Arctic and Atlantic waters in the Norwegian Basin, between year variability and potential ecosystem implications

The ocean climate of the southern Norwegian Sea - the Norwegian Basin - is largely set by the relative amount of Atlantic Water in the eastern and Arctic Water in the western region. Here we utilized hydrographic data from repeated sections, together with annually gridded survey data of the upper 1000 m, to resolve the main hydrographic changes over the period 1995-2019. Based on integrated heat -and freshwater content, we divide into three periods. The first period 1995-2005, denoted Arctic, is characterized by relative fresh and cold Atlantic Water overlaying Arctic Intermediate Water that basically covers the whole Norwegian Basin. Differently, the conditions during the period 2006-2016, denoted Atlantic, are warmer and more saline, and the extent and thickness of Arctic Intermediate Water is greatly reduced. During the most recent period denoted Fresh, 2017-2019, there has been a major freshening of the Atlantic waters, the layer of Arctic Intermediate Water has not recovered, but instead a layer of warmer but relative fresh Arctic Water has expanded. We find that increased abundance of the Arctic zooplankton Calanus hyperboreus in the southern and eastern Norwegian Basin coincides with increased extent of Arctic Water. We also note that the overall mesozooplankton biomass in the Norwegian Basin is significantly higher during periods of relative high amount of Arctic Water. Furthermore, we show that both nitrate and silicate winter (pre-bloom) concentrations are significantly higher in the Arctic Water compared to Atlantic Water, and that there is a reduction in nutrients from the Arctic period compared subsequent Atlantic and Fresh periods. Since these nutrients can be interpreted as the potential for new production, changes in the influx of western Arctic waters are expected to have a bottom-up effect on the Norwegian Sea. Hence, this study indicates that the amount of Arctic waters and their concentration of nutrients and zooplankton are more important for the Norwegian Basin ecosystem functioning rather than the temperature of the Atlantic waters.publishedVersio

The effect of the North Atlantic Subpolar Front as a boundary in pelagic biogeography decreases with increasing depth and organism size

Broad-scale patterns in the distribution of deep-sea pelagic species and communities are poorly known. An important question is whether biogeographic boundaries identified from surface features are important in the deep mesopelagic and bathypelagic. We present community analyses of discrete-depth samples of mesozooplankton and micronekton to full-ocean depth collected in the area where the Mid-Atlantic Ridge is crossed by the Subpolar Front. The results show that the distributional discontinuity associated with the front, which is strong near the surface, decreases with increasing depth. Both the frontal separation near the surface and the community convergence at increasing depths were clearer for mesozooplankton than for micronekton

The Iceland Greenland Seas Project

A coordinated atmosphere-ocean research project, centered on a rare wintertime field campaign to the Iceland and Greenland Seas, seeks to determine the location and causes of dense water formation by cold-air outbreaks. 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 lifecycle 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 modelling activities underway

Acoustically tracked passive drifters for measurement of oceanic circulation. Adaptation to the Nordic Seas

A review of the technology to use neutrally buoyant subsurface drifters or floats for Lagrangian measurements of ocean circulation was made. Focus was put on the RAFOS technology which uses small floats that listen to low-frequency sound-transmitting sources moored at fixed positions in the deep water. Recent applications of this and similar techniques have been studied. Some suggestions for future float applications with emphasis on studies in Nordic waters are presented. These include measurement of the internal circulation in the deep basins, and the exchange of waters between them. The float technology requires close follow-up during planning and execution of experiments by the scientists involved. It is recommended in the beginning to spend most efforts on tasks related to scientific planning and performance, and less on the technological parts, by applying as much commercially available equipment as possible

On the structure of the Lofoten Basin Eddy

A small anticyclonic eddy of extraordinary intensity sits in the center of the Lofoten Basin near 69°40′N, 3°E. This paper gives a first detailed description of its kinematics and lowest-order dynamic balances. Using a 75 kHz vessel-mounted acoustic Doppler current profiler, hydrography, and six RAFOS floats to probe the eddy, we document a solid body core with 7-8 km radius and relative vorticity very close to -f, where f is the local Coriolis parameter. Maximum orbital velocities close to 0.8 ms-1 were observed at 18 km radius. One float, trapped in the core of the eddy for 9 months, indicated undiminished strength throughout that period, possibly even intensifying in winter. Hydrography revealed adiabatic conditions from the bottom of a shallow seasonal thermocline to 1000 m depth in early July 2010. Thermal convection in winter maintains an already deep pycnostad, and may also play a key role in intensifying the eddy, quite possibly through penetrative convection that deepens and sharpens the underlying pycnocline. Heat lost to the atmosphere has to be replenished from warm anticyclonic eddies shed off the eastern branch of the Norwegian Atlantic Current, but the mechanism(s) by which it is added to the eddy remains to be studied. Examination of historical data sets suggests the eddy is a permanent feature of the Lofoten Basin. Two hydrocasts from the 1960s also show a similar adiabatic mixed layer albeit 1°C cooler than in 2010, perhaps reflecting the generally cold winter conditions that prevailed then. © 2013. American Geophysical Union. All Rights Reserved

