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P. Flament and M. Sawyer

Abstract

The thermohaline response of the ocean to a short (10 h) but intense (95 mm) nighttime rainfall event was observed during a transit through the ITCZ. Two CTD profiles and shipboard measurements of air–sea fluxes were consistent with the assumption that rain temperature equals the wet-bulb temperature, within measurement errors. Although the net freshwater input and the net heat loss inferred from the TS characteristics of the surface layer were ∼30% smaller than those obtained by integrating the measured air–sea fluxes, owing to different spatial sampling, inherent limitations of rain measurement from ship, and contamination by internal waves, the two independent estimates of the net heat deficit agreed remarkably well, within 2.4%, when expressed per unit mass of rain (∼72kj kg−1). The heat flux due to the temperature of the rain accounted for about 40% of the net heat flux during rain, and therefore cannot be neglected.

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P. Flament, J. Firing, M. Sawyer, and C. Trefois

Abstract

Intense diurnal warming of the ocean surface was observed in April 1982 off California, using a combination of mooring, hydrographic, and satellite infrared and satellite pigment measurements. The event corresponded to a spatial and temooral minimum of the wind stress. The diurnal surface temperature amplitude exceeded 6.6°C locally despite a 490-nm optical depth of 20 m, suggesting that phytoplankton was not responsible for the shallow heat trapping. Coherent horizontal temperature streaks at least 50 km long and 4-8 km wide formedduring the subsequent erosion of the shallow warm layers. It is hypothesized that thcfr scale was set by planetary boundary-layer circulations.

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C. Chavanne, P. Flament, D. Luther, and K-W. Gurgel

Abstract

Observations of semidiurnal surface currents in the Kauai Channel, Hawaii, are interpreted in the light of the interaction of internal tides with energetic surface-intensified mesoscale currents. The impacts on internal tide propagation of a cyclone of 55-km diameter and ∼100-m vertical decay scale, as well as of vorticity waves of ∼100-km wavelength and 100–200-m vertical decay scales, are investigated using 3D ray tracing. The Doppler-shifted intrinsic frequency is assumed to satisfy the classic hydrostatic internal wave dispersion relation, using the local buoyancy frequency associated with the background currents through thermal-wind or gradient-wind balance. The M 2 internal tide rays with initial horizontal wavelength of 50 km and vertical wavelength of O(1000 m) are propagated from possible generation locations at critical topographic slopes through idealized mesoscale currents approximating the observed currents. Despite the lack of scale separation between the internal waves and background state, which is required by the ray-tracing approximation, the results are qualitatively consistent with observations: the cyclone causes the energy of internal tide rays propagating through its core to increase near the surface (up to a factor of 15), with surfacing time delayed by up to 5 h (∼150° phase lag), and the vorticity waves enhance or reduce the energy near the surface, depending on their phase. These examples illustrate the fact that, even close to their generation location, semidiurnal internal tides can become incoherent with astronomical forcing because of the presence of mesoscale variability. Internal tide energy is mainly affected by refraction through the inhomogeneous buoyancy frequency field, with Doppler shifting playing a secondary but not negligible role, inducing energy transfers between the internal tides and background currents. Furthermore, the vertical wavelength can be reduced by a factor of 6 near the surface in the presence of the cyclone, which, combined with the energy amplification, leads to increased vertical shear within the internal tide rays, with implications for internal wave-induced mixing in the ocean.

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F. I. M. Thomas, S. A. McCarthy, J. Bower, S. Krothapalli, M. J. Atkinson, and P. Flament

Abstract

Response characteristics of a microhole potentiostatic oxygen sensor and a Beckman membrane oxygen sensor were measured in a laboratory over temperatures ranging from 1° to 21°C. The response term τ of the microhole sensor changed 1.7-fold over this temperature range, and τ of the membrane sensor changed 1.6-fold. For the microhole sensor, the effect of temperature on τ can be modeled as lnτ+−6.5 + 1618T −1. For the membrane sensor the temperature effect on τ can be modeled as lnτ = −5.8 + 2116T −1, where T is temperature in kelvins.

