William R. Turrell

Decreasing overflow from the Nordic seas into the Atlantic Ocean through the Faroe Bank channel since 1950.

The overflow of cold, dense water from the Nordic seas, across the Greenland–Scotland ridge and into the Atlantic Ocean is the main source for the deep water of the North Atlantic Ocean. This flow also helps drive the inflow of warm, saline surface water into the Nordic seas. The Faroe Bank channel is the deepest path across the ridge, and the deep flow through this channel accounts for about one-third of the total overflow. Previous work has demonstrated that the overflow has become warmer and less saline over time. Here we show, using direct measurements and historical hydrographic data, that the volume flux of the Faroe Bank channel overflow has also decreased. Estimating the volume flux conservatively, we find a decrease by at least 20 per cent relative to 1950. If this reduction in deep flow from the Nordic seas is not compensated by increased flow from other sources, it implies a weakened global thermohaline circulation and reduced inflow of Atlantic water to the Nordic seas.

Upper layer cooling and freshening in the Norwegian Sea in relation to atmospheric forcing

Abstract Several time series in the Norwegian Sea indicate an upper layer decrease in temperature and salinity since the 1960s. Time series from Weather Station “M”, from Russian surveys in the Norwegian Sea, from Icelandic standard sections, and from Scottish and Faroese observations in the Faroe–Shetland area have similar trends and show that most of the Norwegian Sea is affected. The reason is mainly increased freshwater supply from the East Icelandic Current. As a result, temperature and salinity in some of the time series were lower in 1996 than during the Great Salinity Anomaly in the 1970s. There is evidence of strong wind forcing, as the NAO winter index is highly correlated with the lateral extent of the Norwegian Atlantic Current. Circulation of Atlantic water into the western Norwegian and Greenland basins seems to be reduced while circulation of upper layer Arctic and Polar water into the Norwegian Sea has increased. The water-mass structure is further affected in a much wider sense by reduced deep-water formation and enhanced formation of Arctic intermediate waters. A temperature rise in the narrowing Norwegian Atlantic Current is strongest in the north.

Decadal variability in the composition of Faroe Shetland Channel bottom water

Two standard sections across the deep water channel separating the Faroese Plateau from the Scottish continental shelf have been surveyed regularly since the start of the 20th century. There have been significant changes in the characteristics of surface, intermediate and deep water masses during this period. At intermediate depths, the presence of Norwegian Sea Arctic Intermediate Water (NSAIW) was evident as a salinity minimum during the first decade of the century. During the decades 1960–1980 this salinity minimum disappeared, and only four water types were identified in the Channel. Since 1980 the salinity of the intermediate water has again decreased, due to changes in the atmospheric forcing over the Nordic Seas, and it is again evident on a θS curve as a distinct minimum. The salinity of the bottom water in the Channel has also decreased (0.01/decade) linearly since the mid-1970s, although at a slower rate than the intermediate water (0.02/decade). The decline in salinity of the bottom water cannot be accounted for by changes in the salinity of upper Norwegian Sea Deep Water (NSDW), which Faroe Shetland Channel Bottom Water (FSCBW) has traditionally been assumed to be composed of. There is evidence that the upper level of NSDW has become deeper outside the Channel owing to a reduced supply from the Greenland Sea. This has resulted in a change in the composition of FSCBW, from being approximately 60% NSDW during the period 1970–1985 to 40% NSDW since 1990. Thus, the thermohaline circulation of the Nordic Seas has lost its deep water connection. The associated freshening of FSCBW has propagated out through the Channel into the North Atlantic and has resulted in a reduction of the salinity (0.02/decade) and transport (1–7%/decade) of Iceland Scotland Overflow Water (ISOW) into the North Atlantic.

The use of simple models in the regulation of the impact of fish farms on water quality in Scottish sea lochs

Abstract In Scotland, the fish farming industry has developed over the past two decades to the point where farms are now located in the majority of sea lochs. The expansion of the industry, in terms of new farm sites and increased production at existing ones, is controlled by several regulatory authorities. As part of this regulatory process, a suite of simple box models has been developed to provide a more scientific and consistent basis for the assessment of the relative potential environmental impact of new and existing farms. The potential enhancement of nutrient levels within sea lochs due to fish farms is predicted. Simulations of the dispersion of chemicals following treatments for sea lice infestation allow regulation of the use of these chemicals to ensure compliance with environmental quality standards (EQS). The models make many simplifying assumptions about the underlying hydrography of sea lochs. However, they provide a first estimate of possible effects, and as such, have proven a useful management tool.

Deep-Water Renewal in the Upper Basin of Loch Sunart, a Scottish Fjord

Abstract Recording current meters were deployed near the surface and bottom in the upper basin of Loch Sunart during the summers of 1987, 1989, and 1990. The measurements revealed frequent, though irregular, deep-water renewal events when the basin water was replaced by density driven intrusions. The data suggested that these events were induced by wind-enhanced estuarine circulation during periods of low fresh water runoff. A semi-implicit, laterally integrated estuarine model was modified to allow variable vertical grid spacing and to permit the sea surface to move vertically through the grid, both modifications being necessary in order to apply the model to the sea loch environment. A turbulence closure scheme was also incorporated. The model can simulate the evolution of the salinity field over periods of several weeks and was used to examine the mechanisms that induce deep-water renewal. Model results suggested that very weak stratification in the basin is required for renewal to occur and indicated ...

