Decadal variability in the outflow from the Nordic seas to the deep Atlantic Ocean

Recent evolution of 129I levels in the Nordic Seas and the North Atlantic Ocean

Most of the anthropogenic radionuclide ¹²⁹I released to the marine environment from the nuclear fuel reprocessing plants (NFRP) at Sellafield (England) and La Hague (France) is transported to the Arctic Ocean via the North Atlantic Current and the Norwegian Coastal Current. ¹²⁹I concentrations in seawater provides a powerful and well-established radiotracer technique to provide information about the mechanisms which govern water mass transport in the Nordic Seas and the Arctic Ocean and is gaining importance when coupled with other tracers (e.g. CFC, ²³⁶U). In this work, ¹²⁹I concentrations in surface and depth profiles from the Nordic Seas and the North Atlantic (NA) Ocean collected from four different cruises between 2011 and 2012 are presented. This work allowed us to i) update information on ¹²⁹I concentrations in these areas, required for the accurate use of ¹²⁹I as a tracer of water masses; and ii) investigate the formation of deep water currents in the eastern part of the Nordic Seas, by the analysis of ¹²⁹I concentrations and temperature-salinity (T-S) diagrams from locations within the Greenland Sea Gyre. In the Nordic Seas, ¹²⁹I concentrations in seawater are of the order of 10⁹ at·kg^-1, one or two orders of magnitude higher than those measured at the NA Ocean, not so importantly affected by the releases from the NFRP. ¹²⁹I concentrations of the order of 10⁸atoms·kg^-1 at the Ellet Line and the PAP suggest a direct contribution from the NFRP in the NA Ocean. An increase in the concentrations in the Nordic Seas between 2002 and 2012 has been detected, which agrees with the temporal evolution of the ¹²⁹I liquid discharges from the NFRPs in years prior to this. Finally, ¹²⁹I profile concentrations, ¹²⁹I inventories and T-S diagrams suggest that deep water formation occurred in the easternmost area of the Nordic Seas during 2012.

Fluid mixing in stratified gravity currents: the Prandtl mixing length

Shear-induced vertical mixing in a stratified flow is a key ingredient of thermohaline circulation. We experimentally determine the vertical flux of momentum and density of a forced gravity current using high-resolution velocity and density measurements. A constant eddy-viscosity model provides a poor description of the physics of mixing, but a Prandtl mixing length model relating momentum and density fluxes to mean velocity and density gradients works well. For the average gradient Richardson number Ri(g) approximately 0.08 and a Taylor Reynolds number Re(lambda) approximately 100, the mixing lengths are fairly constant, about the same magnitude, comparable to the turbulent shear length.

A review of the possible impacts of long-term oceanic and climate changes and fishing mortality on recruitment of anguillid eels of the Northern Hemisphere

Possible causes of declines in recruitment of European, American and Japanese eels to continental waters are reviewed. Negative correlations between the Den Oever glass eel recruitment index (DOI) and the North Atlantic Oscillation Index since 1938 are discussed, together with older anecdotal evidence. Correlations are established between the DOI and sea surface temperature anomalies at 100-250 m between 1952 and 1995 in the Sargasso Sea/Sub-Tropical Gyre (STG) spawning area. It is hypothesised that, associated with global warming trends, STG warming inhibits spring thermocline mixing and nutrient circulation, with negative impacts on productivity and hence food for leptocephalus larvae. Concurrent gyre spin-up also affects major currents and slowing of oceanic migration has probably enhanced starvation and predation losses. Local factors, such as unfavourable wind-driven currents, can also affect recruitment of glass eels on continental shelves. In contrast, evidence is discussed that indicates fishing mortality and continental climate change appear to have had lesser impacts. Similar starvation-advection explanations for declines in Japanese eel recruitment are proposed. Predictions for the future are made and multidisciplinary and integrated monitoring and research are recommended for managing eel stocks and fisheries.

Ocean circulation and climate during the past 120,000 years

Oceans cover more than two-thirds of our blue planet. The waters move in a global circulation system, driven by subtle density differences and transporting huge amounts of heat. Ocean circulation is thus an active and highly nonlinear player in the global climate game. Increasingly clear evidence implicates ocean circulation in abrupt and dramatic climate shifts, such as sudden temperature changes in Greenland on the order of 5-10 degrees C and massive surges of icebergs into the North Atlantic Ocean --events that have occurred repeatedly during the last glacial cycle.

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.

Temporal trends in deep ocean Redfield ratios

The Redfield ratio [carbon:nitrogen:phosphorus (C:N:P)] of particle flux to the deep ocean is a key factor in marine biogeochemical cycling. Changes in oceanic carbon sequestration have been linked to variations in the Redfield ratio on geological time scales, but this ratio generally is assumed to be constant with time in the modern ocean. However, deep-water Redfield ratios in the northern hemisphere show evidence for temporal trends over the past five decades. The North Atlantic Ocean exhibits a rising N:P ratio, which may be related to increased deposition of atmospheric nitrous oxides from anthropogenic N emissions. In the North Pacific Ocean, increasing C:N and C:P ratios are accompanied by rising remineralization rates, which suggests intensified export production. Stronger export of carbon in this region may be due to enhanced bioavailability of aeolian iron. These findings imply that the biological part of the marine carbon cycle currently is not in steady state.

Slope water current over the laurentian fan on interannual to millennial time scales

The strength and position of surface and deep currents in the slope water south of Newfoundland are thought to vary as a coupled system in relation to the dipole in atmospheric sea level pressure known as the North Atlantic oscillation (NAO). Paleoceanographic data from the Laurentian Fan, used as a proxy for sea surface temperature, reveal that surface slope waters north of the Gulf Stream experienced warming during the Little Ice Age of the 16th to 19th centuries and support the notion of an NAO-driven coupled system. The NAO may be a useful model for millennial-scale ocean variability during interglacial climate states.