than changes of the individual sources. Further, an anticorrelation with the Faroe Bank Channel Overflow is found, with the total dense water outflow from the Nordic Seas being almost constant at 5.5 Sv from 1999 to 2003.

Ocean bottom pressure (OBP) variability serves as a proxy of ocean mass variability, the knowledge of which is needed in geophysical applications. The question of how well it can be modeled by the present general ocean circulation models on time scales in excess of 1 day is addressed here by comparing the simulated OBP variability with the observed one. To this end, a new multiyear data set is used, obtained with an array of bottom pressure gauges deployed deeply along a transect across the Southern Ocean. We present a brief description of OBP data and show large-scale correlations over several thousand kilometers at all time scales using daily and monthly averaged data. Annual and semiannual cycles are weak. Close to the Agulhas Retroflection, signals of up to 30 cm equivalent water height are detected. Further south, signals are mostly intermittent and noisy. It is shown that the models simulate consistent patterns of bottom pressure variability on monthly and longer scales except for areas with high mesoscale eddy activity, where high resolution is needed to capture the variability due to eddies. Furthermore, despite good agreement in the amplitude of variability, the in situ and simulated OBP show only modest correlation.

Ocean warm and saline Atlantic water (AW) flows northward towards the Arctic. This water crosses the Greenland‐Scotland Ridge in three inflow branches:  the Iceland branch,  the Faroe branch and  the Shetland branch. The first monitoring of these branches was obtained along standard hydrographic sections and in the 1990s these observations were complemented by – at that time the state‐of‐the‐art technology – Acoustic Doppler Current Profilers (ADCPs) that could measure ocean currents directly. For many years the ADCPs were the backbone in transport estimates of the inflowing AW, but in order to get reliable estimates, a high number of moorings were necessary which was costly both in consumables and man‐power. Alternative methods were therefore needed. The process to optimise the inflow arrays began several years ago by the integration of Satellite Altimetry data . Over the years, more data have been obtained at the inflow arrays, including new data types, and within Blue‐Action analyses have been performed utilizing the available data in order to optimise the monitoring of the inflow arrays both with respect to cost and in order to produce more accurate estimates of AW volume, heat and salt transports. Resulting from the work undertaken in Blue‐Action, the recommendations for future monitoring the three inflow branches are as follows: Iceland branch: Combined observations from one or two ADCP moorings (including hydrographicobservations at intermediate depth) and four annual hydrographic surveys. Faroe branch: Combined observations from satellite altimetry, one ADCP mooring, three PIES (Pressure Inverted Echo Sounders), one bottom temperature logger and at least three annual hydrographic surveys. Shetland branch: A combination of gridded geostrophic surface velocities from satellite altimetry, at least three annual hydrographic cruises along the section and continued ADCP deployments at key sites (such as in the Shetland slope current)., The Blue-Action project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 727852

Warm and saline water of Atlantic origin is transported across the Greenland Scotland Ridge into the Arctic Mediterranean. This inflow has a large impact on e.g. the climate and sea-ice in the Arctic and the knowledge of its variability and possible trend is therefore of huge importance in predicting Arctic climate change. The inflow has been monitored since the late 1990s with moored instrumentation combined with regular hydrographic cruises and data from satellite altimetry, but deploying moorings in the heavily fished region close to the Greenland Scotland Ridge is highly demanding in terms of manpower and funding. Efforts have therefore been made to optimize the monitoring systems, lately within the H2020 Blue-Action project. This has led to systems, which rely heavily on satellite altimetry. More recently moored PIES (Pressure Inverted Echo Sounders) have been used in a pilot project to monitor short-term variations of the temperature and salinity fields and these results look promising.

The Iceland Greenland Seas Project (IGP) is a coordinated atmosphere–ocean research program investigating climate processes in the source region of the densest waters of the Atlantic meridional overturning circulation. During February and March 2018, a field campaign was executed over the Iceland and southern Greenland Seas that utilized a range of observing platforms to investigate critical processes in the region, including a research vessel, a research aircraft, moorings, sea gliders, floats, and a meteorological buoy. A remarkable feature of the field campaign was the highly coordinated deployment of the observing platforms, whereby the research vessel and aircraft tracks were planned in concert to allow simultaneous sampling of the atmosphere, the ocean, and their interactions. This joint planning was supported by tailor-made convection-permitting weather forecasts and novel diagnostics from an ensemble prediction system. The scientific aims of the IGP are to characterize the atmospheric forcing and the ocean response of coupled processes; in particular, cold-air outbreaks in the vicinity of the marginal ice zone and their triggering of oceanic heat loss, and the role of freshwater in the generation of dense water masses. The campaign observed the life cycle of a long-lasting cold-air outbreak over the Iceland Sea and the development of a cold-air outbreak over the Greenland Sea. Repeated profiling revealed the immediate impact on the ocean, while a comprehensive hydrographic survey provided a rare picture of these subpolar seas in winter. A joint atmosphere–ocean approach is also being used in the analysis phase, with coupled observational analysis and coordinated numerical modeling activities underway., Bulletin of the American Meteorological Society, 100 (9), ISSN:0003-0007, ISSN:1520-0477