Beschreibung: For more than fifty years, it has been generally accepted by oceanographers that the Deep Western Boundary Current (DWBC) is the principal conduit of recently-convected Labrador Sea Water (LSW) exported from the high-latitude North Atlantic to the equator. Supporting this supposition is observational evidence that the waters of the DWBC have consistently greater equatorward velocities, higher concentrations of passive tracers, and younger ages compared to ocean interior waters. However, recent observations and simulations of floats launched in the DWBC in the Labrador Sea show that most water parcels are quickly ejected from the DWBC and follow instead interior pathways to the subtropics. Here, we show that tracer observations from the last three decades are compatible with the existence of both DWBC and basin-interior export pathways. From analyses of observational data and model output, we find that equatorward transport in the basin interior is consistent with the large-scale vorticity balance at mid-depth. Furthermore, from the modeling analysis we show that despite higher, localized concentrations of tracer and particles in the DWBC, only 5% of particles released in the Labrador Sea are transported from the subpolar to subtropical gyre via a continuous DWBC pathway. Thus, the interior pathway is a significant contributor to LSW export. Highlights: - Lagrangian observations of Labrador Sea Water match Eulerian observations - There is deep equatorward flow in the basin interior - This interior pathway is significant compared to the pathway along the boundary

Beschreibung: A sequence of global ocean circulation models, with horizontal mesh sizes of 0.5°, 0.25° and 0.1°, are used to estimate the long-term dispersion by ocean currents and mesoscale eddies of a slowly decaying tracer (half-life of 30 years, comparable to that of 137Cs) from the local waters off the Fukushima Dai-ichi Nuclear Power Plants. The tracer was continuously injected into the coastal waters over some weeks; its subsequent spreading and dilution in the Pacific Ocean was then simulated for 10 years. The simulations do not include any data assimilation, and thus, do not account for the actual state of the local ocean currents during the release of highly contaminated water from the damaged plants in March–April 2011. An ensemble differing in initial current distributions illustrates their importance for the tracer patterns evolving during the first months, but suggests a minor relevance for the large-scale tracer distributions after 2–3 years. By then the tracer cloud has penetrated to depths of more than 400 m, spanning the western and central North Pacific between 25°N and 55°N, leading to a rapid dilution of concentrations. The rate of dilution declines in the following years, while the main tracer patch propagates eastward across the Pacific Ocean, reaching the coastal waters of North America after about 5–6 years. Tentatively assuming a value of 10 PBq for the net 137Cs input during the first weeks after the Fukushima incident, the simulation suggests a rapid dilution of peak radioactivity values to about 10 Bq m−3 during the first two years, followed by a gradual decline to 1–2 Bq m−3 over the next 4–7 years. The total peak radioactivity levels would then still be about twice the pre-Fukushima values.

Beschreibung: Formed under low temperature – high pressure conditions vast amounts of methane hydrates are considered to be locked up in sediments of continental margins including the Arctic shelf regions [1, 2]. Because the Arctic has warmed considerably during the recent decades and because climate models predict accelerated warming if global greenhouse gas emissions continue to rise, it is debated whether shallow Arctic hydrate deposits could be destabilized in the near future [3, 4]. Methane (CH4), a greenhouse gas with a global warming potential about 25 times higher than CO2, could be released from the melting hydrates and enter the water column and atmosphere with uncertain consequences for the environment. Here we present the results of a recent comprehensive study of the future fate of Arctic methane hydrates [5]. Our multi-disciplinary analysis provides a closer look into regional developments of submarine Arctic gas hydrate deposits under future global warming scenarios and reveals where and over which time scales gas hydrates could be destabilized and affect oceanic pH, oxygen, and atmospheric methane. Arctic bottom water temperatures and their future evolution are projected by a climate model. Predicted bottom water warming is spatially inhomogeneous, with strongest impact on shallow regions affected by Atlantic inflow. Within the next 100 years, the warming affects 25% of shallow and mid-depth regions (water depth 〈 600 m) containing methane hydrates. We have quantified methane release from melting hydrates using transient models resolving the change in stability zone thickness. Due to slow heat diffusion rates, the change in stability zone thickness over the next 100 years is small and methane release limited. Even if these methane emissions were to reach the atmosphere, their climatic impact would be negligible as a climate model run confirms. However, the released methane, if dissolved into the water column, may contribute to ocean acidification and oxygen depletion in the water column. [1]Hester, K.C. and P.G. Brewer, Clathrate Hydrates in Nature. Annual Review of Marine Science, 2009. 1: p. 303-327. [2]Buffett, B.A. and D. Archer, Global inventory of methane clathrate: Sensitivity to changes in the deep ocean. Earth and Planetary Science Letters, 2004. 227: p. 185 - 199. [3]Reagan, M.T. and G.J. Moridis, Oceanic gas hydrate instability and dissociation under climate change scenarios. 2007. 34: p. L22709. [4]Kerr, R.A., 'Arctic Armageddon'Needs More Science, Less Hype. Science, 2010. 329: p. 620. [5]Biastoch, A., et al., Rising Artic ocean temperatures cause gas hydrate destabilization and ocean acidification. Geophysical Research Letters, 2011. 38(L08602).