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R. M. Samelson

Abstract

The generation of continental shelf currents by wind forcing is investigated by analytical and numerical methods. The investigation is motivated by observations from the Coastal Ocean Dynamics Experiment. A central assumption is that the vertical structure of the response over the inshore half of the shelf is controlled by the vertical distribution of the turbulent stress. This suggests a two-layer model of the wind-driven circulation, in which the upper layer represents a surface wind-mixed layer, and the lower layer represents the remainder of the fluid. The response of this idealized dynamical model to wind forcing is examined and compared with observations in the 2–7-day period band. For the alongshore velocity gain relative to local wind stress, an onshore surface maximum and an offshore interior maximum are robustly reproduced by the model. These features are evidently related to a dynamical transition over the inner half of the shelf, in which the alongshore wind stress is balanced more by acceleration of near-surface alongshore flow and less by time-dependent Ekman transport as the coast is approached. This differs from a previous hypothesis, based on a linear model in which the turbulent stress was confined to infinitesimally thin surface and bottom boundary layers, which related the alongshore flow structure to the cross-shore profile of the alongshore wind amplitude. In the present model, the cross-shore velocity variances are roughly comparable to those observed over the onshore half of the shelf. This also contrasts with the previous model results, which underpredicted cross-shore velocity variances by more than an order of magnitude. However, the present agreement is probably fortuitous, as the enhanced lower-layer cross-shore flow is frictionally driven, and should probably be confined to a bottom boundary layer as it was in the previous model. The results demonstrate that the response of these models over the inner half of the shelf depends strongly on a poorly understood coastal boundary condition.

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R. M. Samelson

Abstract

Two idealized, three-dimensional, analytical models of middepth meridional overturning in a basin with a Southern Hemisphere circumpolar connection are described. In the first, the overturning circulation can be understood as a “pump and valve” system, in which the wind forcing at the latitudes of the circumpolar connection is the pump and surface thermodynamic exchange at high northern latitudes is the valve. When the valve is on, the overturning circulation extends to the extreme northern latitudes of the basin, and the middepth thermocline is cold. When the valve is off, the overturning circulation is short-circuited and confined near the circumpolar connection, and the middepth thermocline is warm. The meridional overturning cell in this first model is not driven by turbulent mixing, and the subsurface circulation is adiabatic. In contrast, the pump that primarily drives the overturning cell in the second model is turbulent mixing, at low and midlatitudes, in the ocean interior. In both models, however, the depth of the midlatitude deep layer is controlled by the sill depth of the circumpolar gap. The thermocline structures in these two models are nearly indistinguishable. These models suggest that Northern Hemisphere wind and surface buoyancy forcing may influence the strength and structure of the circumpolar current in the Southern Hemisphere.

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R. M. Samelson

Abstract

A simple dynamical model is proposed for the near-surface drift current in a homogeneous, equilibrium sea. The momentum balance is formulated for a mass-weighted mean in curvilinear surface-conforming coordinates. Stokes drifts computed analytically for small wave slopes by this approach for inviscid linear sinusoidal and Pollard–Gerstner waves agree with the corresponding Lagrangian means, consistent with a mean momentum balance that determines mean parcel motion. A wave-modified mixing length model is proposed, with a depth-dependent eddy viscosity that depends on an effective ocean surface roughness length z 0 o , distinct from the atmospheric bulk-flux roughness length z 0 a , and an additional wave-correction factor ϕw . Kinematic Stokes drift profiles are computed for two sets of quasi-equilibrium sea states and are interpreted as mean wind drift profiles to provide calibration references for the model. A third calibration reference, for surface drift only, is provided by the traditional 3%-of-wind rule. For 10-m neutral wind U 10 N ≤ 20 m s−1, the empirical z 0 o ranges from 10−4 to 10 m, while ϕw ranges from 1.0 to 0.1. The model profiles show a shallow log-layer structure at the smaller wind speeds and a nearly uniform near-surface shear at the larger wind speeds. Surface velocities are oriented 10°–20° from downwind for U 10 N ≤ 10 m s−1 and 20°–35° from downwind for 10 ≤ U 10 N ≤ 20 m s−1. A small correction to the drag coefficient is implied. The model predictions show a basic consistency with several sets of previously published near-surface current measurements, but the comparison is not definitive.

