Evolution and Transformation of the North Icelandic Irminger Current Along the North Iceland Shelf

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−1 per 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.publishedVersio

Icelandic coastal sea surface temperature records constructed: putting the pulse on air-sea-climate interactions in the northern North Atlantic. Part I: Comparison with HadISST1 open-ocean surface temperatures and preliminary analysis of long-term patterns and anomalies of SSTs around Iceland

A new comprehensive record of long-term Icelandic sea surface temperature measurements, which have been updated and filled in with reference to air temperature records, is presented. The new SST series reveal important features of the variability of climate in Iceland and the northern North Atlantic. This study documents site histories and possible resulting inconsistencies and biases, for example, changes in observing sites and instruments. A new 119-yr continuous time series for north Iceland SST is presented, which should prove particularly useful for investigating air-sea ice interactions around northern Iceland. As this is the only part of the country to be regularly engulfed by winter and/or spring sea ice, it is therefore highly sensitive to climatic change. The coastal series correlate well overall with independent Hadley Centre Sea Ice and SST dataset version 1 (HadISST1) series from the adjacent open ocean (mean r = 0.59), although correlations are generally higher in summer than winter and for south and east Iceland compared with the west and north. The seasonal temperature range is generally twice as large at the coastal sites because of differential effects of radiation, melting, mixing, and advection of warmer or colder air or water masses, as well as spatial resolution differences and smoothing in HadISST1. The long-term climatological averages and graphs for the 10 SST stations and/or their composites reveal decadal variations and trends that are generally similar to Icelandic air temperature records: a cold late-nineteenth-century, rapid warming around the 1920s, an overall warm peak circa 1940, cooling until an "icy" period circa 1970, followed by warming. Regional differences between sites include relatively greater (lesser) long-term variations for the eastern and southern (western and northern) Icelandic coasts, suggesting greater variability and influence of ocean current advection in the southeast. Moreover, Vestmannaeyjar SST data reveal that the late-nineteenth-century cold period in the ocean was not confined to the cold currents off north and east Iceland but also affected the south coast markedly. The Stykkishólmur, Iceland, SST record is relatively noisy and shows very little decadal variation, which may largely be due to fjord ice in cold winters suppressing low temperatures. It is anticipated that researchers may find these Icelandic SST series of practical use as a historic measure of air-sea-climate interactions around Iceland. © 2006 American Meteorological Society

Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic

The Atlantic Ocean overturning circulation is important to the climate system because it carries heat and carbon northward, and from the surface to the deep ocean. The high salinity of the subpolar North Atlantic is a prerequisite for overturning circulation, and strong freshening could herald a slowdown. We show that the eastern subpolar North Atlantic underwent extreme freshening during 2012 to 2016, with a magnitude never seen before in 120 years of measurements. The cause was unusual winter wind patterns driving major changes in ocean circulation, including slowing of the North Atlantic Current and diversion of Arctic freshwater from the western boundary into the eastern basins. We find that wind-driven routing of Arctic-origin freshwater intimately links conditions on the North West Atlantic shelf and slope region with the eastern subpolar basins. This reveals the importance of atmospheric forcing of intra-basin circulation in determining the salinity of the subpolar North Atlantic

Model-observation and reanalyses comparison at key locations for heat transport to the Arctic: Assessment of key lower latitude influences on the Arctic and their simulation

Blue-Action Work Package 2 (WP2) focuses on lower latitude drivers of Arctic change, with a focus on the influence of the Atlantic Ocean and atmosphere on the Arctic. In particular, warm water travels from the Atlantic, across the Greenland-Scotland ridge, through the Norwegian Sea towards the Arctic. A large proportion of the heat transported northwards by the ocean is released to the atmosphere and carried eastward towards Europe by the prevailing westerly winds. This is an important contribution to northwestern Europe's mild climate. The remaining heat travels north into the Arctic. Variations in the amount of heat transported into the Arctic will influence the long term climate of the Northern Hemisphere. Here we assess how well the state of the art coupled climate models estimate this northwards transport of heat in the ocean, and how the atmospheric heat transport varies with changes in the ocean heat transport. We seek to improve the ocean monitoring systems that are in place by introducing measurements from ocean gliders, Argo floats and satellites. These state of the art computer simulations are evaluated by comparison with key trans-Atlantic observations. In addition to the coupled models ‘ocean-only’ evaluations are made. In general the coupled model simulations have too much heat going into the Arctic region and the transports have too much variability. The models generally reproduce the variability of the Atlantic Meridional Ocean Circulation (AMOC) well. All models in this study have a too strong southwards transport of freshwater at 26°N in the North Atlantic, but the divergence between 26°N and Bering Straits is generally reproduced really well in all the models. Altimetry from satellites have been used to reconstruct the ocean circulation 26°N in the Atlantic, over the Greenland Scotland Ridge and alongside ship based observations along the GO-SHIP OVIDE Section. Although it is still a challenge to estimate the ocean circulation at 26°N without using the RAPID 26°N array, satellites can be used to reconstruct the longer term ocean signal. The OSNAP project measures the oceanic transport of heat across a section which stretches from Canada to the UK, via Greenland. The project has used ocean gliders to great success to measure the transport on the eastern side of the array. Every 10 days up to 4000 Argo floats measure temperature and salinity in the top 2000m of the ocean, away from ocean boundaries, and report back the measurements via satellite. These data are employed at 26°N in the Atlantic to enable the calculation of the heat and freshwater transports. As explained above, both ocean and atmosphere carry vast amounts of heat poleward in the Atlantic. In the long term average the Atlantic ocean releases large amounts of heat to the atmosphere between the subtropical and subpolar regions, heat which is then carried by the atmosphere to western Europe and the Arctic. On shorter timescales, interannual to decadal, the amounts of heat carried by ocean and atmosphere vary considerably. An important question is whether the total amount of heat transported, atmosphere plus ocean, remains roughly constant, whether significant amounts of heat are gained or lost from space and how the relative amount transported by the atmosphere and ocean change with time. This is an important distinction because the same amount of anomalous heat transport will have very different effects depending on whether it is transported by ocean or the atmosphere. For example the effects on Arctic sea ice will depend very much on whether the surface of the ice experiences anomalous warming by the atmosphere versus the base of the ice experiencing anomalous warming from the ocean. In Blue-Action we investigated the relationship between atmospheric and oceanic heat transports at key locations corresponding to the positions of observational arrays (RAPID at 26°N, OSNAP at ~55N, and the Denmark Strait, Iceland-Scotland Ridge and Davis Strait at ~67N) in a number of cutting edge high resolution coupled ocean-atmosphere simulations. We split the analysis into two different timescales, interannual to decadal (1-10 years) and multidecadal (greater than 10 years). In the 1-10 year case, the relationship between ocean and atmosphere transports is complex, but a robust result is that although there is little local correlation between oceanic and atmospheric heat transports, Correlations do occur at different latitudes. Thus increased oceanic heat transport at 26°N is accompanied by reduced heat transport at ~50N and a longitudinal shift in the location of atmospheric flow of heat into the Arctic. Conversely, on longer timescales, there appears to be a much stronger local compensation between oceanic and atmospheric heat transport i.e. Bjerknes compensation

