Lofoten Basin Research Articles

Norwegian Sea Oceanic Basin and Prograded Margins Composite Tectono-Sedimentary Element

Abstract The Norwegian Sea oceanic basins and prograded margins have developed since NE Atlantic break-up in the earliest Eocene. Significant amounts of sediments were fed to the regionally subsiding and widening Norwegian Sea during the Cenozoic as a result of several phases of uplift and erosion of the bounding shelves and their hinterland. Despite an overall passive-margin evolution, the area experienced tectonic events and associated processes that interrupted the regional subsidence, causing contraction/inversion and tilting. The post-break-up depositional history of the mid-Norwegian margin comprises two main stages: (1) middle Eocene–Pliocene margin subsidence and relatively modest sedimentation during a period of climatic decline; and (2) latest Pliocene–Pleistocene full-scale northern hemisphere glaciations that resulted in deep erosion of shelves and hinterlands, and very high sedimentation rates and large-scale continental margin progradation. Slope failures within rapidly deposited glacial sediments affected both prograded margins, releasing large slides that travelled downslope into the oceanic Norway and Lofoten basins. Despite a long exploration history for hydrocarbon prospects in deeper waters and large amounts of data acquisition, no significant discovery has been made.

Research on the change of the storage volume of the Nordic Seas Overflows over the last 40 years

The Nordic Sea overflow, being a significant driver of the thermohaline circulation, exerts a substantial influence on environmental dynamics in the Arctic and globally. A better understanding about the trend of the storage volume of the Nordic Seas Overflows is of paramount importance to a realistic assessment of the North Atlantic circulation and variability. EN4.2.2 reanalysis data were utilized to acquire the monthly average time series of overflow water storage volume in the Nordic Sea from 1950 to 2022. The storage volume demonstrates seasonal variations, with fluctuations of approximately 10% around the average. Over the period from 1980 to 2022, the average volume per decade exhibited a consistent decrease. Linear fitting of the annual average data estimated the overall relative change trend in the last 43 years to be -7.2 ± 2.6%. However, the quality of the EN4.2.2 data brings the average error of about 12% in the calculation of the overflow storage volume, which implies that the downward trend requires further validation. From a spatial standpoint, the Norwegian Sea, particularly the Lofoten Basin, is the primary region where overflow water storage volume in the Nordic seas have decreased. This decrease corresponds to an increase in ocean temperatures within the upper layer (0-600m) of both the Lofoten Basin and the Norwegian Basin. The warming of these regions has directly impacted the overflow water storage volume, leading to its reduction. It is noteworthy that the ocean temperature rise in the upper layer is more influenced by the Atlantic inflow rather than air-sea flux, particularly in the Norwegian Basin. This is attributed to the substantial increase in SST in the North Atlantic, which aligns with the warm current regions in the Atlantic Ocean, and the lack of statistical significance in linear trend of the air-sea heat flux. Furthermore, there has been a more rapid reduction in the overflow storage volume in the Norwegian Sea from 2011 to 2022. Specifically, the annual average overflow volume from 2018 to 2022 dropped below the previous lowest value. Concurrently, the salinity of the upper Norwegian Sea and the Atlantic inflow decreased considerably, while there has been no significant change in ocean temperature. Therefore, this short-term fluctuation is predominantly attributed to the decrease in Atlantic inflow salinity. Interestingly, the decrease in overflow water volume in the Nordic Seas does not follow a linear pattern in relation to increasing ocean temperature; instead, it exhibits an accelerating trend. If the ocean temperature rises by 1°C, the overflow water volume in the Nordic Seas will decrease to 86% of the total volume. Overall, the overflow water storage in the Norwegian Sea undergoes complex interannual variations and is notably influenced by the influx from the Atlantic Ocean.