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R. M. Samelson
and
A. M. Rogerson

Abstract

A recent climatology of observed coastal-trapped disturbances in the marine atmospheric boundary layer along the United States west coast motivates the detailed examination, for a specific form of imposed forcing, of a linear shallow-water boundary layer model. The model is forced by a time-dependent pressure field, imposed at a fixed level above the boundary layer, that is an idealized representation of the climatological synoptic evolution: a low pressure center translates westward across the coastal boundary, corresponding to the observed offshore extension of a continental thermal trough. The alongshore structure of the model disturbance is characterized by enhanced northerly flow, a depressed marine layer, and low surface pressure to the north; and southerly flow, a raised marine layer, and high surface pressure to the south. Initially, the marine-layer thickness along the coast responds predominantly to convergence of the ageostrophic cross-shore flow driven by the imposed cross-shore pressure gradient and to convergence (to the south of the low pressure center) and divergence (to the north) of the geostrophic cross-shore flow balanced by the imposed alongshore pressure gradient, lifting in the central and southern parts of the forcing region and failing north of the forcing region. For the parameter values considered here, the amplitude of the coastal-trapped thickness response to the geostrophic cross-shore flow is roughly three times as large as that due to the ageostrophic cross-shore flow, but this ratio is likely to be sensitive to the cross-shore/alongshore aspect ratio of the pressure forcing. The coastal-trapped alongshore velocity disturbance is dominated by the response to the alongshore pressure gradient. There is no alongshore propagation in thickness disturbance during the initial stage of the event, while the alongshore velocity and surface pressure exhibit only weak propagation. In the later stages of the event, when the imposed coastal pressure gradients relax (as the low translates offshore), the cross-shore flow weakens, and the response at the coast is controlled by the convergence and divergence of the alongshore flow. The thickness disturbance, alongshore velocity reversal, and surface pressure perturbation propagate northward along the coast essentially as a Kelvin wave in the later stages of the event. Although both the model and the imposed pressure forcing are highly idealized, the model response is qualitatively and, to some degree, quantitatively consistent with many aspects of existing observations of coastal-trapped wind reversals along the United States west coast.

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A. M. Rogerson
and
R. M. Samelson

Abstract

Motivated by recent observations along the west coast of the United States, the authors investigate the generation and propagation of coastal-trapped disturbances in the marine atmospheric boundary layer. Analytic solutions are obtained in a linear, shallow water, reduced-gravity model of the flow subject to forcing by upper-level pressure disturbances and dissipation in the form of wind stress at the sea surface. It is found that unless the mean alongshore flow is to the south with speeds larger than the gravity wave phase speed, a northward propagating coastal-trapped response develops. The superposition of cross-shore propagating forcing and the northward propagating response in marine-layer thickness can give rise to surface pressure ridges at the coast with both narrow and broad cross-shore extent. Wind reversals associated with the disturbance lead the change in surface pressure at the coast. The magnitude of the response increases for weaker inversion strength, greater undisturbed marine-layer depth, and, to some extent, with weaker dissipation. For periodic forcing, the near-resonant response propagates steadily up the coast with the inviscid free Kelvin wave phase speed and has a cross-shore length scale equal to the Rossby deformation radius, while the off-resonant response possesses cross-shore length scales that differ from the Rossby radius, and propagates unsteadily up the coast with an average speed determined by forcing parameters. It is also found that the alongshore length scale of the disturbance depends on the propagation speed of the forcing, and may appear more mesoscale-like for fast-moving pressure systems. The results illustrate that unsteady propagation of the coastal-trapped disturbance can result from the linear response to synoptic forcing.

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R. M. Samelson
and
P. L. Barbour

Abstract

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−1 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 monthly-mean 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−1, 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.

