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Andrew C. Vastano
and
Robert O. Reid

Abstract

Sea surface flow derived from displacements of surface patterns in sequential NOAA-6 AVHRR (11 micron band) satellite images yield coherent nonuniform distributions of velocity vectors, An analytic representation of flow over the region of the distribution is obtained by performing a least-squares regression analysis for coefficients of a streamfunction expansion that is expressed in terms of trigonometric bash functions. Sea surface topography is estimated with the streamfunction by employing a geostrophic approximation. An application is made to a portion of the Oyashio Frontal Zone in the northwestern Pacific that includes the First and Second Oyashio Intrusions and an anticyclonic eddy. A horizontal map of a local rotational perturbation property is calculated for this region as a further example of the use of the streamfunction analysis.

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James J. O'Brien
and
Robert O. Reid

Abstract

This study is concerned with the theoretical description of upwelling induced in a stratified, rotating, two-layer ocean by momentum transfer from an intense stationary, axially-symmetric atmospheric vortex. The dynamic internal response of the ocean is assumed to be axially-symmetric which permits consideration of the solution in two independent variables, radial distance and time. Numerical integration via the method of characteristics is utilized to obtain values of radial velocity, tangential velocity, and depth of the upper layer for a period of two days. Transfer of momentum between the air and the sea and between the upper and lower layers are allowed. Transfer of heat and moisture with the atmosphere is not considered.

A general model is derived which leads to a hierarchy of models of increasing complexity. The detailed solution of the first of these is illustrated.

Results agree qualitatively with observations taken in the Gulf of Mexico following hurricane Hilda, 1964. Intense upwelling is confined to within twice the radius of maximum winds. The displaced warm central waters produce some downwelling adjacent to the upwelled region. The degree of upwelling is time-dependent and the hurricane-force winds must act on the ocean for several hours before significant upwelling occurs. The model indicates a strong coupling of the radially propagating internal wave mode and the vortex mode of the system. This coupling confines the significant internal disturbances to within the wind-forced region.

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Wensu Wang
,
Worth D. Nowlin Jr.
, and
Robert O. Reid

Abstract

The primary objective of this work is to formulate surface meteorological fields over the northwestern Gulf of Mexico for the period from April 1992 through November 1994 useful for the study of mesoscale processes and for model forcing of the near-coastal circulation. Observations were adjusted to standard heights, and a method of statistical interpolation was applied to time series of in situ observations to produce the required surface fields. Resulting monthly and seasonal mean fields show two principal patterns over the Texas-Louisiana shelf region—for summer and nonsummer. From June through August, surface winds are relatively constant, with alongshore wind components generally directed upcoast (from Mexico toward the Mississippi Delta). In other (nonsummer) months, surface winds are much more variable with alongshore wind components generally directed downcoast. The relatively large interannual variability is illustrated. Using these meteorological fields together with rather complete oceanographic data available from the same period, the effects of episodic atmospheric events on the circulation and properties of the Texas-Louisiana shelf may be examined. As examples, two extreme atmospheric events are characterized in terms of wind, surface air temperature, SST, and sensible heat flux fields: a cold air outbreak in November 1992 and a cyclone generated in March 1993 known as the “Storm of the Century.”

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Artemio Gallegos-Garcia
,
William J. Emery
,
Robert O. Reid
, and
Lorenz Magaard

Abstract

Frequency-wavenumber spectra of sea surface temperature and wind-stress curl are computed from 11 years of surface marine observations taken in the eastern North Pacific. These data were averaged by month and 2° quadrangles to yield spectra with periods from 2 to 48 months and zonal wavelengths from 400 to 4000 km. Spectra were computed for all 2° zonal bands between 16 and 40°N using data from the area between 120 and 160°W. Missing monthly values led to the computation of these spectra using a least-squares Fourier expansion which eliminated the need for temporal interpolation. Frequency spectra computed with this technique compare well with spectra using standard Fourier methods.

The resulting spectra were found to separate naturally into two regions; one between 29 and 40°N and the second between 15 and 29°N. Even within these zonal bands there were some important north–south changes. The annual signal was found to dominate the spectra of sea surface temperature at almost all wavelengths. The semiannual and 2-year periods were often also significant in sea surface temperature spectra. The annual peak dominated many of the wind-stress curl spectra at the longest wavelengths (∼2000–4000 km). Most of the energetic peaks in all spectra were symmetric with respect to east–west wavenumber. There were, however, some asymmetries suggesting both east and westward phase propagation. Generally, wind-stress curl spectra were white in frequency and red in wavenumber while sea surface temperature spectra were red in wavenumber but dominated by the 2-year, annual and semiannual periods in frequency.

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James H. Saylor
,
Joseph C. K. Huang
, and
Robert O. Reid

Abstract

Current velocities and water temperatures were observed in southern Lake Michigan with an array of AMF vector-averaging current meters during late spring, summer and fall 1976. Analyses of the recorded current data have revealed that persistent oscillations of nearly 4 days in period were at least as energetic as inertial oscillations in the kinetic energy spectra and current hodographs. The 4-day oscillations were present at all stations, including a very clear signal at stations near the center of the lake basin. This lake-wide oscillation was present during both stratified and unstratified seasons and current vectors rotated cyclonically near the center of the lake and anticyclonically elsewhere. The observed rotational oscillations closely fit the characteristics of barotropic second-class motions of a basin with variable depth first described by Lamb (1932). While such topographic vortex modes are of the same class as low-frequency shelf waves, their kinematic properties and natural period are governed by the lake shape as well as the bathymetry. Moreover, the gravest mode is unique among these waves in having nonzero velocity at the lake center. The present observations give clear evidence for the existence of the gravest mode of such oscillations in southern Lake Michigan.

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