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C. A. Paulson

Abstract

Analytical expressions which specify non-dimensionalized wind speed and potential temperature gradients as functions of stability are integrated. The integrated equations are tested against Swinhank's wind and temperature profiles measured at Kerang, Australia. It is found that a representation suggested independently by Businger and by Dyer gives the best fit to temperature profiles and describes the wind profiles equally as well as a relation suggested by Panofsky et al.

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E. Leavitt and C. A. Paulson

Abstract

Atmospheric surface layer turbulent statistics measured during the Barbados Oceanographic and Meteorological Experiment 8 and 30 m above mean sea level are presented. The budget equations of turbulent kinetic energy, humility variance and temperature variance are examined. Within discussed limitations it is concluded that production equals dissipation in the case of turbulent kinetic energy and humidity variance. The analysis of the temperature variance budget revealed large differences between productions and dissipations computed assuming standard similarity functions derived from other data sets. Initial computation of fluxes revealed large systematic decreases with height in the shear stress and heat flux. Comparison with other results suggested corrections which would eliminate these differences. Comparison between profile fluxes and direct measurements suggests strong similarity of momentum and water vapor transfer.

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H. W. Wijesekera, C. A. Paulson, and A. Huyer

Abstract

Measurements of a fresh surface anomaly (fresh lens) produced by rainfall during a westerly wind burst have been analyzed. The measurements were made in December 1992 as part of the Coupled Ocean–Atmosphere Response Experiment in the western equatorial Pacific (2°S, 156°E). Measurements included radar estimates of rainfall, upper-ocean temperature (T), salinity (S), horizontal velocity, and microstructure. In situ observations of the fresh lens were made 5 to 7 hours after its formation. In the 5 hours after formation, the lens deepened to a depth of 40 m as indicated by its salinity anomaly. Salinity and temperature were highly correlated within the lens, consistent with its initial formation by cold rainfall. The T–S relation exhibited curvature, which can be explained by surface cooling and upper-ocean mixing subsequent to formation of the lens. The lens exhibited a horizontal velocity anomaly in the direction of wind, which extended down to a depth of 40 m. The horizontal velocity anomaly is consistent with momentum being trapped near the surface due to rain-induced stratification. Vertical velocity, estimated from the divergence of zonal velocity, showed downwelling at the leading edge of the lens and upwelling at the trailing edge. The magnitude of vertical velocity at a depth of 20 m is 20 m day−1. Richardson numbers within the lens were low (0.25 to 0.5), suggesting that turbulent mixing was governed by critical-Ri instability. Wavenumber spectra of T and S in the upper 20 m exhibit a −5/3 range, which extends to wavenumbers below the range of local isotropy. Spectral levels were used to estimate turbulent dissipation rates of T and S, which were in turn used to estimate turbulent fluxes of heat and salt. Turbulent fluxes were also estimated from microstructure observations between depths of 10 and 60 m. Fluxes within the fresh lens were nearly uniform from 2 m to 35 m depth, then decreased to near zero at 45 m. The lifetime of fresh lenses during westerly wind bursts appears to be less than one day.

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Clive E. Dorman, C. A. Paulson, and W. H. Quinn

Abstract

Meteorological and oceanographic data for Ocean Station Vessel N (30N, 14OW) are analyzed over 20 years (1951–70) and 7 years (1964–70), respectively. A rainfall estimate is constructed for the 20-year period. The yearly average rainfall is 23 cm, far less than existing estimates. Daily and seasonal variations are presented. Heat budgets of the surface show that the two decades (1951–60, 1961–70) are distinctly different. Anomalies of the 20 years of all meteorological variables are calculated. The pressure anomaly appears to be loosely correlated with anomalous large-scale events in the equatorial Pacific. Time series cross sections are shown of the mixed layer depth, bottle temperature and salinity. The near-surface density appears to be largely controlled by temperature.

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C. A. Paulson, E. Leavitt, and R. G. Fleagle

Abstract

One hundred forty-one simultaneous wind speed, temperature and humidity profiles measured during the Barbados Oceanographic and Meteorological Experiment (BOMEX) are analyzed. The observations were from heights 2–11 m above MSL and were made from the R/V Flip, a research vessel specially designed to be stable at sea. The wind measurements are corrected for the interference of Flip's hull with the air flow. Evaporation estimates from the profiles are in fair agreement with simultaneous estimates by the eddy-correlation method. However, the heat fluxes estimated by the two methods are in poor agreement. There appear to be diurnal variations in air temperature, sea surface temperature and stress. The flux of latent heat is large, averaging 17 mW cm−2, while the flux of sensible heat is always upward and ∼1 mW cm−2.