NORWEGIAN SEA HOLOCENE ENVIRONMENTS BASED ON THE FOSSIL MICROALGAE ASSEMBLAGES

Detailed micropaleontological analysis was performed on the sediment core AMK-6142 collected in the deep-sea southwestern part of the Lofoten Basin in the Norwegian Sea. Summer sea-surface temperature for the last 7 cal. ky was reconstructed from dinocyst assemblages using the modern analogue technique (MAT). Sea-surface temperature reconstruction and dinocysts species composition indicate repeated changes in the surface environments during the Middle and Late Holocene. Episodes of cooling and probable displacement of the Arctic Front to the southwestern part of the Lofoten Basin were recorded for 5–7, 1.6–2.1, and 0.1–1.0 cal. ka BP.

Analysis of Seasonal and Long-Term Variations in the Surface and Vertical Structures of the Lofoten Vortex

The Lofoten Vortex (LV) is a quasi-permanent anticyclonic eddy with the characteristic of periodic regeneration in the Lofoten Basin (LB), which is one of the major areas of deep vertical mixing in the Nordic Sea. Our analysis of the LV contributes to our understanding of the variations in convective mixing in the LB. Based on drifter data and satellite altimeter data, the climatological results show that the LV has the sea surface characteristics of relative stability in terms of its spatial position and significant seasonal variations in its physical characteristics. Combined with the temperature and salinity data of Argo profiles, the vertical structures of the LV are presented here in terms of their spatial distribution and monthly variations. The wavelet analysis of the satellite sea surface temperature (SST) data shows that the period of SST anomaly (SSTA) in the LV sea area is 8–16 years. In the stage marked by a decreasing (increasing) trend of SSTA, the vertical mixing is strengthened (weakened). Current vertical mixing is clearly revealed by the Argo profiles, and the SSTA shows a significant impact of cooling. However, against a background of warming and freshening, this vertical mixing will be greatly weakened in the next increasing trending stage of the SSTA.

Decadal Changes in Ventilation and Anthropogenic Carbon in the Nordic Seas

AbstractWe evaluate the decadal evolution of ventilation and anthropogenic carbon (Cant) in the Nordic Seas between 1982 and the 2010s. Ventilation changes on decadal timescale are identified by evaluating decadal changes in mean ages and apparent oxygen utilization in each of the four main basins of the Nordic Seas (the Greenland and Iceland Seas, and the Norwegian and Lofoten Basins). The ages are derived from the transient time distribution approach, based on the transient tracers chlorofluorocarbon‐12 (CFC‐12) and sulfur hexafluoride (SF6). The different decades show different phases in ventilation, with the 2000s being overall better ventilated than the 1990s in all basins. For the Greenland Sea, we also show that the 2010s are better ventilated than the 2000s, with a clear shift in hydrographic properties. The evolution of concentrations and inventory of Cant is linked to the ventilation state. The deep waters get progressively older over the analyzed period, which is connected to the increased fraction of deep water from the Arctic Ocean.

Application to the World Ocean of the Theory of Transformation of a Mesoscale Vortex into a Submesoscale Vortex Thread When the Vortex Elongates by an Inhomogeneous Barotropic Flow

This paper is a continuation of the authors’ research [5]. We apply some aspects of the theory about strong horizontal stretching of large-scale mesoscale eddies in the World ocean based on ‘the ellipsoidal eddies’ theory to the ocean conditions. Two areas are considered, characterized by increased eddy manifestation. They are the Lofoten Basin in the Norwegian Sea and the Agulhas Current area. We also discuss the condition of the Atlantic Ocean and the World Ocean. The main purpose of this paper is to check the feasibility of the physical conditions for unbounded eddy elongation in the real ocean and estimate the fraction of mesoscale ocean eddies that are elongating into filaments and redistributing energy from the mesoscale to the submesoscale. We estimate the share of eddies in the World Ocean that are strongly elongated. We propose the maps of the geographical location of unlimited eddy elongation zones and give the interpretation of the obtained results. We also describe the effect of a decrease in the vortices’ self-energy due to the tensile action of the background flow. We suppose that it is a possible mechanism for energy transfer from the vortex to the flow and a manifestation of the negative viscosity effect.