NACLIM - Fluxes: Hornbanki Section Atlantic inflow volume and heat fluxes

<p><strong>Last Update: 31 October 2014</strong></p> <p><strong>Data set:</strong> Atlantic inflow volume and heat fluxes at Hornbanki section  </p> <p><strong>Description:</strong> Monthly mean of Atlantic inflow volume and heat transports </p> <p><strong>Period: </strong>January 1994 – July 2014</p> <p><strong>Location:</strong> 66°50′ N   21°30′ W (map)  </p> <p><strong>Instruments: </strong>CTD, moored current meters </p> <p><strong>Variables:</strong> AW_transp, Heat_transp  - Atlantic Water volume transport [Sv] and respective heat transport [TW]</p> <p><strong>Source:</strong> Steingrímur Jónsson and Hedinn Valdimarsson (MRI)</p

The Emergence of the North Icelandic Jet and Its Evolution from Northeast Iceland to Denmark Strait

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 ≡ 106 m3 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

Abundance and productivity of the pelagic ecosystem along a transect across the northern Mid Atlantic Ridge in June 2003

A research cruise was conducted into the Irminger Sea west and southwest of Iceland on the Icelandic vessel Árni Friðriksson, from 4-30 June 2003, investigating redfish, Sebastes mentella, other pelagic fishes, zooplankton, phytoplankton and the hydrography of the area. Part of the cruise was devoted to a special study on the physical and chemical factors as well as the abundance of phytoplankton, meso- and macrozooplankton and planktivorous fish on a transect across the northern part of the Mid-Atlantic Ridge (MAR). This research is a part of the MAR-ECO project which aims to study the ecosystem associated with the northern MAR. In this paper we analyse the organisation of the pelagic ecosystem on the transect over the northern MAR, from phytoplankton to fish as apex predators. Keywords: MAR-ECO, Mid-Atlantic Ridge, environmental factors, phytoplankton, zooplankton, fish

The Emergence of the North Icelandic Jet and Its Evolution from Northeast Iceland to Denmark Strait

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 ≡ 106 m3 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

The East Greenland Boundary Current System South of Denmark Strait

Four repeat sections across the East Greenland shelf and slope south of Denmark Strait are analysed to investigate the components of the boundary current system. The sections were occupied in summer 2001, 2003, 2004 and 2007, and included use of a vessel-mounted acoustic Doppler current profiler, enabling the computation of absolute geostrophic velocities. The components of the boundary current are the East Greenland/Irminger Current (EGIC) in the upper layer, the Deep Western Boundary Current (DWBC) at the base of the continental slope, and the East Greenland spill jet which resides inshore and beneath the EGIC. Special emphasis is placed on the spill jet, a recently discovered feature about which relatively little is known. The spill jet was observed in each occupation, transporting 5.0±2.2 Sv equatorward in the mean, which is similar to the DWBC at this latitude (4.9±1.4 Sv). The spill jet displayed considerable variability between sections, which appears to be linked to the geographical location of the upper-layer hydrographic front associated with the EGIC. When the front is located near the shelfbreak, the spill jet is confined to the outer shelf/upper slope and its transport is smaller. During these times there is less mixing and the water advected by the jet is generally lighter than that transported by the DWBC. In contrast, when the front is located seaward of the shelfbreak, the spill jet extends farther down the continental slope and its volume flux is larger. At these times, there is stronger mixing and the spill jet can transport water as dense as the Denmark Strait Overflow Water. A vorticity analysis indicates that the jet is susceptible to a variety of instability processes including baroclinic, barotropic and symmetric instability. In addition, it is subject to double diffusive mixing that may influence its downstream evolution. It appears that the spill jet is a permanent feature of the summertime circulation in this region and contributes significantly to the intermediate, and at times deep, limb of the Atlantic Meridional Overturning Circulation