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X. Flores-Vidal, P. Flament, R. Durazo, C. Chavanne, and K.-W. Gurgel

Abstract

Linear array antennas and beamforming techniques offer some advantages compared to direction finding using squared arrays. The azimuthal resolution depends on the number of antenna elements and their spacing. Assuming an ideal beam pattern and no amplitude taper across the aperture, 16 antennas in a linear array spaced at half the electromagnetic wavelength theoretically provide a beam resolution of 3.5° normal to the array, and up to twice that when the beam is steered within an azimuthal range of 60° from the direction normal to the array. However, miscalibrated phases among antenna elements, cables, and receivers (e.g., caused by service activities without recalibration) can cause errors in the beam-steering direction and distortions of the beam pattern, resulting in unreliable ocean surface current and wave estimations. The present work uses opportunistic ship echoes randomly received by oceanographic high-frequency radars to correct an unusual case of severe phase differences between receiver channels, leading to a dramatic improvement of the surface current patterns. The method proposed allows for simplified calibrations of phases to account for hardware-related changes without the need to conduct the regular calibration procedure and can be applied during postprocessing of datasets acquired with insufficient calibration.

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C. Chavanne, P. Flament, G. Carter, M. Merrifield, D. Luther, E. Zaron, and K-W. Gurgel

Abstract

Observations of semidiurnal currents from high-frequency radio Doppler current meters and moored acoustic Doppler current profilers (ADCPs) in the Kauai Channel, Hawaii, are described and compared with two primitive equation numerical models of the tides. The Kauai Channel, separating the islands of Oahu and Kauai, is a site of strong internal tide generation by the barotropic tides flowing over Kaena Ridge, the subsurface extension of Oahu. The nature and impacts of internal tide generation in the Kauai Channel were intensively studied during the 2002–03 near-field component of the Hawaii Ocean Mixing Experiment.

Comparisons of observed coherent (i.e., phase locked to the astronomical forcing) M 2 and S 2 surface currents with model predictions show good agreement for the phases, indicating propagation of internal tides away from the ridge. Although the predicted M 2 and S 2 surface currents are similar (except for their magnitudes), as expected for internal waves at periods closer to each other (12.4 and 12 h, respectively) than to the inertial period (33 h), the observed M 2 and S 2 surface currents differ significantly. The S 2 kinetic energy pattern resembles the predicted pattern. In contrast, the observed structure and magnitude of the more important M 2 kinetic energy pattern differs significantly from the model predictions. The models predict a band of enhanced M 2 surface kinetic energy 30–40 km from the ridge axis, corresponding to the first surface reflection of internal tide beams generated on the ridge flanks. The beams are clearly observed by the moored ADCPs, albeit with weaker amplitudes than predicted. Observations at the surface show an area of enhanced kinetic energy that is 10–20 km farther away from the ridge than predicted, with weaker magnitude. Observed M 2 surface currents also exhibit apparent seasonal variability, with magnitudes weaker in spring 2003 than in fall 2002.

Complex-demodulated semidiurnal currents exhibit significant temporal variability in amplitude and phase, not only because of the interference between semidiurnal constituents (e.g., the spring–neap cycle) but also on shorter and irregular time scales. The result is that ∼20% of semidiurnal energy is incoherent with astronomical forcing. Furthermore, the temporal variability is not spatially coherent; the spatial patterns of semidiurnal kinetic energy resemble those predicted by the numerical models during the strongest spring tides but differ from them at other times. As a result, M 2 and S 2 kinetic energy patterns phase locked to the astronomical forcing differ from each other. Some features of the observed spatial pattern and amplitude modulations can be qualitatively reproduced by a simple analytical model of the effects of homogeneous barotropic background currents on internal tide beams.

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Cédric P. Chavanne, Pierre Flament, Douglas S. Luther, and Klaus-Werner Gurgel

Abstract

Short-wavelength (L ∼ 100 km) Rossby waves with an eastward zonal phase velocity were observed by high-frequency radio Doppler current meters and moored ADCPs west of Oahu, Hawaii, during spring 2003. They had Rossby numbers Ro = |ζ/f| = O(1), periods of 12–15 days, and phase speeds of 8–9 cm s−1, and they were surface trapped with vertical e-folding scales of 30–170 m. They transferred horizontal kinetic energy to the background flow of a mesoscale cyclone lying 160–190 km west of Oahu, revealed by altimetry. The waves approximately satisfied the dispersion relation of vortex Rossby waves propagating through the radial gradient of potential vorticity associated with the cyclone. Vertical shear of the background currents may also affect wave propagation. Theoretical studies have shown that vortex Rossby waves provide a mechanism by which perturbed vortices axisymmetrize and strengthen and may be important to the dynamics of oceanic vortices.

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