Abstract. 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 threedimensional, 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.

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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. q

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[1] 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.

A regional coupled ice‐ocean model for the Kara Sea, forced with boundary conditions from a large‐scale North Atlantic/Arctic Ocean Model, is used to study dispersion and export of freshwater from Ob and Yenisei rivers toward the Arctic Ocean and the Laptev Sea, for the period 1996–2001. The years 1998 and 1999 were characterized by a strong positive sea level pressure (SLP) anomaly in the Siberian part of the Arctic Ocean. Owing to prevailing northeasterly winds, the SLP anomaly caused a blocking situation, which suppressed the otherwise eastward freshwater export to the Arctic Ocean and Laptev Sea. This reversal of the prevailing circulation scheme led to a dramatic freshening of the Kara Sea through accumulation of low‐saline river water in the central and western parts. Additionally, the Kara Strait inflow from the Barents Sea, which presents the main source for saline Atlantic derived water, was reduced and partly even reversed. The suppressed freshwater export during winter 1998/1999 recovered in the following winter 1999/2000 when a significant pulse of low‐saline surface water left toward the Laptev Sea. The variability of the river discharge plays a minor role for the investigated period because the interannual variability of runoff rates is generally too low to explain the observed hydrographic changes. The results underline the importance of local shelf sea processes for the variability of the freshwater export from the Arctic Shelves to the central Arctic Ocean.