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  • 1
    Publication Date: 2008-01-01
    Description: Results from new experiments on baroclinic instability of a coastal jet demonstrate that this almost balanced flow spontaneously emits inertial waves when the Rossby radius of deformation is relatively small such that the characteristics of baroclinic meanders match the dispersion relation for the inertial waves. The energy of the waves is small compared to the energy of the flow. A single event of wave emission is identified in the experiment with larger radius of deformation and is interpreted in terms of vorticity dynamics. The flows are generated on a laboratory polar β plane where the Coriolis parameter varies quadratically with latitude. A new method for imaging the rotating flows, which the authors call “altimetric imaging velocimetry,” is employed. Optical color coding of slopes of the free-surface elevation field allows the authors to derive the fields of pressure, surface elevation, geostrophic velocity, or the “gradient wind” velocity with very high spatial resolution (typically several million vectors) limited largely by the pixel resolution of the available imaging sensors. The technique is particularly suited for the investigations of small-amplitude waves, which are often difficult to detect by other methods.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
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  • 2
    Publication Date: 1986-12-01
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
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  • 3
    Publication Date: 2007-10-01
    Description: This paper describes qualitative features of the generation of jetlike concentrated circulations, wakes, and blocks by simple mountainlike orography, both from idealized laboratory experiments and shallow-water numerical simulations on a sphere. The experiments are unstratified with barotropic lee Rossby waves, and jets induced by mountain orography. A persistent pattern of lee jet formation and lee cyclogenesis owes its origins to arrested topographic Rossby waves above the mountain and potential vorticity (PV) advection through them. The wake jet occurs on the equatorward, eastern flank of the topography. A strong upstream blocking of the westerly flow occurs in a Lighthill mode of long Rossby wave propagation, which depends on βa2/U, the ratio of Rossby wave speed based on the scale of the mountain, to zonal advection speed, U (β is the meridional potential vorticity gradient, f is the Coriolis frequency, and a is the diameter of the mountain). Mountains wider (north–south) than the east–west length scale of stationary Rossby waves will tend to block the oncoming westerly flow. These blocks are essentially β plumes, which are illustrated by their linear Green function. For large βa2/U, upwind blocking is strong; the mountain wake can be unstable, filling the fluid with transient Rossby waves as in the numerical simulations of Polvani et al. For small values, βa2/U ≪ 1 classic lee Rossby waves with large wavelength compared to the mountain diameter are the dominant process. The mountain height, δh, relative to the mean fluid depth, H, affects these transitions as well. Simple lee Rossby waves occur only for such small heights, δh/h ≪ aβ/f, that the f/h contours are not greatly distorted by the mountain. Nongeostrophic dynamics are seen in inertial waves generated by geostrophic shear, and ducted by it, and also in a texture of finescale, inadvertent convection. Weakly damped circulations induced in a shallow-water numerical model on a sphere by a lone mountain in an initially simple westerly wind are also described. Here, with βa2/U ∼1, potential vorticity stirring and transient Rossby waves dominate, and drive zonal flow acceleration. Low-latitude critical layers, when present, exert strong control on the high-latitude waves, and with no restorative damping of the mean zonal flow, they migrate poleward toward the source of waves. While these experiments with homogeneous fluid are very simplified, the baroclinic atmosphere and ocean have many tall or equivalent barotropic eddy structures owing to the barotropization process of geostrophic turbulence.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
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  • 4
    Publication Date: 2013-11-01
    Description: This paper presents new observations of the overflow waters downstream of the Faroe Bank Channel (FBC) and the Iceland–Faroe Ridge (IFR). Between 2006 and 2009, over 17 400 hydrographic profiles were collected during quarterly deployments in the region by autonomous gliders, providing previously unrealized spatial resolution to observations downstream of the FBC. Observations show that the second sill of the FBC coincides with the largest changes in the overflow plume, including significant thinning, widening, and entrainment. Between the second sill and a topographic feature 75 km downstream, the plume bifurcates with the densest portion (65% of the transport), descending below 1000 m. On the IFR, near-bottom velocities are directed alongslope with speeds averaging 21.5 cm s−1. Observations indicate that 80% of baroclinic velocities associated with mesoscale variability of the overflow plume are smaller than the alongslope topographically induced circulation. Evidence of overflow is found at all locations on the Atlantic flank of the IFR. However, the meridionally oriented portion at 13°W has anomalously warm bottom water and divides FBC and eastern IFR overflow from overflow found in the Western Valley. Individual Seaglider sections identify IFR overflow in a narrow current (8–14 km wide) along the Iceland shelf with a mean transport of 0.43 Sv (1 Sv ≡ 106 m3 s−1) with significant variability from days to weeks. A lower-bound estimate of 0.8 Sv of total IFR overflow is presented. These results provide constraints on regional models that inform the representation of this crucial, yet underresolved, region in large-scale ocean and climate models.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
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  • 5
    Publication Date: 1982-06-01
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
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