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 ...
Abstract 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. ; Funding was also received from the Danish Ministry of Climate, Energy and Utilities (FARMON & FARMON II projects) and the Marine Scotland Science's Offshore Monitoring Programme (Scottish Government).
The Arctic Mediterranean (AM) is the collective name for the Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Water enters into this region through the Bering Strait (Pacific inflow) and through the passages across the Greenland–Scotland Ridge (Atlantic inflow) and is modified within the AM. The modified waters leave the AM in several flow branches which are grouped into two different categories: (1) overflow of dense water through the deep passages across the Greenland–Scotland Ridge, and (2) outflow of light water – here termed surface outflow – on both sides of Greenland. These exchanges transport heat and salt into and out of the AM and are important for conditions in the AM. They are also part of the global ocean circulation and climate system. Attempts to quantify the transports by various methods have been made for many years, but only recently the observational coverage has become sufficiently complete to allow an integrated assessment of the AM exchanges based solely on observations. In this study, we focus on the transport of water and have collected data on volume transport for as many AM-exchange branches as possible between 1993 and 2015. The total AM import (oceanic inflows plus freshwater) is found to be 9.1 Sv (sverdrup, 1 Sv =106 m3 s−1) with an estimated uncertainty of 0.7 Sv and has the amplitude of the seasonal variation close to 1 Sv and maximum import in October. Roughly one-third of the imported water leaves the AM as surface outflow with the remaining two-thirds leaving as overflow. The overflow water is mainly produced from modified Atlantic inflow and around 70 % of the total Atlantic inflow is converted into overflow, indicating a strong coupling between these two exchanges. The surface outflow is fed from the Pacific inflow and freshwater (runoff and precipitation), but is still approximately two-thirds of modified Atlantic water. For the inflow branches and the two main overflow branches (Denmark Strait and Faroe Bank Channel), systematic monitoring of volume transport has been established since the mid-1990s, and this enables us to estimate trends for the AM exchanges as a whole. At the 95 % confidence level, only the inflow of Pacific water through the Bering Strait showed a statistically significant trend, which was positive. Both the total AM inflow and the combined transport of the two main overflow branches also showed trends consistent with strengthening, but they were not statistically significant. They do suggest, however, that any significant weakening of these flows during the last two decades is unlikely and the overall message is that the AM exchanges remained remarkably stable in the period from the mid-1990s to the mid-2010s. The overflows are the densest source water for the deep limb of the North Atlantic part of the meridional overturning circulation (AMOC), and this conclusion argues that the reported weakening of the AMOC was not due to overflow weakening or reduced overturning in the AM. Although the combined data set has made it possible to establish a consistent budget for the AM exchanges, the observational coverage for some of the branches is limited, which introduces considerable uncertainty. This lack of coverage is especially extreme for the surface outflow through the Denmark Strait, the overflow across the Iceland–Faroe Ridge, and the inflow over the Scottish shelf. We recommend that more effort is put into observing these flows as well as maintaining the monitoring systems established for the other exchange branches. ; This study was supported by European Framework Programmes under grant agreement no. GA212643 (THOR) and grant agreement no. 308299 (NACLIM), and from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 727852 (Blue-Action). Bering Strait data and analysis were supported by NSF-Office of Polar Programs Arctic Observing Network grants PLR-1304052 & PLR1758565. The Davis Strait program (CML and BC) was supported by the US National Science Foundation under grants OPP0230381, ARC0632231, and ARC1022472, with additional support from the Department of Fisheries and Oceans, Canada. We thank WilkenJon von Appen and an anonymous referee for very constructive comments. ; Peer reviewed
