I. H. Harms

Modelling Siberian river runoff — implications for contaminant transport in the Arctic Ocean

Abstract This model study investigates the role of Siberian river runoff for the transport of possible river contaminants in the Arctic Ocean. Three-dimensional coupled ice-ocean-models of different horizontal resolution are applied to simulate the dispersion of river water from Ob, Yenisei and Lena. These Siberian rivers are supposed to be important sources for various contaminants. The relevant processes which are considered in this study include the dispersion of dissolved or suspended contaminants in the water column and the transport of contaminated particles, incorporated into drifting sea ice. Circulation model results from both spatial scales explain the main pathways and transit times of Siberian river water in the Arctic Ocean. Kara Sea river water clearly dominates in the Siberian branch of the Transpolar Drift, while the Lena water dominates in the Canadian branch. River water concentrations in Nares Strait, Canadian Archipelago, are similar to those in the northern Fram Strait. Special emphasis is given to the seasonal variability of the river plume in the Kara Sea. Particle tracking simulations on the regional scale illustrate that Ob and Yenisei tracers behave differently. Yenisei tracers leave the Kara Sea quite fast towards the Arctic Ocean or the Laptev Sea, but Ob tracers spread also in the southern Kara Sea, in particular at lower levels. A comparison of simulated freezing rates and particle concentrations in Siberian coastal waters suggests that during autumn, the incorporation of particles into freezing sea ice near the estuaries of Ob and Yenisei is very likely. Simulated ice trajectories, started close to the Lena river delta easily reach the multi-year Transpolar Drift within one winter. Ice trajectories from Ob and Yenisei estuaries, however, mostly drift towards the Barents Sea where the ice melts close to Svalbard. The model study confirms that contaminant transport through sediment-laden sea ice offers a short and effective pathway for pollutant transport from Siberian rivers to the Barents and Nordic Seas.

Modeling the seasonal variability of hydrography and circulation in the Kara Sea

In the frame of a project on transport of contaminants in the Arctic, the Hamburg shelf ocean model (HAMSOM) is applied to the Kara Sea. The HAMSOM system consists of a three-dimensional, baroclinic circulation model coupled to a thermodynamic and dynamic sea ice model. The Kara Sea model is forced with climatological winds, atmospheric heat fluxes, river runoff, and tides. The obtained results reveal no typical Kara Sea circulation that prevails throughout the year. Instead, the model showed a strong seasonal variability in circulation and hydrography due to winds, freshwater runoff, and ice formation. The circulation is weakest in spring when the wind speeds are low and horizontal density gradients are small. Fresh water from the rivers spreads toward the north and northwest rather than forming a coastally trapped current that flows to the east. In autumn, the circulation is significantly enhanced because of increasing wind speeds and strong horizontal density gradients. Good agreement was found between model results and recent observations. The “classical” cyclonic current pattern in the southern Kara Sea, however, was not reproduced by the model.

Modelling the dispersion of 137Cs and 239Pu released from dumped waste in the Kara Sea

Abstract Three-dimensional, baroclinic, circulation models are applied to study the dispersal of radioactivity in the Barents Sea and Kara Sea. The release is supposed to occur at underwater dump sites for radioactive waste in the Kara Sea, used by the former Soviet Union. Two different spatial scales of dispersion are considered: the regional scale (the far field), which covers the shelves of the Barents and Kara Seas and the local scale (the near field) which is focused mainly on Abrasimov Bay where the dumping partly took place. The regional-scale model results suggest that, even for a worst case scenario, the radioactive contamination of Siberian coastal waters would be relatively small compared to observations in other marine systems (e.g the Baltic Sea and the Irish Sea). Realistic gradual release scenarios show very low concentrations in the central and eastern Kara Sea. A significant contamination of surrounding seas like the Laptev Sea, the Arctic Ocean or the Barents Sea by radioactive waste dispersion from the Kara Sea seems to be unlikely.

