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The North Icelandic Irminger Current (NIIC) flowing northward through Denmark Strait is the main source of salt and heat to the north Iceland shelf. We quantify its along‐stream evolution using the first high‐resolution hydrographic/velocity survey north of Iceland that spans the entire shelf along with historical hydrographic measurements as well as data from satellites and surface drifters. The NIIC generally follows the shelf break. Portions of the flow recirculate near Denmark Strait and the Kolbeinsey Ridge. The current's volume transport diminishes northeast of Iceland before it merges with the Atlantic Water inflow east of Iceland. The hydrographic properties of the current are modified along its entire pathway, predominantly because of lateral mixing with cold, fresh offshore waters rather than air‐sea interaction. Progressing eastward, the NIIC cools and freshens by approximately 0.3°C and 0.02–0.03 g kg⁻¹per 100 km, respectively, in both summer and winter. Dense‐water formation on the shelf is limited, occurring only sporadically in the historical record. The hydrographic properties of this locally formed water match the lighter portion of the North Icelandic Jet (NIJ), which emerges northeast of Iceland and transports dense water toward Denmark Strait. In the region northeast of Iceland, the NIIC is prone to baroclinic instability. Enhanced eddy kinetic energy over the steep slope there suggests a dynamical link between eddies shed by the NIIC and the formation of the NIJ as previously hypothesized. Thus, while the NIIC rarely supplies the NIJ directly, it may be dynamically important for the overturning circulation in the Nordic Seas.

 

The North Icelandic Jet (NIJ) is an important source of dense water to the overflow plume passing through Denmark Strait. The properties, structure, and transport of the NIJ are investigated for the first time along its entire pathway following the continental slope north of Iceland, using 13 hydrographic/velocity surveys of high spatial resolution conducted between 2004 and 2018. The comprehensive dataset reveals that the current originates northeast of Iceland and increases in volume transport by roughly 0.4 Sv (1 Sv ≡ 10 6 m 3 s −1 ) per 100 km until 300 km upstream of Denmark Strait, at which point the highest transport is reached. The bulk of the NIJ transport is confined to a small area in Θ– S space centered near −0.29° ± 0.16°C in Conservative Temperature and 35.075 ± 0.006 g kg −1 in Absolute Salinity. While the hydrographic properties of this transport mode are not significantly modified along the NIJ’s pathway, the transport estimates vary considerably between and within the surveys. Neither a clear seasonal signal nor a consistent link to atmospheric forcing was found, but barotropic and/or baroclinic instability is likely active in the current. The NIJ displays a double-core structure in roughly 50% of the occupations, with the two cores centered at the 600- and 800-m isobaths, respectively. The transport of overflow water 300 km upstream of Denmark Strait exceeds 1.8 ± 0.3 Sv, which is substantially larger than estimates from a year-long mooring array and hydrographic/velocity surveys closer to the strait, where the NIJ merges with the separated East Greenland Current. This implies a more substantial contribution of the NIJ to the Denmark Strait overflow plume than previously envisaged. 

Abstract. The Arctic Mediterranean (AM) is the collective name forthe Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Water enters into thisregion through the Bering Strait (Pacific inflow) and through the passages across theGreenland–Scotland Ridge (Atlantic inflow) and is modified within the AM. The modifiedwaters leave the AM in several flow branches which are grouped into two differentcategories: (1) overflow of dense water through the deep passages across theGreenland–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 ofthe AM and are important for conditions in the AM. They are also part of the global oceancirculation and climate system. Attempts to quantify the transports by various methodshave been made for many years, but only recently the observational coverage has becomesufficiently complete to allow an integrated assessment of the AM exchanges based solelyon observations. In this study, we focus on the transport of water and have collecteddata on volume transport for as many AM-exchange branches as possible between 1993 and2015. The total AM import (oceanic inflows plusfreshwater) is found to be 9.1 Sv (sverdrup,1 Sv =10⁶ m³ s⁻¹) with an estimated uncertainty of 0.7 Sv and hasthe 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 theremaining two-thirds leaving as overflow. The overflow water is mainly produced frommodified Atlantic inflow and around 70 % of the total Atlantic inflow is convertedinto overflow, indicating a strong coupling between these two exchanges. The surfaceoutflow is fed from the Pacific inflow and freshwater (runoff and precipitation), but isstill approximately two-thirds of modified Atlantic water. For the inflowbranches and the two main overflow branches (Denmark Strait and Faroe Bank Channel),systematic monitoring of volume transport has been established since the mid-1990s, andthis 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 astatistically significant trend, which was positive. Both the total AM inflow and thecombined transport of the two main overflow branches also showed trends consistent withstrengthening, but they were not statistically significant. They do suggest, however,that any significant weakening of these flows during the last two decades is unlikely andthe overall message is that the AM exchanges remained remarkably stable in the periodfrom the mid-1990s to the mid-2010s. The overflows are the densest source water for thedeep 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 tooverflow weakening or reduced overturning in the AM. Although the combined data set hasmade it possible to establish a consistent budget for the AM exchanges, the observationalcoverage for some of the branches is limited, which introduces considerable uncertainty.This lack of coverage is especially extreme for the surface outflow through the DenmarkStrait, the overflow across the Iceland–Faroe Ridge, and the inflow over the Scottishshelf. We recommend that more effort is put into observing these flows as well asmaintaining the monitoring systems established for the other exchange branches.