Synoptic Forcing Of Coastal-Trapped Disturbances in the Marine Atmospheric Boundary Layer

A. M. Rogerson Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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R. M. Samelson Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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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.

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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