On the long-term stability of the Lofoten Basin Eddy

In recent years, several studies have identified an area of intense anticyclonic activity about 500 km straight west of the Lofoten Islands at 70°N in the northern Norwegian Sea. Now recognized as the coherent Lofoten Basin Eddy (LBE), it is maintained by a supply of anticyclonic eddies that break away from the Norwegian Atlantic Current. Here we show from ship-based surveys of its velocity field that it is quite stable with a central core in solid body rotation ∼1000 m deep, ∼8 km radius, and a relative vorticity close to its theoretical limit –f. The surveys also show the LBE typically has a \u3e60 km radius with maximum swirl velocities at 17–20 km radius. From the velocity field, we estimate the dynamic height amplitude at the surface to be about ∼0.21 ± 0.03 dyn. m. Second, altimetry from the last 20 years shows the extremum in sea surface height relative to the surrounding waters to be about the same, 0.2 dyn. m. Third, a float trapped in the LBE for many months reveals a clear cyclonic wandering of the eddy over the deepest parts of the basin. Last, three hydrographic sections from the 1960s show the dynamic height signal to be virtually the same then as it is now. From these observations, we conclude that the LBE is a permanent feature of the Nordic Seas and plays a central role in maintaining the pool of warm water in the western Lofoten Basin

The subsurface circulation of the Iceland Sea observed with RAFOS floats

The pathways of dense waters located above the sill depth of Denmark Strait were investigated in the Iceland Sea using 52 acoustically tracked RAFOS floats. These floats were deployed in summer 2013 and 2014, with a target depth of 500 m, resulting in a total of 40.9 float-years of track data covering the Iceland Sea basin. In the interior Iceland Sea basin, the float tracks showed a double gyre circulation, out of which floats eventually escaped towards the Norwegian Sea in the East Icelandic Current, with some appearing to be en route to the Faroe Bank Channel. Four floats exited through Denmark Strait and surfaced in the Labrador and Irminger Seas. Four other floats deployed west of the Kolbeinsey Ridge at 70°N show the connection between the East Greenland Current and the East Icelandic Current. Floats deployed east of the Kolbeinsey Ridge along the Icelandic slope were captured in a region with no clear main flow. Eddy motions, mainly small scale (radii of 0.5–3 km), are seen throughout the Iceland Sea. Several floats were grounded on the Icelandic slope both east and west of the Kolbeinsey Ridge due to upslope currents, which created a rim of cold water along the slope. While this water was indicative of the presence of the North Icelandic Jet, no connection between the eastern Iceland Sea and Denmark Strait sill was found. Our investigation of wind stress curl fields from atmospheric reanalysis data suggests that high wind stress curl conditions may have been unfavorable for a westward connection by the North Icelandic Jet at the time of the float observations

Zeepipe 2B rørledning fra draupner til Kollsnes. Vurdering av risiko for inntrenging av sedimentpartikler under fri fylling. (Suction of sediment particles into offshore pipeline. Assessment of risk)

Det er foretatt beregninger for sannsynlighet for suspensjon av sedimenter ved Draupner plattforma som følge av overflatebølger. Beregningene er basert på modeller for bunnstress og kritisk bunnstrøm for å mobilisere sediment med gitt karakteristikk, samt på bølgestatistikk for Sleipner fra Værvarslinga på Vestlandet. Resultatene viser at mobilisering tar til ved vedvarende bølgehøyder på rundt 6 meter, tilsvarende signifikant bølgehøyde på 7-8 meter. Vindgenererte bølger av denne størrelsesordenen opptrer sjeldnere enn 1.5 % av tiden i månedene september-desember. Lengste beregnede varighet av perioder med 7 m bølger er 54 timer, og for 8 m bølger 42 timer, med kun 1-2 slike perioder registrert innenfor 40 års statistikk. Berregningene endikerer at det er en viss risiko for at partikler kan mobiliseres, men at denne risikoen er liten. Det knytter seg en viss usikkerhet til beregningene på grunn av mulig effekt av lange bølger (dønning) og av bakgrunnsstrømmen, og det anbefales om mulig å foreta en feltmessig verifisering av disse effektene.STATOIL Z2B prosjekte

Zeepipe 2B rørledning fra draupner til Kollsnes. Vurdering av risiko for inntrenging av sedimentpartikler under fri fylling. (Suction of sediment particles into offshore pipeline. Assessment of risk)

Det er foretatt beregninger for sannsynlighet for suspensjon av sedimenter ved Draupner plattforma som følge av overflatebølger. Beregningene er basert på modeller for bunnstress og kritisk bunnstrøm for å mobilisere sediment med gitt karakteristikk, samt på bølgestatistikk for Sleipner fra Værvarslinga på Vestlandet. Resultatene viser at mobilisering tar til ved vedvarende bølgehøyder på rundt 6 meter, tilsvarende signifikant bølgehøyde på 7-8 meter. Vindgenererte bølger av denne størrelsesordenen opptrer sjeldnere enn 1.5 % av tiden i månedene september-desember. Lengste beregnede varighet av perioder med 7 m bølger er 54 timer, og for 8 m bølger 42 timer, med kun 1-2 slike perioder registrert innenfor 40 års statistikk. Berregningene endikerer at det er en viss risiko for at partikler kan mobiliseres, men at denne risikoen er liten. Det knytter seg en viss usikkerhet til beregningene på grunn av mulig effekt av lange bølger (dønning) og av bakgrunnsstrømmen, og det anbefales om mulig å foreta en feltmessig verifisering av disse effektene