Anthropogenic radioactivity in the Arctic Ocean } review of the results from the joint German project

The paper presents the results of the joint project carried out in Germany in order to assess the consequences in the marine environment from the dumping of nuclear wastes in the Kara and Barents Seas. The project consisted of experimental work on measurements of radionuclides in samples from the Arctic marine environment and numerical modelling of the potential pathways and dispersion of contaminants in the Arctic Ocean. Water and sediment samples were collected for determination of radionuclide such as 137Cs, 90Sr, 239 + 240Pu, 238Pu, and 241Am and various organic micropollutants. In addition, a few water and numerous surface sediment samples collected in the Kara Sea and from the Kola peninsula were taken by Russian colleagues and analysed for artificial radionuclide by the BSH laboratory. The role of transport by sea ice from the Kara Sea into the Arctic Ocean was assessed by a small subgroup at GEOMAR. This transport process might be considered as a rapid contribution due to entrainment of contaminated sediments into sea ice, following export from the Kara Sea into the transpolar ice drift and subsequent release in the Atlantic Ocean in the area of the East Greenland Current. Numerical modelling of dispersion of pollutants from the Kara and Barents Seas was carried out both on a local scale for the Barents and Kara Seas and for long range dispersion into the Arctic and Atlantic Oceans. Three-dimensional baroclinic circulation models were applied to trace the transport of pollutants. Experimental results were used to validate the model results such as the discharges from the nuclear reprocessing plant at Sellafield and subsequent contamination of the North Sea up the Arctic Seas.

SNAC: a statistical emulator of the north-east Atlantic circulation

Abstract The three-dimensional statistical emulator SNAC (Statistical North-east Atlantic Circulation) for operational computation of the oceanic circulation in the north-eastern North Atlantic is presented. SNAC was trained with the output of the numerical three-dimensional ocean model HAMSOM (HAMburg Shelf Ocean Model) and determines the most probable circulation from a prescribed transient weather situation. Air pressure distributions derived from a network of eight weather stations (i.e. airports) provide the forcing. The emulator uses the statistical correlation between air pressure differences amongst selected stations and oceanic currents. Horizontal currents and sea surface elevation are computed on a spherical grid with a resolution of 0.25° (zonal) and 0.125° (meridional). Vertically, the water column is resolved by 11 layers. The emulator domain extends from 44° W to 15.75° E, and from 48.5° to 70° N. SNAC is able to reproduce the temporal variability simulated by numerical models on time scales between 2 months and 1 year, though the program runs up to 500 times faster. Optionally, a simple M2 tide model can be integrated into the output. As an alternative to a more complex numerical circulation model, SNAC requires far less atmospheric forcing data and much less computational effort. Presently, SNAC provides circulation fields for a tracer model used in simulating the dispersal of fish larvae around Iceland and on the north-west European continental shelf. It might equally well be used to provide boundary values for regional hydrodynamic ocean models. The program can be downloaded from www.ifm.uni-hamburg.de/~logemann/snac/ .

Recent freshening in the Kara Sea (Siberia) recorded by stable isotopes in Arctic bivalve shells

Oxygen and stable carbon isotope records along the growth direction on shells of the bivalve species Astarte borealis and Serripes groenlandicus reliably record all important aspects of the bottom water hydrography in the shallow southeastern Kara Sea, despite uncertainties about the isotopic range due to sparse sampling and the possibility of growth rate changes. Changing freshwater supply from the rivers Ob and Yenisei is the main cause for seasonal temperature and salinity variations near the three sampling locations in 20 to 70 m water depth as suggested by CTD measurements and modeling. Peak winter salinity of the simulated hydrographic data series and peak winter values in the isotope records follow negative trends, which indicate a freshening of the bottom water due to an increasing fraction of river water during the 1990s. This freshening affected the whole Kara Sea, and coincided with a lowering of regional air pressure gradients, as indicated by the declining Arctic oscillation index. The resulting weakening of the prevailing southwesterly winds diminished the inflow of saline Atlantic-derived water from the Barents Sea through the Kara Strait in the southwest, and, additionally, reduced the export of river water toward the north and northeast into the Arctic basin. Saline Atlantic-derived water thus was replaced by freshwater, which was successively accumulated in the Kara Sea and accordingly imprinted on the stable isotope composition of the bivalve shells. The 1990s freshening in the Kara Sea thus may be caused by natural variations rather than being a signal for global change.

On the potential for climate change impacts on marine anthropogenic radioactivity in the Arctic regions

Current predictions as to the impacts of climate change in general and Arctic climate change in particular are such that a wide range of processes relevant to Arctic contaminants are potentially vulnerable. Of these, radioactive contaminants and the processes that govern their transport and fate may be particularly susceptible to the effects of a changing Arctic climate. This paper explores the potential changes in the physical system of the Arctic climate system as they are deducible from present day knowledge and model projections. As a contribution to a better preparedness regarding Arctic marine contamination with radioactivity we present and discuss how a changing marine physical environment may play a role in altering the current understanding pertaining to behavior of contaminant radionuclides in the marine environment of the Arctic region.

