Combining in situ measurements and altimetry to estimate volume

From 1994 to 2011, instruments measuring ocean currents (Acoustic Doppler Current Profilers; ADCPs) have been moored on a section crossing the Faroe–Shetland Channel. Together with CTD (Conductivity Temperature Depth) measurements from regular research vessel occupations, they describe the flow field and water mass structure in the channel. Here, we use these data to calculate the average volume transport and properties of the flow of warm water through the channel from the Atlantic towards the Arctic, termed the Atlantic inflow. We find the average volume transport of this flow to be 2.7 ± 0.5 Sv (1 Sv = 106 m3 s–1) between the shelf edge on the Faroe side and the 150 m isobath on the Shetland side. The average heat transport (relative to 0 °C) was estimated to be 107 ± 21 TW (1 TW = 1012 W) and the average salt import to be 98 ± 20 × 106 kg s−1. Transport values for individual months, based on the ADCP data, include a large level of variability, but can be used to calibrate sea level height data from satellite altimetry. In this way, a time series of volume transport has been generated back to the beginning of satellite altimetry in December 1992. The Atlantic inflow has a seasonal variation in volume transport that peaks around the turn of the year and has an amplitude of 0.7 Sv. The Atlantic inflow has become warmer and more saline since 1994, but no equivalent trend in volume transport was observed

Discovery of an unrecognized pathway carrying overflow waters toward the Faroe Bank Channel

The dense overflow waters of the Nordic Seas are an integral link and important diagnostic for the stability of the Atlantic Meridional Overturning Circulation (AMOC). The pathways feeding the overflow remain, however, poorly resolved. Here we use multiple observational platforms and an eddy-resolving ocean model to identify an unrecognized deep flow toward the Faroe Bank Channel. We demonstrate that anticyclonic wind forcing in the Nordic Seas via its regulation of the basin circulation plays a key role in activating an unrecognized overflow path from the Norwegian slope – at which times the overflow is anomalously strong. We further establish that, regardless of upstream pathways, the overflows are mostly carried by a deep jet banked against the eastern slope of the Faroe-Shetland Channel, contrary to previous thinking. This deep flow is thus the primary conduit of overflow water feeding the lower branch of the AMOC via the Faroe Bank Channel

The Iceland-Faroe slope jet: a conduit for dense water toward the Faroe Bank Channel overflow

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Semper, S., Pickart, R. S., Vage, K., Larsen, K. M. H., Hatun, H., & Hansen, B. The Iceland-Faroe slope jet: a conduit for dense water toward the Faroe Bank Channel overflow. Nature Communications, 11(1), (2020): 5390, doi:10.1038/s41467-020-19049-5.Dense water from the Nordic Seas passes through the Faroe Bank Channel and supplies the lower limb of the Atlantic Meridional Overturning Circulation, a critical component of the climate system. Yet, the upstream pathways of this water are not fully known. Here we present evidence of a previously unrecognised deep current following the slope from Iceland toward the Faroe Bank Channel using high-resolution, synoptic shipboard observations and long-term measurements north of the Faroe Islands. The bulk of the volume transport of the current, named the Iceland-Faroe Slope Jet (IFSJ), is relatively uniform in hydrographic properties, very similar to the North Icelandic Jet flowing westward along the slope north of Iceland toward Denmark Strait. This suggests a common source for the two major overflows across the Greenland-Scotland Ridge. The IFSJ can account for approximately half of the total overflow transport through the Faroe Bank Channel, thus constituting a significant component of the overturning circulation in the Nordic Seas.Support for this work was provided by the Bergen Research Foundation Grant BFS2016REK01 (S.S. and K.V.), the U.S. National Science Foundation Grants OCE-1558742 and OCE-1259618 (R.S.P.), the Danish Ministry of Climate, Energy and Utilities (K.M.H.L., H.H., and B.H.) and the European Union’s Horizon 2020 research and innovation programme under grant agreement 727852 (Blue-Action) (K.M.H.L., H.H., and B.H.)

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

Circulation and exchange of water masses on the Faroe Shelf and the impact on the Shelf ecosystem