Heat Transport by Mesoscale Eddies in the Norwegian and Greenland Seas

AbstractOcean vortices are an important regional agent of water transport and cross‐frontal exchange. This study is a first attempt to compare statistics of 3D properties of mesoscale eddies over the Norwegian and Greenland Seas. Results suggest that eddies in the central Greenland Sea are less intense, have smaller vertical extent and much smaller heat anomalies in their cores compared to eddies in the Lofoten Basin of the Norwegian Sea. In addition, these results suggest a relatively small inter‐basin eddy exchange. The large‐scale pattern of eddy translations shows that eddies cyclonically skirt the Norwegian‐Greenland region. There is also a regional cyclonic pattern in the Lofoten Basin with a consistent signature of eddy merger in the northern part of the basin. We confirm that eddies generated from the Norwegian Atlantic Slope Current (NwASC) have a significant effect on the amount of heat the NwASC brings to the Arctic. The heat lost from the NwASC between the Svinoy and Sorkapp sections associated with the westward eddy heat transport translates to 70 ± 23 TW, 90% of which occurs in the Lofoten Basin. This accounts to 35% of the heat advected by the NwASC across the Svinoy section and is comparable with the heat loss in the Barents Sea. Interannual variability of the heat flux due to a change in the number of generated eddies is relatively small (∼10 TW). Nevertheless, our estimates suggest that, by varying temperature of their cores, the generated eddies can effectively damp temperature anomalies that propagate north along the NwASC.

CAGE22-5 Cruise Report: High-resolution 2D and 3D seismic investigations on the Vøring Margin

The overall goal of this expedition was the acquisition of geophysical data on the Vøring Plateau and the southernmost part of the Lofoten Basin to improve the understanding of the subsurface structures and geological processes at four sites (U1571-U1574) where scientific drilling was recently successfully completed as a part of the the International Ocean Discovery Program (IODP) Expedition 396
 The cruise may be known as: CAGE22_5

Dynamical Controls of the Eastward Transport of Overwintering Calanus finmarchicus From the Lofoten Basin to the Continental Slope

AbstractDiapausing populations of Calanus finmarchicus at depth in the Lofoten Basin (LB) return to the continental shelf and slope off the Lofoten‐Vesterålen Islands during the phytoplankton spring bloom to feed and spawn, forming surface swarms with a great abundance. To study how overwintering populations of C. finmarchicus move with the deep currents and return to the shelf, Lagrangian transport characteristics of particles in deep water between 2008 and 2019 were analyzed using Global Ocean Reanalysis and Simulation re‐analysis data and Lagrangian Coherent Structures (LCSs). Our analyses revealed that persistent eastward transport of diapausing C. finmarchicus between LB and continental slope occurred mainly between 600 and 1,100 m in the Arctic Intermediate Water. The consistency of the vertical distributions of C. finmarchicus abundance and salinity further suggests that physical factors control the horizontal distribution of the species. Hovmöller diagrams of kinetic energy indicate that there is an eastward advection of mean current at depth. The co‐occurrence between the eastward transport of LCSs and the eastward advection of the mean current provides direct evidence that the life history of C. finmarchicus is subjected to physical control in the Norwegian Sea.

Вклад мезомасштабных вихрей Лофотенской котловины в ее энергетику

A comparative assessment of the contribution of eddies and other dynamic structures to the energy of the Lofoten Basin is carried out. The basis of the study is the data of global ocean reanalysis GLORYS12V1 for 1993–2019. We estimate the total kinetic and potential energy as well as the corresponding contribution of eddy energy for a region bounded by an isobath of 3000 m. We use the method of automatic identification of vortices for the analysis which allows us to calculate the eddy kinetic and potential energy in the automatically selected areas of the eddies. We establish that the potential energy of both cyclones and anticyclones is on average 2–3 times higher than the kinetic
 energy values, and the energy values of kinetic and potential for anticyclones dominate relative to the energy of cyclones. We also consider the interannual variability and seasonal course of eddy kinetic and potential energy. The seasonal course revealed an increase in both types of energy in the winter months. It is established that the contribution of eddies to the total energy of the basin is small. The contribution of eddy kinetic energy to the total energy of the basin is 7.3%, and the contribution of eddy potential energy is 8.4%. This means that the main contribution to the energy of the basin is made not by mesoscale eddies, but by other dynamic structures i.e. filaments and background flow.