Transport of radionuclides by sea-ice and dense-water formed in western Kara Sea flaw leads

A transport assessment of particle-bound and dissolved artificial radionuclides (137Cs and 239,240Pu) by sea-ice and dense-water formed in western Kara Sea flaw leads close to the Novaya Zemlya dumping sites is presented in this study. We both performed a “best estimate” based on available data, and a “maximum assessment” relying on simulated constant releases of 1 TBq 137Cs and 239,240Pu from individual dumping bays. The estimates are based on a combination of (i) the content of particulate matter in sea-ice; (ii) analytical data and numerical simulations of radionuclide concentrations in shelf surface deposits, suspended particulate matter (SPM), and the dissolved phase; and (iii) estimates of lead-ice and dense-water formation rates as well as modeling results of local ice drift pathways. In the “best estimate” case, 2.90 GBq 137Cs and 0.51 GBq 239,240Pu attached to sea-ice sediments can be exported from the lead areas toward the central Arctic basin. The radionuclide burden of the annually formed dense lead water in the “best estimate” amounts to 4.68 TBq 137Cs and 0.014 TBq 239,240Pu. In the “maximum assessment”, potential export-rates of ice-particle bound 137Cs and 239,240Pu toward the central Arctic would amount to 0.64 and 0.16 TBq, respectively. As much as ≈900 TBq 137Cs and ≈6.75 TBq 239,240Pu could be annually taken up by 34.75 dense-water rejected in the lead area. Assuming the (unlikely) instantaneous release of the total 137Cs and 239,240Pu inventories (≈1 PBq and 10 TBq, respectively) from the Novaya Zemlya dumping sites into the dissolved phase, the dense lead water locally formed during one winter season could take up ≈90% of the Cs and ≈68% of the Pu released.

The outflow of radionuclides from Novaya Zemlya bays: Modeling and monitoring strategies

Hydrodynamic model results are used to evaluate possible monitoring strategies for a continuous survey of underwater dump sites. The Hamburg Shelf Ocean Model (HAMSOM) is applied to Abrosimov Bay and forced with realistic, transient wind fields and air temperatures. The three-dimensional circulation model is coupled to a dynamic-thermodynamic ice model that accounts for surface heat fluxes, fractional ice cover and ice thickness. Model results show significant variations in the bay circulation due to a pronounced seasonality in the wind forcing and the ice cover. The circulation is weakest in early summer when wind speeds are low and the ice still covers most parts of the bay. In autumn, circulation and flushing of the bay is most enhanced, due to increasing wind speeds and the absence of an ice cover. Dispersion scenarios were carried out assuming a leakage at dumped objects. During most of the year the obtained tracer concentrations in the bay are higher in the upper layers than close to the bottom, indicating an outflow at the surface and a compensatory inflow below. This general pattern is only reversed during spring and early summer, when the wind directions change. Since ice problems make it almost impossible to monitor surface waters or even the whole water column in a shallow bay, the only way to install a monitoring system, is at the bottom of the bay, as close as possible to dumped objects. Data transmission via satellite or radio could be realized from a small station located on the bays edge.

Chapter 10 Pathways of anthropogenic radioactivity in the Northern Oceans

Publisher Summary This chapter explains the pathways of anthropogenic radioactivity in the Northern Oceans. The case studies present in the chapter, describes the application of hydrodynamic circulation models to transport and dispersion of anthropogenic radioactivity in the North Atlantic and Arctic Ocean. The input of anthropogenic radioactivity to the marine environment can be divided basically into two types: marine sources and atmospheric sources. Atmospheric sources usually appear as fallout, that is, contaminated rain, stemming from bomb testing or from accidents. Marine sources discharge directly into the marine environment. They mostly represent point sources such as outlets of reprocessing plants, leaking nuclear waste units or sunken nuclear ship reactors. The chapter illustrates that the model studies confirm that the present radiological situation in the Arctic is far from being dramatic. However, the density of nuclear facilities in this part of the world ocean is remarkable and higher than anywhere else. There still is a potential risk of nuclear accidents and failures with considerable consequences for the environment.