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JAMES J. O'BRIEN

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James J. O'Brien

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James J. O'Brien

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James J. O'Brien

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This paper is a continuation of a theoretical description of upwelling and mixing induced in a stratified, rotating, two-layer ocean by momentum transfer from an intense, stationary, axially-symmetric atmospheric vortex. A second model which includes mixing is considered. 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 depth-averaged radial and tangential velocities, depth of the upper layer, and density contrast for a period of two days. Transfer of momentum between the air and the sea and between the upper and lower layers of the ocean is included. Transfer of heat and salt between the two ocean layers is simulated. Transfer of heat and moisture with the atmosphere is not considered.

The mechanism of energy transfer to and from the atmosphere and to and from the lower layer is examined in detail. This indicates that the total energy varies only with the inertial period. The energy associated with the effect of mixing is an order of magnitude smaller than that associated with turbulent dissipation. However, turbulent mixing of heat and salt modifies the density structure throughout the wind-forced region of the ocean, while intense upwelling is confined to within twice the radius of maximum hurricane winds.

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James J. O'Brien

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The kinematic method for determining vertical velocity ω in pressure coordinates is reviewed. Alternative objective procedures are derived for obtaining ω, and an analytical solution to the pressure-differentiated continuity equation is found. A variational formulation leads to a generalized objective adjustment for divergence estimates which yields improved, physically realistic estimates of ω. Case studies for intense mesoscale convection demonstrate the utility of an adjustment scheme based on the simplest hypothesis, namely, that the errors in divergence estimates are a linear function of pressure.

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James J. O'Brien

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James J. O'Brien and Robert O. Reid

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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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Antonio J. Busalacchi and James J. O'Brien

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A simple linear model of the tropical Pacific Ocean is used to simulate the oceanic response to time-dependent wind stress forcing. A linear, one-layer, reduced-gravity transport model on an equatorial beta-plane is incorporated. The non-rectangular model basin extends from 18°N to 12°S. Bottom topography, thermohaline and thermodynamic effects are neglected.

The equatorial response, particularly at the eastern boundary, is studied along the same lines as Kindle. Annual and semiannual harmonics of the zonal equatorial wind stress calculated by Meyers are used to force the model. The east-west slope of the model pycnocline is compared with depth observations of the 14°C isotherm. The linear model generates a semiannual eastern boundary response remote from any region with strong second harmonies of the zonal wind stress. This response supports Meyers' hypothesis that at the eastern boundary the semiannual displacement of the thermocline is due to remote forcing.

The major application of the model is forced by mean monthly wind stresses based on 10 years of observations over the tropical Pacific. The resulting meridional profile of the pycnocline depth is similar to Wyrtki's profile of dynamic height. The equatorial system of troughs and ridges is evident in the pycnocline profile. The seasonal variation of the major equatorial surface currents is compared with the observations. An annual Rossby wave emanating from the eastern boundary is found to modify the location and variability of the Countercurrent Trough. The presence of an anomalous eastward flow centered south of the equator in the eastern equatorial Pacific is supported by Tsuchiya's maps of the dynamic topography of this region.

The results of the two model applications indicate that the dynamics inherent in linear theory are capable of simulating some of the major features of the equatorial response and those of the equatorial surface current system.

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Dennis L. Elliott and James J. O'Brien

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The mesoscale structure of the lowest 1500 m of the atmosphere for the central Oregon coast region was investigated during August 1973, using meteorological and sea surface temperature data obtained primarily by an instrumented aircraft. Two types of aircraft flight patterns were utilized. The horizontal variability of wind velocity, air temperature, moisture and sea surface temperature are discussed from data collected during level flights at 150 m over the ocean. The vertical structure of temperature, mixing ratio and wind, extending from 50 km inland to 40 km seaward, is examined from a series of horizontal traverses and aircraft soundings made throughout the lowest 1500 m.

The mesoscale wind field over the upwelling region has a structure which depends, to a large extent, on the basic stratification and depth of the marine layer. On days with moderate to strong northerly winds, three separate patterns in the seaward variation of wind speed at 150 m above the surface were observed: a maximum along the coast which was the most frequently observed pattern; a maximum 10–30 km offshore oriented parallel to the coast; and otherwise a mostly uniform pattern. Those cases with a maximum offshore were characterized by a shallow marine layer with no significant inversion in the lowest 1500 m and a protrusion of relatively dry air (presumed subsidence from aloft)associated with the wind maximum all other cases were characterized by either a deep marine layer or strong marine inversion.

Horizontal gradients in the ambient air temperature appear to reflect the sign of the sea surface temperature gradient to a height of only 150 m or less, as horizontal gradients in the air temperature at 150 m were typically quite small, 1°C (30 km)−1, even days when sea surface temperature gradients were as large as 5°C (30 km)−1.

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J. Dana Thompson and James S. O'Brien

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Linear and nonlinear two-layer ocean circulation models of coastal upwelling on an f-plane are driven by time-dependent winds and solved numerically. Longshore variations in the circulation are neglected and offshore variations in the winds are specified. A technique for generating a realistic broad frequency-band wind stress from a kinetic energy spectrum of wind speed is developed.

When results from the two models are compared, nonlinearities are found to be unimportant in explaining the basic upwelling dynamics. However, they do provide a mechanism for wave-wave interactions which broaden all spectral peaks. In the nonlinear model coherence-squared spectra between the winds and zonal current components in the upwelling zone indicate highest coherence at lowest frequencies for both layers, accompanied by a 180° phase shift from upper to lower layer at frequencies <3 cycles per day. Similar analyses for winds vs meridional current components and winds vs pycnocline height anomalies show a coherence squared maximum for winds of 5-day period. In the frequency band below and including the inertial, remarkable similarities are observed between the results of the nonlinear model and actual ocean current autospectra.

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