Peculiarities of Formation of the Density Field in Mesoscale Eddies of the Lofoten Basin: Part 2

Peculiarities of Formation of the Density Field in Mesoscale Eddies of the Lofoten Basin: Part 2

Seasonal variability of the potential vorticity in the Lofoten vortex

The anticyclonic quasi-permanent Lofoten vortex is a strong dynamic formation in the center of the Lofoten Basin in the Norwegian Sea. We used the oceanic reanalysis data GLORYS12V1. We analyzed a seasonal distribution of the relative and potential vorticity, as well as the orbital velocities and the potential density in the Lofoten vortex. The main points of the potential vorticity calculating are considered. The values of the Lofoten vortex volume, the horizontal dimension (the diameter) as well as the vertical scale for the period from 2000 to 2019 are obtained, as well as the method for estimates of these characteristics is described in detail. It was shown that the intensification of the vortex occurs in the summer, and its relaxation -- in the winter.

Peculiarities of Formation the of Density Field in Mesoscale Eddies of the Lofoten Basin: Part 1

Peculiarities of Formation the of Density Field in Mesoscale Eddies of the Lofoten Basin: Part 1

A Satellite‐Based Lagrangian Perspective on Atlantic Water Fractionation Between Arctic Gateways

AbstractWarm Atlantic Water reaches the Arctic Ocean via two gateways: the Barents Sea Opening (BSO) and Fram Strait. Here, we study the near‐surface flow of the Atlantic Water in the Nordic Seas and its fractionation between these Arctic gateways, using simulated Lagrangian trajectories based on satellite altimetry for 1994–2018. Lagrangian particles are released in the eastern Nordic Seas, where Atlantic Water flows poleward in two current cores: an inner branch along the Norwegian Continental Slope and an outer sea ward branch. The trajectories toward Fram Strait and the BSO are, in an averaged sense, largely steered by the bottom topography, and on inter‐annual timescales we find an anticorrelation in the number of particles that reach the two gateways. Most of the particles released in the inner branch enter the Barents Sea and most of the particles seeded in the outer branch reach Fram Strait. However, there is a significant cross‐over of particles from the outer to the inner branch in the Lofoten Basin, and nearly half of the total number of particles entering the BSO originate in the outer branch. This cross‐over is accomplished solely by the time‐fluctuating part of the velocity field, and it becomes stronger when the eddy kinetic energy in the Lofoten Basin is anomalously high. Thus, the outer branch may, via processes in the Lofoten Basin, be important for Barents Sea climate variability.

Lagrangian modeling of water circulation in the Lofoten Basin

Lagrangian particle tracking is implemented for the Lofoten Basin of the Norwegian Sea. The ocean dynamic fields are obtained from the GLORYS 12V1 reanalysis available by the Copernicus Marine Environment Monitoring Service. Spatial distributions of the Lagrangian particles during May-November 2014 are analyzed for two depth layers: the sea-surface (0.5 m) and 266 m. The results show a significant impact of the Norwegian Coastal Current (NCC) on the thermohaline structure of the upper Lofoten Basin, underestimated previously. The NCC penetrates deep into the central Lofoten basin as far as the longitude 0°. In the subsurface layer, the area over which the NCC influences water structure is comparable to the area of the Norwegian Atlantic Slope Current (NASC), as well as to that of the Norwegian Atlantic Frontal Current (NAFC). The NCC maximum influence on the surface water of the Basin is reached in August. The inflow of the NCC is associated with relatively fresh water intrusions (0.5–2‰ fresher than the surrounding waters) moving from the coast to the central part of the Basin. The NASC and NAFC form two main sources of the Atlantic Water in the Lofoten Basin. At 266 m level, the NASC and NAFC waters dominate water structure in the basin. Herewith the NASC influence prevails over that of the NAFC, the latter being limited to the western periphery of the Basin. At this level, the NCC is observed only along a narrow band following the eastern coast. During summer, the core of the Lofoten Vortex (LV) at 266 m is mainly composed of the NAFC water. This fact contradicts the previous point of view of the dominance of the NASC in the LV core at all depth levels. Using two types of Lagrangian maps, we highlighted the summer and the autumn periods in the LV annual lifecycle. The summer LV is characterized by high orbital velocities, which are several times higher than those of the currents along the basin boundaries. The monthly mean orbital velocities in the LV reach 35 сm s−1. To the end of autumn, the LV weakens with the monthly mean orbital velocities below 10 cm s−1.

