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Rory O. R. Y. Thompson

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Rory O. R. Y. Thompson

Abstract

Waves are forced by moving sources and sinks of mass on a beta-plane with Ekman friction. If the motion is retrograde and not too fast, these are Rossby waves with a nonzero Reynolds stress uv′¯ . This stress forces a meridional mass flux which builds up a pressure gradient to allow a compensating mass flux in the Ekman layer. In a rotating system, this meridional pressure gradient requires a zonal flow
uyuvE½
which is also the momentum accumulated by the Reynolds stress in one spindown time. An example shows how excitation of Rossby waves brings momentum into the source of energy, causing a prograde jet there, with retrograde currents on each side. A laboratory experiment confirms the theory, which is proposed as an explanation of the observed “negative viscosity” of the jet stream in the atmosphere and the Gulf Stream in the ocean.
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Rory O. R. Y. Thompson

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Rory O. R. Y. Thompson

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Rory O. R. Y. Thompson

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A systematic way is given to design digital filters which allow clear separation of signals with periods of a few days from noise of higher frequency, particularly tidal and inertial. Several examples are given which pass little high-frequency power and none at the principal tidal frequencies. The Lanczos–cosine filter passes too much energy near diurnal frequencies; the Godin filter is better but not optimal. A longer filter is recommended, with flat low-frequency response, a sharp cut-off and very low noise. For current meter records containing inertial motions, it appears desirable to design a filter which specifically suppresses the local inertial frequency.

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Rory O. R. Y. Thompson

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Shipboard observations made in May 1982 showed a definite poleward surface flow (the Leeuwin Current) over the West Australian shelf from 22°S to 28°S. The surface current was relatively fresh, warm, low in dissolved oxygen concentration, and high in nutrients. The current flowed against a strong wind. Only a small portion of its flux of 4 × 106 m3 s−1 came from the Northwest Shelf. There was a subsurface equatorward current at a few hundred meters depth which was salty, high in oxygen concentration and low in nutrients. Observations from previous cruises show a surface geopotential gradient that could drive the surface current. It is suggested that winter deepening of the mixed layer may allow the geopotential gradient to overcome the wind stress.

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Rory O. R. Y. Thompson

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It is suggested that baroclinic currents may he expected to he sharper where deeper water or a decreasing Coriolis parameter lies in the direction of the Coriolis force. This is the case for the Gulf Stream between Hatteras and the Grand Banks. The suggested mechanism is that, in this case, Rossby waves propagating away must carry momentum up-gradient into the mean current. In the South Atlantic, they may carry momentum down-gradient, diffusing the mean flow.

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Rory O. R. Y. Thompson

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Three slab models of the surface mixed layer of the ocean are given simple and fast computer implementations. Actual meteorological data from Ocean Weather Station N are used for a year-long forecast. The results compare quite well with the observations of vertical temperature profiles, with correlations up to 0.98 between predicted and observed sea-surface temperature and of 0.8 between predicted and observed mixed-layer depths. Temperature anomalies introduced in the spring can be covered up in the summer, yet reappear in the winter. A constant-thickness slab is suitable as a lower boundary for some atmospheric climatological studies, if a depth of 25 m is used. The model based on a Foude number criterion worked best for the available data set; this is physically appealing since the model contains no adjustable parameters.

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Colleen Leary
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Rory O. R. Y. Thompson

Abstract

A currently used objective analysis scheme was tested by comparing a known function to an objective analysis of its values at the locations of radiosonde observing stations. A surface spherical harmonic of wavenumber 2 produced little spectral distortion, and 87% of the input amplitude squared appeared in wavenumber 2 in the objectively analyzed field. In contrast, only 13% of the input amplitude squared for a function of wavenumber 12 survived the analysis to appear in wavenumber 12. These results raise the possibility that spectral analyses of the kinetic energy in the atmospheric wind field may more properly be described by a less steep drop-off rate, for example, −2 rather than the −3 power law value measured from spectra derived from objectively analyzed date.

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Harald Svendsen
and
Rory O. R. Y. Thompson

Abstract

Currents, temperature, salinity, wind, runoff and water level were observed for a month in the Jøsen-fjord of southern Norway. Tide gages and currents show little semidiurnal tide. There is a strong diurnal signal in the upper 20 m, which a linear model shows to be caused by the wind. There is a week-long event in which the entire water-mass above the sill is flushed out; this is interpreted to be caused by downwelling outside the fjord. The strong stratification near the surface of the fjord greatly modifies the diurnal response of the fjord, but any density-driven mean circulation is at least an order of magnitude smaller than the wind-driven currents.

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