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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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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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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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James J. O'Brien

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J. C. O'BRIEN

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J. M. Caron
and
J. J. O’Brien

Abstract

Synthetic monthly SST anomaly data are constructed using frequency domain analyses of significant principle components derived from reconstructed SST data in the equatorial Pacific Ocean. The model provides insight into the dominant physical processes contained in each component and retains the relevant statistical properties of the original data, such as the mean, variance, and autocorrelation. Thus, numerous sets of synthetic SST anomaly data can be produced for the equatorial Pacific that are statistically indistinguishable from the original SST anomaly data.

The spatial and temporal SST signatures of the biennial, intradecadal, and decadal pseudoperiodicities are reproduced, including their frequency and duration of occurrence. Specifically, the ENSO warm and cold event signatures recur in the synthetic data at peak return periods of 2.4, 3.5, 5.0, and 6.4 yr. Moreover, the anticipated return period of an extreme ENSO event with a maximum SST anomaly magnitude of 1.7°C is approximately every five warm events and every seven cold events.

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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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Andrew Johnson Jr.
and
James J. O'brien

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A series of meteorological observations including aircraft, pilot balloon (pibal), rawinsonde, surface buoy, and special land-based surface observations was taken on 23–24 August 1972, on the central Oregon coast, to investigate the mesoscale thermal and kinematic responses of the lowest 4 km of the atmosphere during a sea breeze event.

A description of those field observations is given. Vertical cross sections of the wind field on a line perpendicular to the coast, extending 60 km inland from data obtained at three pibal stations, are presented and discussed. Time sections of the wind field and temperature fields at the coast are discussed. Mesoscale features are presented and related to prevailing synoptic-scale changes occurring aloft during the observational period.

The sea breeze event on 23 August exhibited the following important characteristics: 1) a sea breeze front, distinguishable in the zonal wind field, which penetrated more than 60 km inland; 2) a distinct wind maximum which followed the front inland; 3) the surface onshore flow at the coast which took place below the main inversion, deepening the marine layer at the onset; and 4) a return flow above the inversion which appeared in quasi-periodic surges in response to surges in the sea breeze flow.

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