The climatically sensitive zone of the Arctic Ocean lies squarely within the domain of the North Atlantic oscillation (NAO), one of the most robust recurrent modes of atmospheric behavior. However, the specific response of the Arctic to annual and longer-period changes in the NAO is not well understood. Here that response is investigated using a wide range of datasets, but concentrating on the winter season when the forcing is maximal and on the postwar period, which includes the most comprehensive instrumental record. This period also contains the largest recorded low-frequency change in NAO activity-from its most persistent and extreme low index phase in the 1960s to its most persistent and extreme high index phase in the late 1980s/early 1990s. This longperiod shift between contrasting NAO extrema was accompanied, among other changes, by an intensifying storm track through the Nordic Seas, a radical increase in the atmospheric moisture flux convergence and winter precipitation in this sector, an increase in the amount and temperature of the Atlantic water inflow to the Arctic Ocean via both inflow branches (Barents Sea Throughflow and West Spitsbergen Current), a decrease in the late-winter extent of sea ice throughout the European subarctic, and (temporarily at least) an increase in the annual volume flux of ice from the Fram Strait.

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[1] In multicategory sea ice models the compressive strength of the ice pack is often assumed to be a function of the potential energy of pressure ridges. This assumption, combined with other standard features of ridging schemes, allows the ice strength to change dramatically on short timescales. In high-resolution ($10 km) sea ice models with a typical time step ($1 hour), abrupt strength changes can lead to large internal stress gradients that destabilize the flow. The unstable flow is characterized by large oscillations in ice concentration, thickness, strength, velocity, and strain rates. Straightforward, physically motivated changes in the ridging scheme can reduce the likelihood of abrupt strength changes and improve stability. In simple test problems with flow toward and around topography, stability is significantly enhanced by eliminating the threshold fraction G* in the ridging participation function. Use of an exponential participation function increases the maximum stable time step at 10-km resolution from less than 30 min to about 2 hours. Modifying the redistribution function to build thinner ridges modestly improves stability and also gives better agreement between modeled and observed thickness distributions. Allowing the ice strength to increase linearly with the mean ice thickness improves stability but probably underestimates the maximum stresses.

[1] As a part of the Arctic Ocean Model Intercomparison Project, results from 10 Arctic ocean/ice models are intercompared over the period 1970 through 1999. Models' monthly mean outputs are laterally integrated over two subdomains (Amerasian and Eurasian basins), then examined as functions of depth and time. Differences in such fields as averaged temperature and salinity arise from models' differences in parameterizations and numerical methods and from different domain sizes, with anomalies that develop at lower latitudes carried into the Arctic. A systematic deficiency is seen as AOMIP models tend to produce thermally stratified upper layers rather than the ''cold halocline'', suggesting missing physics perhaps related to vertical mixing or to shelf-basin exchanges. Flow fields pose a challenge for intercomparison. We introduce topostrophy, the vertical component of VÂr r r rD where V is monthly mean velocity and r r r rD is the gradient of total depth, characterizing the tendency to follow topographic slopes. Positive topostrophy expresses a tendency for cyclonic ''rim currents''. Systematic differences of models' circulations are found to depend strongly upon assumed roles of unresolved eddies.

[1] The northward flow of Atlantic Water via the Barents Sea and Fram Strait is modeled, and climatological volume, heat, and salt fluxes into the Arctic Ocean are investigated. We argue that understanding of climate change in the region requires the knowledge of the mean circulation before its variability can be determined. Since estimates of long-term mean fluxes in the region are not available from observations, we present a modeling approach to quantify the climatological circulation and northward transports from the Norwegian Sea into the Arctic Ocean. A coupled ice-ocean model of the pan-Arctic region is configured at a 1/12°and 45-level grid and is integrated for 7 decades using a combination of daily-averaged 1979-2001 European Centre for Medium-Range Weather Forecasts data. Simulated water mass characteristics are compared with climatological atlas and selected observational data. The separation of the Norwegian Atlantic Current into Barents Sea and Fram Strait branches and their relative contributions to the total mass and property input into the Arctic Ocean are quantified. We emphasize the Barents Sea because fewer direct measurements of transports exist there and because water masses are significantly altered along this path by the seasonal ice melt/formation and the freshwater inputs. Under the given atmospheric forcing the Barents Sea outflow is shown to significantly contribute to the boundary flow continuing along the slopes of the Arctic Ocean. On the basis of model results, we argue that the contribution of the Barents Sea branch of Atlantic Water into the Arctic Ocean is equally, if not more, important than the Fram Strait branch.