Observed transport estimates between the North Atlantic and the Arctic Mediterranean in the Iceland–Scotland region

The Arctic Mediterranean is the ocean area north of the Greenland-Scotland Ridge. Exchanges between this region and the North Atlantic both provide the main source for production of North Atlantic Deep Water and supply heat and salt to the northern oceans. The exchange occurs through several gaps in the ridge; in terms of volume flux the Iceland-Scotland Gap is the most important one as it carries more than half the total, with approximately three quarters of the total inflow and one third of the total outflow. The Nordic WOCE observational system was initiated to monitor the exchanges through this gap and it has provided data that allow estimates of typical fluxes and their seasonal variation. The flux measurements show that most of the Atlantic inflow to the Arctic Mediterranean returns as overflow and hence the processes forming intermediate and deep waters in the Arctic Mediterranean are the main forcing mechanism for the Atlantic inflow. The inflow between Iceland and Scotland seems to be a maximum in late winter while the Faroe Bank Channel overflow is strongest in late summer. Using the results from the Nordic WOCE system it has been possible to interpret historical observations from Ocean Weather Ship Station M and conclude that the flux of the Faroe Bank Channel overflow decreased in magnitude from 1950 to 2000.

Wind-driven monthly variations in transport and the flow field in the Faroe-Shetland Channel

The transport of water from the North Atlantic to the Nordic seas through the Faroe–Shetland Channel is analysed from a decade of conductivity, temperature and depth (CTD) and acoustic Doppler current profiler (ADCP) data. The long-term mean transport, integrated over the upper 500 m, is 3.5 ± 0.1 Sv (1 Sv =106m3s-1), of which 2.1 Sv is barotropic flow and 1.4 Sv is baroclinic flow. Short-term variability leads to a standard deviation of ca. 2.2 Sv in 3-day averages of the ADCP-measured transport. The barotropic transport is located over the upper part of the slope region of the Shetland Shelf, but sometimes broadens over deeper water. There is a peak surface baroclinic transport above the foot of the slope, and a weak recirculation of Modified North Atlantic Water (MNAW), which enters from the north, on the Faroese side. In September, when isobars downwell on the eastern side, the strong transport (ca. 4 Sv) is barotropic and evenly distributed across the Shetland slope, and both recirculation of MNAW from the Faroe side and mesoscale activity are weak. In spring, the net transport is small (ca. 2.5 Sv), the MNAW recirculation is strong and mesoscale activity is relatively large. These seasonal variations appear to correlate with the local south-west wind stress, which may contribute to nearly half of the long-term transport in the channel.

Disentangling habitat concepts for demersal marine fish management

Fishing and other anthropogenic impacts have led to declines in many fish stocks and modification of the seabed. As a result, efforts to restore marine ecosystems have become increasingly focused on spatially explicit management methods to protect fish and the habitats they require for survival. This has led to a proliferation of investigations trying to map ‘habitats’ vulnerable to anthropogenic impacts and identify fish resource requirements in order to meet conservation and management needs. A wide range of habitat-related concepts, with different uses and understandings of the word ‘habitat’ itself has arisen as a consequence. Inconsistencies in terminology can cause confusion between studies, making it difficult to investigate and understand the ecology of fish and the factors that affect their survival. Ultimately, the inability to discern the relationships between fish and their environment clearly can hinder conservation and management measures for fish populations. This review identifies and addresses the present ambiguity surrounding definitions of ‘habitat’ and habitat-related concepts currently used in spatial management of demersal marine fish populations. The role of spatial and temporal scales is considered, in addition to examples of how to assess fish habitat for conservation and management purposes.

Operational oil spill trajectory modelling using HF radar currents: A northwest European continental shelf case study

This paper presents a novel operational oil spill modelling system based on HF radar currents, implemented in a northwest European shelf sea. The system integrates Open Modal Analysis (OMA), Short Term Prediction algorithms (STPS) and an oil spill model to simulate oil spill trajectories. A set of 18 buoys was used to assess the accuracy of the system for trajectory forecast and to evaluate the benefits of HF radar data compared to the use of currents from a hydrodynamic model (HDM). The results showed that simulated trajectories using OMA currents were more accurate than those obtained using a HDM. After 48h the mean error was reduced by 40%. The forecast skill of the STPS method was valid up to 6h ahead. The analysis performed shows the benefits of HF radar data for operational oil spill modelling, which could be easily implemented in other regions with HF radar coverage.

An assessment of juvenile Atlantic cod Gadus morhua distribution and growth using diver operated stereo-video surveys.

Stereo‐video scuba transects were conducted during daylight hours from June to September 2013 within a proposed marine protected area (MPA) in the Firth of Clyde, west of Scotland. More juvenile Atlantic cod Gadus morhua of fork length (L F) range 6–11 cm were observed in substrata containing mixed gravel, including maerl, than in boulder‐cobble substrata with high algal cover, or sand with low density seagrass. Community composition was significantly different between substratum types. A decrease in G. morhua abundance was observed over the period of data collection. Over time, mean and variance in G. morhua L F increased, indicating multiple recruitment events. Protecting mixed gravel substrata could be a beneficial management measure to support the survival and recruitment of juvenile G. morhua; other substrata might be important at night given their diel migratory behaviour. Stereo‐video cameras provide a useful non‐destructive fisheries‐independent method to monitor species abundance and length measurements.