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Theoretical studies of the circulation of the Subarctic Pacific Region and the generation of Kelvin type waves by atmospheric distrubances Thomson, Richard Edward


Theoretical studies of two problems concerned with the surface forced, large-scale motions in bounded oceanic regions are presented. In Part I, such motions are considered for a particular area of the North Pacific Ocean known as the Subarctic Pacific Region. Discussion is based on the assumption that the velocity components may be separated into a time-averaged or quasi-steady flow about which fluctuations occur in the form of transient planetary waves. Some of the characteristics of the latter are briefly outlined. Several aspects of the time-averaged motions are then considered. A simple circulation, driven by the vertical velocity structure, is presented for the interior region of the ocean below the upper frictional layer. Also, using observational data to obtain the depth of the layer between the suface [sic] and the main halocline, this upper layer is found to behave as a geostrophic layer of fluid when averaged over many years. Combination of the above observed depths with the mean calculated Ekman divergences permitted calculation of a mean eddy coefficient of diffusivity for density. The results agree very well with those obtained by Veronis for similar oceanic situations. An explanation for the variations in the intrusion of 'warm' water along the top and bottom of the halocline off the coast of British Columbia is also given. The two final sections of Part I deal with the overall, quasi-steady circulation of the Subarctic Pacific Region. Here, a theoretical study is combined with the mean-monthly values of the calculated surface forcing. Curvilinear coordinates are used in order to model the northern boundary formed by the Aleutian-Komandorski island chain. The interior quasi-steady flow, which satisfies a Sverdrup-type balance of vorticity, is closed to the north by a frictional boundary layer. Using mean-monthly values for the surface winds over the region, the observed separation of the eastward flowing West Wind Drift into a northern and southern tending flow is found to correspond to the zero of the mean wind-stress curl. In the northern boundary layer, the characteristics of the westward flowing boundary current there, are shown to change downstream from a Western' to a 'zonal', type boundary current. The stability of the latter is dependent upon vorticity of appropriate sign being added to the boundary layer flow to balance that generated by friction along the coast. Discussion is also given for the effect of passes between the Aleutian islands on the zonal boundary current. Through a type of boundary layer 'suction' or, alternately, by mass transport into the boundary layer, the effect of these passes would seem to be to keep the boundary flow attached to the coast. Finally, spectral analysis of the wind-stress curl data, obtained from the mean-monthly surface pressure, is performed to determine its frequency distribution. A demodulation technique is used to determine the time variations of six of the frequency bands obtained in the spectral analysis. These results are then applied to the circulation in the Subarctic Pacific Region in an attempt to relate variations and spatial distribution in the circulation with the applied winds. The generation by the atmosphere of a type of long, boundary waves, known as Kelvin waves, is considered in Part II. In particular, it is shown that for a general large-scale distribution of wind and pressure systems that only the longshore component of the wind-stress and pressure can generate such waves. Examples are presented for a semi-infinite wind and moving pressure pattern. Kelvin waves are shown to move away from the force discontinuities at the speed of shallow-water waves. These waves are further found to exhibit a frequency shift, typical of non-dispersive waves from a moving source. Using some observed parameters for the atmospheric forcing terms off the Oregon coast of the United States, numerical values for the wave amplitudes for both examples are given. Part II has been published in the form presented here. Reference: J. Fluid Mech. C1970), 42C4), 657-670.

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