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Hongbo Qi, Roland A. De Szoeke, Clayton A. Paulson, and Charles C. Eriksen

Abstract

Current meter data from two sites were analyzed for near-inertial motions generated by storm during the ten-month period of the Ocean Storms Experiment in the northeast Pacific Ocean. The most striking feature of the inertial wave response to storms was the almost instantaneous generation of waves in the mixed layer, followed by a gradual propagation into the thermocline that often lasted many days after the initiation of the storm. The propagation of near-inertial waves generated by three storms in October, January, and March was studied by using group propagation theory based on the WKB approximation. It was found that wave frequencies were slightly superinertial, with inertial shifts 1%–3% in October and March and around 1% in January. The phase of near-inertial currents propagated upward below the mixed layer, confirming the downward radiation of energy by these waves. The average downward energy flux during the storm periods was between 0.5 and 2.8 mW m−2. The vertical wavelengths indicated by the vertical phase differences ranged from 150 to 1500 m. The vertical group velocity was estimated from the arrival times of the groups at successive depths. Using this in the dispersion relation, horizontal wavelengths ranging from 140 to 410 km were obtained. A relation between density and velocity that gives the horizontal directionality of internal waves was derived. During the storm periods examined, the propagation directions of near-inertial waves mainly lay between northeast and south, indicating sources west of moorings. The directions tended to rotate clockwise with increasing depth, consistent with the expected effect of the earth's curvature. The estimated horizontal wavelength and propagation direction were consistent with the horizontal phase difference between inertial currents at the two sites.

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J. N. Moum, D. R. Caldwell, C. A. Paulson, T. V. Chereskin, and L. A. Regier

Abstract

A 3°N to 3°S transect of the equator at 140°15'W was made in November 1984. Vertical profiles of temperature, conductivity and turbulent dissipation were obtained at approximately 1 km intervals. Contrary to previous results, we found no obvious peak in dissipation either at the equator or clearly associated with the Equatorial Undercurrent. A thermistor chain towed behind the ship indicated the rich (and previously unseen) variability of the hydrophysical field of the equatorial ocean. Some of this variability (especially, internal waves) is intimately linked to mixing processes.

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J. N. Moum, D. Hebert, C. A. Paulson, and D. R. Caldwell

Abstract

High correlations between turbulent dissipation rates and high-wavenumber internal waves and the high values of turbulent dissipation associated with internal wave activity suggest that internal waves are the main direct source of mixing in the thermocline above the core of the Equatorial Undercurrent. An extensive dataset obtained using a microstructure profiler and thermistor chain towed along the equator was analyzed to examine the correspondence between turbulent mixing and high-wavenumber internal waves. In the low Richardson number (Ri) thermocline below the mixed layer but above the core of the Equatorial Undercurrent, and when winds were moderate and steadily westward, it was found that:

• the spectrum of vertical isotherm displacement was dominated by a narrow wavenumber band (corresponding to 150–250-m zonal wavelength) of internal waves;

• both turbulence and internal waves varied diurnally—hourly averaged values of turbulent dissipation rate and wave potential energy were greater by a factor of 100 at night; and

• correlations between turbulent dissipation rate and several measures of internal wave activity (wave isotherm displacement, wave slope, and wave potential energy) were high.

Little or no high wavenumber internal wave activity was observed when winds were light or eastward: Superposing plane waves with the observed characteristics on the observed background field suggests that they are inherently unstable to both adjective and shear instability above the core of the Equatorial Undercurrent. These waves are due either to locally generated internal gravity waves or to Kelvin-Helmholz-type instabilities generated in the shear flow; from our measurements these two phenomena could not be distinguished.

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J. N. Moum, D. Hebert, C. A. Paulson, D. R. Caldwell, M. J. McPhaden, and H. Peters

Abstract

Appearing in this issue of the Journal of Physical Oceanography are three papers that present new observations of a distinct, narrow band, and diurnally varying signal in temperature records obtained in the low Richardson number shear flow above the core of the equatorial undercurrent. Moored data suggest that the intrinsic frequency of the signal is near the local buoyancy frequency, while towed data indicate that the horizontal wavelength in the zonal direction is 150–250 m. Coincident microstructure profiling shows that this signal is associated with bursts of turbulent mixing, it seems that this narrowband signal represents the signature of instabilities that ultimately cause the turbulence observed in the equatorial thermocline. Common problems in interpreting the physics behind the signature are discussed here.

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D. Hebert, J. N. Moum, C. A. Paulson, and D. R. Caldwell

Abstract

In the low Richardson number shear flow above the Pacific Equatorial Undercurrent, a single vertical microstructure profile intersected the overturning crest of a packet of high horizontal wavenumber waves. The observed dissipation rates within the overturning wave were so high that if they were representative of the volume-averaged rate, the total wave energy would have been dissipated within a single buoyancy period. The chaotic structure (and temperature fluctuations with horizontal scales less than 2 m) of the two wave crests and troughs west of the overturning wave crest suggest that recent mixing had occurred there. Wave crests and troughs east of the overturning wave crest showed little or no sign of turbulent mixing.

Similar high horizontal wavenumber waves, believed to be shear-instability waves, have been observed in low Richardson number regions of the midlatitude seasonal thermocline. Although the equatorial waves have a horizontal wavelength appropriate for shear-instability waves, their vertical scale is much larger than the vertical extent of the low Richardson region, unlike that found for simple shear-instability waves.

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