Study of the Mechanisms of Vortex Variability in the Lofoten Basin Based on Energy Analysis

Purpose. The Lofoten Basin is one of the most energetic zones of the World Ocean characterized by high activity of mesoscale eddies. The study is aimed at analyzing different components of general energy in the basin, namely the mean kinetic and vortex kinetic energy calculated using the integral of the volume of available potential and kinetic energy of the Lofoten Vortex, as well as variability of these characteristics. Methods and Results. GLORYS12V1 reanalysis data for the period 2010–2018 were used. The mean kinetic energy and the eddy kinetic one were analyzed; and as for the Lofoten Vortex, its volume available potential and kinetic energy was studied. Mesoscale activity of eddies in winter is higher than in summer. Evolution of the available potential energy and kinetic energy of the Lofoten Vortex up to the 1000 m horizon was studied. It is shown that the vortex available potential energy exceeds the kinetic one by an order of magnitude, and there is a positive trend with the coefficient 0,23·1015 J/year. It was found that in the Lofoten Basin, the intermediate layer from 600 to 900 m made the largest contribution to the potential energy, whereas the 0–400 m layer – to kinetic energy. The conversion rates of the mean kinetic energy into the vortex kinetic one, and the mean available potential energy into the vortex available potential one (baroclinic and barotropic instability) were analyzed. It is shown that the first type of transformation dominates in summer, while the second one is characterized by its increase in winter. Conclusions. The vertical profile shows that kinetic energy of eddies in winter is higher than in summer. The available potential energy of a vortex is by an order of magnitude greater than the kinetic energy. Increase in the available potential energy is confirmed by a significant positive trend and by decrease of the vortex Burger number. The graphs of the barotropic instability conversion rate demonstrate the multidirectional flows in the vortex zone with the dipole structure observed in a winter period, and the tripole one – in summer. The barotropic instability highest intensity is observed in summer. The baroclinic instability is characterized by intensification of the regime in winter that is associated with weakening of stratification in this period owing to winter convection.

Study of the Mechanisms of Vortex Variability in the Lofoten Basin Based on Energy Analysis

Study of the Mechanisms of Vortex Variability in the Lofoten Basin Based on Energy Analysis

Extreme Convective Events in the Lofoten Basin

Strong oceanic convection plays a crucial role in the variability of the global climate. In this study, we investigate extreme convection in the Lofoten Basin (LB) of the Norwegian Sea using Argo profiles. Mixed layer depth (MLD) is the main metric characterizing convection intensity. MLD exceeds 1000 m in March–April and December 2010 in the Lofoten Basin Eddy (LBE), whereas the mean MLD is about 200 m and seldom exceeds 400 m in the basin. We connect the extreme convection events with water volume formed at mid-depth of the central LB, between 1000 m depth and the isosteric surface s07. Analytical assessments of final mixing depth demonstrate perfect correspondence to measured values of MLD, indicating variations in the buoyancy flux and stratification as the main reasons for MLD variability in the basin. This variability is easily explained as a result of heat release to the atmosphere over the basin. Atmospheric conditions during extreme convection events are described. Northerly winds are as common as dominating southwesterly winds during the months with extreme convective events. We analyze 32 cases of extreme convective events with MLD exceeding 350 m and reveal that composite maps of sea level pressure (SLP) and surface heat flux match well with the North Atlantic Oscillation (NAO)−/East Atlantic Pattern (EAP)− pattern of atmospheric variability in the Northern Atlantic, with the negative NAO pattern prevailing in the climate during winter–spring. The heat release is the major trigger of extreme convection. We establish the stronger heat release associated with the extreme convection events in the LB.

