A mesoscale atmospheric model, nested in operational global numerical weather prediction fields, is used to estimate low-level winds and surface wind stress through Nares Strait, between Ellesmere Island and Greenland, during 2 yr from August 2003 to July 2005. During most of the year, the model low-level winds are dominated by intense, southward along-strait flow, with monthly-mean southward 10-m winds reaching 10 m s Ϫ1 in winter. Summertime flow is weak and distributions of hourly along-strait winds during the 2-yr period are strongly bimodal. The strong southward low-level winds are associated with ageostrophic, orographically channeled flow down the pressure gradient from the Lincoln Sea to Baffin Bay and are highly correlated with the pressure difference along Nares Strait. The 2-yr means and leading EOFs of monthlymean 10-m winds and wind stress place the strongest winds and stress in the southern parts of Smith Sound and of Kennedy Channel, at the openings to Baffin Bay and Kane Basin, at known sites of polynya formation, including the North Water polynya in Smith Sound, suggesting that the locally intensified winds may cause these persistent polynyas. An intense wind event observed in Nares Strait by a field camp, with surface winds exceeding 30 m s Ϫ1 , generally follows the typical pattern of these low-level flows. Based on the model correlation of winds and pressure difference, a 51-yr time series of estimated winds in Nares Strait is reconstructed from historical surface pressure measurements at Thule, Greenland, and Alert, Canada. The pressure difference and reconstructed wind time series are correlated with the Arctic Oscillation at annual and longer periods, but not on monthly periods.

[1] The wind stress forcing of the Oregon coastal ocean during June -August 1999 is estimated from a regional mesoscale atmospheric model and from satellite scatterometer observations, supplemented by moored and coastal surface data and by a land-based wind profiler. Both the mean and variable components of model alongshore wind stress increase by factors of 3 -4 from north to south along the Oregon coast. There is evidence of orographic intensification near Cape Blanco, which is supported by previous aircraft and ship observations during August 1995. The systematic southward increase of southward stress will drive enhanced ocean upwelling along the southern Oregon coast and suggests the new hypothesis that systematic variations in local wind stress may contribute to the observed offshore displacement of the coastal upwelling jet in this region. It is inferred from an analysis of the model and surface data that ocean upwelling modifies coastal surface air temperatures by 1-5°C over timescales of 12-24 hours.INDEX TERMS: 3329

Air-sea coupling during coastal upwelling was examined through idealized three-dimensional numerical simulations with a coupled atmosphere-ocean mesoscale model. Geometry, topography, and initial and boundary conditions were chosen to be representative of summertime coastal conditions off the Oregon coast. Over the 72-h simulations, sea surface temperatures were reduced several degrees near the coast by a wind-driven upwelling of cold water that developed within 10-20 km off the coast. In this region, the interaction of the atmospheric boundary layer with the cold upwelled water resulted in the formation of an internal boundary layer below 100-m altitude in the inversion-capped boundary layer and a reduction of the wind stress in the coupled model to half the offshore value. Surface heat fluxes were also modified by the coupling. The simulated modification of the atmospheric boundary layer by ocean upwelling was consistent with recent moored and aircraft observations of the lower atmosphere off the Oregon coast during the upwelling season. For these 72-h simulations, comparisons of coupled and uncoupled model results showed that the coupling caused measurable differences in the upwelling circulation within 20 km off the coast. The coastal Ekman transport divergence was distributed over a wider offshore extent and a thinner ocean surface boundary layer, with consistently smaller offshore and depth-integrated alongshore transport formed in the upwelling region, in the coupled case relative to the uncoupled case. The results indicate that accurate models of coastal upwelling processes can require representations of ocean-atmosphere interactions on short temporal and horizontal scales.

Numerical simulations of boundary layer evolution in offshore flow of warm air over cool water are conducted and compared with aircraft observations of mean and turbulent fields made at Duck, North Carolina. Two models are used: a two-dimensional, high-resolution mesoscale model with a turbulent kinetic energy closure scheme, and a three-dimensional large-eddy simulation (LES) model that explicitly resolves the largest turbulent scales. Both models simulate general aspects of the decoupling of the weakly convective boundary layer from the surface, as it is advected offshore, and the formation of an internal boundary layer over the cool water. Two sets of experiments are performed, which indicate that complexities in upstream surface conditions play an important role in controlling the observed structure. The first (land–sea) experiments examine the transition from a rough surface having the same temperature as the ambient lower atmosphere, to a smooth ocean surface that is 5°C cooler. In the second (barrier island) experiment, a 4-km strip along the coastline having surface temperature 5°C warmer than the ambient atmosphere is introduced, to represent a narrow, heated barrier island present at the Duck site. In the land–sea case, it is found that the mesoscale model overpredicts turbulent intensity in the upper half of the boundary layer, forcing a deeper boundary layer. Both the mesoscale and LES models produce only a small change in the boundary layer shear and tend to decrease the momentum flux near the surface much more rapidly than the observations. Results from the barrier-island case are more in line with the observed momentum and turbulence structure, but still have a reduced momentum flux in the lower boundary layer in comparison with the observations. The authors find that turbulence in the LES model generated by convection over the heated land surface is stronger than in the mesoscale model, and tends to persist offshore for greater distances because of greater shear in the upper boundary layer winds. Analysis of the mesoscale model results suggests that better estimation of the mixing length could improve the turbulence closure in regions where the surface fluxes are changing rapidly.

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