The findings in this work can be listed as follows: FSW: Based on CTD data, the FSW is found to be relatively vertically homogeneous through-out the year inside the 100 m depth contour. Temperature measurements at coastal stations reveal that the FSW is also horizontally homogeneous especially in the central and northern part of the Shelf. The southern part can become isolated from the rest of the Shelf during winter. Temperature disturbances originate mainly at the offshelf boundaries and are transmitted to the inner shelf. The seasonal range in temperature is approximately 4 °C. From a coastal station and a standard CTD station on the Shelf, the FSW is found to be fairly homogeneous in salinity too, with a seasonal range on the order of 0.1. Tidal currents are strong and predominantly semidiurnal. The residual currents follow the topography in a clockwise manner and are on the order of 10 cm s-¹. A correlation and regression analysis verifies that the residual circulation can be generated by tidal rectification. A seasonal variation in the residual current is found in two out of five sites. FSF: The FSF experiences a seasonal variation such that it has the largest cross-frontal temperature difference (~2 °C) in spring induced by effective atmospheric cooling over the shallow shelf during winter. The temperature difference is maintained, although diminished, during summer, and the lowest temperature differences are observed in autumn. During a short period in autumn, the cross-frontal temperature difference is close to zero and may occasionally be reversed, so that the on-shelf water becomes slightly lighter than the off-shelf water. Salinity differences experience a small seasonal variation on the order of 0.1. Density differences are, therefore, mainly determined by temperature variations. The bottom depth where the centre of the FSF is observed, varies around the Shelf, and increases from east through north to the west and south. Tidal currents determine the location of the FSF and the theoretically predicted location of the front (Soulsby, 1983) fits fairly well with the observed location. The tilting angle of the front during spring is found to be on the order of 0.02 and in near geostrophic balance. The tilt induces a bottom contact some 5 km farther offshelf compared to the surface expression. Exchange: The typical exchange rate of FSW is found to be equivalent to a turbulent diffusivity of 67 m² s-¹, estimated using a temperature and a salinity budget. Indications in temperature and salinity observations reveal that the exchange rate is highly variable and typically varies by a factor of five. Continuous exchange through the front can be achieved through advection and turbulent diffusion. The associated on-shelf advection of an off-shelf transport in the bottom Ekman layer is found to be large enough to supply the heat needed to maintain the front in a quasistationary balance during spring. Extreme exchange is likely to be induced by episodic exchange events. Two extreme exchange processes are indicated: cascading through a canyon and an extreme intrusion of off-shelf water into coastal waters. The importance of these processes is uncertain. Biological impact: The relationship between the primary production on the Shelf and the magnitude of the exchange rate was investigated using a PP-model. When the exchange rate is small, the plankton is mainly confined to the FSW, and a strong spring bloom develops. If the exchange rate is high, the plankton is exported off the Shelf, delaying the spring bloom by up to three weeks. This is a general relationship and may be important for other island or bank systems, as well. With updated observational material, the hypothesised relationship between the primary production and the physical parameters is still found to be statistically significant. Years with an intensive spring blooms, thus, seem to occur when there is a large density difference across the front due to strong winter cooling

An investigation of the Faroe Shelf Front

The water mass on the Faroe Shelf is distinct from the off-shelf water surrounding the shelf. This difference of water masses is reflected in the temperature and salinity distributions. The on-shelf water is colder and fresher than the off-shelf water throughout most of the year. A temperature/salinity front thus forms, where the on-shelf water meets the off-shelf water. The waters inside the front have a different cycle of primary production and support a different ecosystem from the off-shelf waters and they are important nursery areas for larvae of many commercially important fish stocks. Sea surface temperature measurements from the R/V Magnus Heinason in the period February 1999 to November 2000 show the existence of the front throughout the year except for a short period in autumn, and the largest cross-frontal gradients are found in the spring. Also, the measurements are used to find typical values for the frontal location and width in various directions across the shelf. The observed characteristics of the front are discussed in relation to bottom topography and proximity to a shelf edge, to the heating/cooling cycle driven by the air-sea heat flux, and to various theories for fronts generated by tidal mixing

NACLIM - Fluxes: Iceland-Faroe Atlantic inflow transport

<p><strong>Last update: 31 October 2014</strong></p> <p><strong>Data set: </strong>Iceland-Faroe Atlantic inflow transport </p> <p><em>Preliminary results: HAV is in the process of recalculating the whole series (Update 31 October 2014)</em></p> <p><strong>Description: </strong>Weekly averaged volume flux of Atlantic water through section N (see map) crossing the Faroe Current</p> <p><strong>Period:</strong> June 1997 – May 2014 </p> <p><strong>Location:</strong> 62°20′ – 63°40′ N   6°05′ W</p> <p><strong>Instruments:</strong> Moored Acoustic Doppler Current Profilers (ADCP) and CTD sections </p> <p><strong>Variables: </strong>Atlantic water component of transport (Sv) through the section, which has come across the Iceland-Faroe Ridge </p> <p><strong>Source:</strong> Bogi Hansen and Karin Margretha H. Larsen (HAV)</p

The Iceland–Faroe inflow of Atlantic water to the Nordic Seas

The flow of Atlantic water between Iceland and the Faroe Islands is one of three current branches flowing from the Atlantic Ocean into the Nordic Seas across the Greenland–Scotland Ridge. By the heat that it carries along, it keeps the subarctic regions abnormally warm and by its import of salt, it helps maintain a high salinity and hence density in the surface waters as a precondition for thermohaline ventilation. From 1997 to 2001, a number of ADCPs have been moored on a section going north from the Faroes, crossing the inflow. Combining these measurements with decade-long CTD observations from research vessel cruises along this section, we compute the fluxes of water (volume), heat, and salt. For the period June 1997–June 2001, we found the average volume flux of Atlantic water to be 3.5 ± 0.5 Sv (1 Sv = 106 m3·s1). When compared to recent estimates of the other branches, this implies that the Iceland– Faroe inflow is the strongest branch in terms of volume flux, transporting 47% of the total Atlantic inflow to the Arctic Mediterranean (Nordic Seas and Arctic Ocean with shelf areas). If all of the Atlantic inflow were assumed to be cooled to 0 °C, before returning to the Atlantic, the Iceland–Faroe inflow carries a heat flux of 124 ± 15 TW (1 TW = 1012 W), which is about the same as the heat carried by the inflow through the Faroe–Shetland Channel. The Iceland–Faroe Atlantic water volume flux was found to have a negligible seasonal variation and to be remarkably stable with no reversals, even on daily time scales. Out of a total of 1348 daily flux estimates, not one was directed westwards towards the Atlantic

The Arctic Mediterranean component of AMOC did NOT weaken during the last two decades

Presentation of new researc