The mesoscale eddy field in the Lofoten Basin from high-resolution Lagrangian simulations

Abstract. Warm Atlantic-origin waters are modified in the Lofoten Basin in the Nordic Seas on their way toward the Arctic. An energetic eddy field redistributes these waters in the basin. Retained for extended periods, the warm waters result in large surface heat losses to the atmosphere and have an impact on fisheries and regional climate. Here, we describe the eddy field in the Lofoten Basin by analyzing Lagrangian simulations forced by a high-resolution numerical model. We obtain trajectories of particles seeded at three levels – near the surface, at 200 m and at 500 m depth – using 2D and 3D velocity fields. About 200 000 particle trajectories are analyzed from each level and each simulation. Using multivariate wavelet ridge analysis, we identify coherent cyclonic and anticyclonic vortices in the trajectories and describe their characteristics. We then compare the evolution of water properties inside cyclones and anticyclones as well as in the ambient flow outside vortices. As measured from Lagrangian particles, anticyclones have longer lifetimes than cyclones (16–24 d compared to 13–19 d), a larger radius (20–22 km compared to 17–19 km) and a more circular shape (ellipse linearity of 0.45–0.50 compared to 0.51–0.57). The angular frequencies for cyclones and anticyclones have similar magnitudes (absolute values of about 0.05f). The anticyclones are characterized by warm temperature anomalies, whereas cyclones are colder than the background state. Along their path, water parcels in anticyclones cool at a rate of 0.02–0.04 ∘Cd-1, while those in cyclones warm at a rate of 0.01–0.02 ∘Cd-1. Water parcels experience a net downward motion in anticyclones and upward motion in cyclones, often found to be related to changes in temperature and density. The along-path changes in temperature, density and depth are smaller for particles in the ambient flow. An analysis of the net temperature and vorticity fluxes into the Lofoten Basin shows that while vortices contribute significantly to the heat and vorticity budgets, they only cover a small fraction of the domain area (about 6 %). The ambient flow, including filaments and other non-coherent variability undetected by the ridge analysis, hence plays a major role in closing the budgets of the basin.

Issledovaniye vikhrevoy izmenchivosti v Lofotenskoy kotlovine na osnove analiza dostupnoy potentsial'noy i kineticheskoy energii

Purpose. The Lofoten Basin is one of the most energetic zones of the World Ocean characterized by high activity of mesoscale eddies. The study is aimed at analyzing different components of general energy in the basin, namely the mean kinetic and vortex kinetic energy calculated using the integral of the volume of available potential and kinetic energy of the Lofoten Vortex, as well as variability of these characteristics. Methods and Results. GLORYS12V1 reanalysis data for the period 2010–2018 were used. The mean kinetic energy and the eddy kinetic one were analyzed; and as for the Lofoten Vortex, its volume available potential and kinetic energy were studied. The mesoscale activity of eddies in winter is higher than in summer. Evolution of the available potential energy and kinetic energy of the Lofoten Vortex up to the 1000 m horizon was studied. It is shown that the vortex available potential energy exceeds the kinetic one by an order of magnitude, and there is a positive trend with the coefficient 0,23⋅1015 J/year. It was found that in the Lofoten Basin, the intermediate layer from 600 to 900 m made the largest contribution to the potential energy, whereas the 0–400 m layer – to kinetic energy. The conversion rates of the mean kinetic energy into the vortex kinetic one and the mean available potential energy into the vortex available potential one (barotropic and baroclinic instability) were analyzed. It is shown that the first type of transformation dominates in summer, while the second one is characterized by its increase in winter. Conclusions. The vertical profile shows that the kinetic energy of eddies in winter is higher than in summer. The available potential energy of a vortex is by an order of magnitude greater than the kinetic energy. An increase in the available potential energy is confirmed by a significant positive trend and by a decrease in the vortex Burger number. The graphs of the barotropic instability conversion rate demonstrate the multidirectional flows in the vortex zone with the dipole structure observed in a winter period, and the tripole one – in summer. The barotropic instability highest intensity is observed in summer. The baroclinic instability is characterized by intensification of the regime in winter that is associated with weakening of stratification in this period owing to winter convection.