Bogi Hansen

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.

Reversal of the 1960s to 1990s freshening trend in the northeast North Atlantic and Nordic Seas

Hydrographic time series in the northeast North Atlantic and Nordic Seas show that the freshening trend of the 1960s–1990s has completely reversed in the upper ocean. Since the 1990s temperature and salinity have rapidly increased in the Atlantic Inflow from the eastern subpolar gyre to the Fram Strait. In 2003–2006 salinity values reached the previous maximum last observed around 1960, and temperature values exceeded records. The mean properties of the Atlantic Inflow decrease northwards, but variations seen in the eastern subpolar gyre at 57°N persist with the same amplitude and pattern along the pathways to Fram Strait. Time series correlations and extreme events suggest a time lag of 3–4 years over that distance. This estimate allows predictions to be made; the temperature of Atlantic water in the Fram Strait may start to decline in 2007 or 2008, salinity a year later, but both will remain high at least until 2010.

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.

Observed and modelled stability of overflow across the Greenland–Scotland ridge

Across the Greenland–Scotland ridge there is a continuous flow of cold dense water, termed ‘overflow’, from the Nordic seas to the Atlantic Ocean. This is a main contributor to the production of North Atlantic Deep Water that feeds the lower limb of the Atlantic meridional overturning circulation, which has been predicted to weaken as a consequence of climate change. The two main overflow branches pass the Denmark Strait and the Faroe Bank channel. Here we combine results from direct current measurements in the Faroe Bank channel for 1995–2005 with an ensemble hindcast experiment for 1948–2005 using an ocean general circulation model. For the overlapping period we find a convincing agreement between model simulations and observations on monthly to interannual timescales. Both observations and model data show no significant trend in volume transport. In addition, for the whole 1948–2005 period, the model indicates no persistent trend in the Faroe Bank channel overflow or in the total overflow transport, in agreement with the few available historical observations. Deepening isopycnals in the Norwegian Sea have tended to decrease the pressure difference across the Greenland–Scotland ridge, but this has been compensated for by the effect of changes in sea level. In contrast with earlier studies, we therefore conclude that the Faroe Bank channel overflow, and also the total overflow, did not decrease consistently from 1950 to 2005, although the model does show a weakening total Atlantic meridional overturning circulation as a result of changes south of the Greenland–Scotland ridge.

8 Ecological features and recent trends in the physical environment, plankton, fish stocks, and seabirds in the Faroe shelf ecosystem

Abstract The Faroe shelf water is relatively well separated from the offshore water by a persistent tidal front, which surrounds the islands. The shelf water has neritic phyto- and zooplankton communities, which to a large extent are separated from the surrounding offshore area, although receiving variable influence from the offshore environment. The shelf production of plankton is the basis for production in the higher trophic levels within the ecosystem. The plankton production is interannually variable and in general monitoring data show simultaneous fluctuations at several trophic levels in the ecosystem, including calculated new primary production, fish recruitment, growth and landings, and seabird recruitment and growth. The paper gives an overview of trophic interactions within the Faroe shelf ecosystem and variability in production in the various trophic levels. The production and harvesting potential of the ecosystem is discussed.

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.

Growth, maturation, diet and distribution of saithe (Pollachius virens) in Faroese waters (NE Atlantic)

Abstract Saithe (Pollachius virens) in Faroese waters is a stock that is very important for the Faroese economy. Previous studies have provided information on juvenile saithe, but only sporadic information on the general biology of adult saithe is available in the published literature. Here, we present the basic biology of Faroe saithe based on data from Faroese groundfish surveys. Spawning appears presently to occur earlier in the year than in the early twentieth century, and the main spawning seems to occur on the eastern part of the Faroe Plateau. There is a gradual movement of saithe into deeper water with increasing size, which may well reflect a shift in diet with age. Together with changes associated with maturation, this may explain the change observed in the growth pattern around the age of 4 years. The diet of adult Faroe saithe is dominated by young blue whiting in August, whereas Norway pout, sandeel and blue whiting are equally important in March, but the stomach fullness is substantially higher in summer.

Using an ''Inverse Dynamic Method'' to Determine Temperature and Salinity Fields from ADCP Measurements

Abstract In heavily fished areas, upward looking acoustic Doppler current profilers (ADCPs), moored at depth, may be the only option for long-term current measurements. Arrays of ADCP moorings that cross a current can thus be the optimal strategy for monitoring the volume flux. These instruments only measure water properties at the instrument, not through the water column, however. By itself, an ADCP array, therefore, does not give flux estimates of specific water masses unless temperature and salinity profiles can be derived from the velocity profiles. This is the opposite of the classical problem of determining currents from temperature and salinity observations, and in principle it should be possible to solve it by inverting the classical dynamic method. As for the classical method, this problem requires additional reference information. Using observations from the Faroe Current between Iceland and the Faroe Islands, it is demonstrated that this procedure can indeed be used by applying empirical orthog...

Biased thermohaline exchanges with the arctic across the Iceland-Faroe Ridge in ocean climate models

The northern limb of the Atlantic thermohaline circulation and its transport of heat and salt towards the Arctic strongly modulate the climate of the Northern Hemisphere. The presence of warm surface waters prevents ice formation in parts of the Arctic Mediterranean, and ocean heat is directly available for sea-ice melt, while salt transport may be critical for the stability of the exchanges. Through these mechanisms, ocean heat and salt transports play a disproportionally strong role in the climate system, and realistic simulation is a requisite for reliable climate projections. Across the Greenland–Scotland Ridge (GSR) this occurs in three well-defined branches where anomalies in the warm and saline Atlantic inflow across the shallow Iceland–Faroe Ridge (IFR) have been shown to be particularly difficult to simulate in global ocean models. This branch (IF-inflow) carries about 40 % of the total ocean heat transport into the Arctic Mediterranean and is well constrained by observation during the last 2 decades but associated with significant interannual fluctuations. The inconsistency between model results and observational data is here explained by the inability of coarse-resolution models to simulate the overflow across the IFR (IF-overflow), which feeds back onto the simulated IFinflow. In effect, this is reduced in the model to reflect only the net exchange across the IFR. Observational evidence is presented for a substantial and persistent IF-overflow and mechanisms that qualitatively control its intensity. Through this, we explain the main discrepancies between observed and simulated exchange. Our findings rebuild confidence in modelled net exchange across the IFR, but reveal that compensation of model deficiencies here through other exchange branches is not effective. This implies that simulated ocean heat transport to the Arctic is biased low by more than 10 % and associated with a reduced level of variability, while the quality of the simulated salt transport becomes critically dependent on the link between IF-inflow and IF-overflow. These features likely affect sensitivity and stability of climate models to climate change and limit the predictive skill.