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David Rivas
and
R. M. Samelson

Abstract

Regional ocean circulation along the Oregon coast is studied numerically for forcing fields derived from year 2005 and climatological-mean conditions. The primary object is to study directly the Lagrangian pathways by which fluid arrives in the Oregon upwelling zone. Roughly half of the upwelling fluid is found to arrive in the regional domain from alongshelf source points, primarily north but also south of the Oregon upwelling zone, while the other half ultimately arrives from points offshore and west of the zone. For both the year 2005 and the climatological simulations, different regimes of dominant alongshelf source waters are found, with Cape Blanco being a dividing point for northern versus southern sources, and with the water parcels coming primarily from depths below 100 m. For the offshore sources, most upwelling fluid originates from depths between 150 and 250 m during 2005, but from within the upper 150 m for the climatological simulations. In both cases there are specific regions along the shelf of enhanced vertical motion, which appear to be associated with topographic features such as submarine banks and canyons. A perhaps surprising result is the apparently small role played by the poleward undercurrent as a direct, immediate source of upwelling fluid; on the seasonal time scale considered here, most trajectories are found to move southward over the slope and shelf, with weaker northward motion only farther offshore, in the deep interior. In 2005 the water parcels cover distances longer than in the climatology, and the meridional exchange along their paths is more vigorous. These differences are likely associated in part with the presence of mesoscale vortical structures seaward of the shelf, which are largely absent in the climatological simulation, perhaps because their formation may depend on the large-amplitude, short-time-scale wind variations and reversals that occur in the year 2005 wind field but not in the multiyear mean climatological wind field.

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Eric D. Skyllingstad
and
R. M. Samelson

Abstract

Interaction between mixed layer baroclinic eddies and small-scale turbulence is studied using a nonhydrostatic large-eddy simulation (LES) model. Free, unforced flow evolution is considered, for a standard initialization consisting of an 80-m-deep mixed layer with a superposed warm filament and two frontal interfaces in geostrophic balance, on a model domain roughly 5 km × 10 km × 120 m, with an isotropic 3-m computational grid. Results from these unforced experiments suggest that shear generated in narrow frontal zones can support weak three-dimensional turbulence that is directly linked to the larger-scale baroclinic waves. Two separate but closely related issues are addressed: 1) the possible development of enhanced turbulent mixing associated with the baroclinic wave activity and 2) the existence of a downscale transfer of energy from the baroclinic wave scale to the turbulent dissipation scale. The simulations show enhanced turbulence associated with the baroclinic waves and enhanced turbulent heat flux across the isotherms of the imposed frontal boundary, relative to background levels. This turbulence develops on isolated small-scale frontal features that form as the result of frontogenetic processes operating on the baroclinic wave scale and not as the result of a continuous, inertial forward cascade through the intermediate scales. Analysis of the spectrally decomposed kinetic energy budget indicates that large-scale baroclinic eddy energy is directly transferred to small-scale turbulence, with weaker forcing at intermediate scales. For fronts with significant baroclinic wave activity, cross-frontal eddy fluxes computed from correlations of fluctuations from means along the large-scale frontal axis generally agreed with simple theoretical estimates.

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K. H. Brink
and
R. M. Samelson

Abstract

No abstract available.

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R. M. Samelson
and
C. L. Wolfe

Abstract

An unstable, nonlinear baroclinic wave-mean oscillation is found in a strongly supercritical quasigeostrophic f-plane numerical channel model with 3840 Fourier components. The growth of linear disturbances to this time-periodic oscillation is analyzed by computing time-dependent normal modes (Floquet vectors). Two different Newton–Picard methods are used to compute the unstable solution, the first based on direct computation of a large set of Floquet vectors, and the second based on an efficient iterative solver. Three different growing normal modes are found, which modify the wave structure of the wave-mean oscillation in two essentially different ways. The dynamics of the instabilities are qualitatively similar to the baroclinic dynamics of the wave-mean oscillation. The results provide an example of time-dependent normal mode instability of a strongly nonlinear time-dependent baroclinic flow.

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