Douglas S. Luther

Barotropic and Baroclinic Tides in the Central North Pacific Ocean Determined from Long-Range Reciprocal Acoustic Transmissions

Abstract Travel times of reciprocal 1000-km range acoustic transmissions, determined from the 1987 Reciprocal Tomography Experiment, are used to study barotropic tidal currents and a large-scale, coherent baroclinic tide in the central North Pacific Ocean. The difference in reciprocal travel times determines the tidal currents, while the sum of reciprocal travel times determines the baroclinic tide displacement of isotachs (or equivalently, isotherms). The barotropic tidal current accounts for 90% of the observed differential travel time variance. The measured harmonic constants of the eight major tidal constituents of the barotropic tide and the constants determined from current meter measurements agree well with the empirical–numerical tidal models of Schwiderski and Cartwright et al. The amplitudes and phases of the first-mode baroclinic tide determined from sum travel times agree with those determined from moored thermistors and current meters. The baroclinic tidal signals are consistent with a large-...

Observations of 20-day period meridional current oscillations in the upper ocean along the Pacific Equator

Abstract Prominent oscillations of the meridional current, with a mean period of approximately 20 days, have been observed in the upper ocean over several years from May 1979 to October 1985 using moored current measurements along the Pacific equator at 95°, 110°, 124°,140°W and 152°W, as well as off (but near) the equator at 110° and 140°W. The fluctuations are relatively narrowband (±0.005 cpd) in frequency. A 95% statistically significant peak in power spectra of meridional current occurred at 110°, 124° and 140°W, but not at 95° and 152°W where the spectral peaks were smaller. The dominant wave period decreased by about 4% from 110° to 140°W. Maximum amplitude was measured at 124°W; the amplitude above 80 m was maximum at the equator and decreased poleward from the equator. At 15 m the annual averaged root-mean-square amplitude was about 20.5 cm s−1, and individual peak-to-trough values reached 150 cm s−1. The wave amplitude decreased with depth and the wave was essentially confined to the upper 80 m....

Zonal Winds in the Central Equatorial Pacific and El Niño

Easterly trade winds from near-equatorial islands in the central Pacific weakened before each El Ni�o between 1950 and 1978, except for the 1963 El Ni�o. The weakening of the easterlies and their later collapse did not occur uniformly over several months, but rather through a series of strong westerly wind bursts lasting 1 to 3 weeks. The bursts may force equatorial Kelvin waves in the ocean that can both initiate and sustain the sea surface warming characteristics of El Ni�o events.

Energetics of M2 Barotropic-to-Baroclinic Tidal Conversion at the Hawaiian Islands

Abstract A high-resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide (M2) over a region of the Hawaiian Ridge from Niihau to Maui. This region includes the Kaena Ridge, one of the three main internal tide generation sites along the Hawaiian Ridge and the main study site of the Hawaii Ocean Mixing Experiment. The 0.01°–horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations, moored ADCP currents, and notably reasonable agreement with microstructure data. Barotropic and baroclinic energy equations are derived from the model’s sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget. The M2 barotropic tide loses 2.7 GW of energy over the study region. Of this, 163 MW (6%) is dissipated by bottom friction and 2.3 GW (85%) is converted into internal tides. Internal tide generation primarily occurs along the flanks of the Kaena Ridge and ...

Observations of Enhanced Diapycnal Mixing near the Hawaiian Ridge

Profiles of potential density obtained from CTD casts at two stations at different distances from the Hawaiian ridge are examined for evidence of diapycnal turbulent mixing as indicated by density inversions and internalwave vertical strain. Results from independent casts are used to produce ensemble-averaged vertical distributions for the number of inversions and the Thorpe scale. Both parameters were found to be higher over the slope of the topography at 2500-m depth than in the deep ocean, 110 km to the north. Thorpe scale‐based estimates of the rate of dissipation of turbulent kinetic energy and turbulent vertical diffusivity are elevated by an order of magnitude over the slope relative to deep ocean background levels. The vertical distributions of these mixing parameters are nonuniform and exhibit signs of locally enhanced dissipation, possibly due to internal tides generated at the ridge. At the deep station, turbulence is at background levels from the surface down to 2000 m. Below this, a localized zone of enhanced mixing is observed, within which the dissipation rate is O(1029 W kg21) and turbulent diffusivity is greater than O(1024 m2 s 21), perhaps due to an internal tide ray originating at the ridge. The full-depth topographical enhancement of mixing near the ridge also appears in the vertical strain field. Estimates of dissipation rate and turbulent diffusivity, based on an internal wave‐wave interaction model, give results similar to direct Thorpe scale methods, except in weakly stratified environments where both methods are subject to uncertainty. Near the topography, the variation in mixing intensity observed between casts is sensitive to sporadic large mixing events, which are triggered by internal waves associated with the spring tide. The upper portion of the water column (stronger stratification) is more responsive to the tide than the deep regions.

Evidence of a 4–6 Day Barotropic, Planetary Oscillation of the Pacific Ocean

Abstract Spectral analysis of scattered island and coastal tide-gage records from the Pacific Ocean reveals the presence of a coherent sea level fluctuation at 4–6 days period. The oscillation is distinct from baroclinic, inertia-gravity wave fluctuations of sea level at the same periods that are trapped to the central Pacific equatorial zone. Concomitant Spectral analysis of island surface weather data demonstrates that sea level is forced by surface atmospheric pressure but does not respond statically like an “Inverted barometer”. The basinwide character and uniform westward propagation of the oscillation suggest the presence of a barotropic, planetary wave(s). However, the oscillation is strongly attenuated. with an estimated energy e-folding time of less than three days.

Incoherent Nature of M2 Internal Tides at the Hawaiian Ridge

AbstractMoored current, temperature, and conductivity measurements are used to study the temporal variability of M2 internal tide generation above the Kaena Ridge, between the Hawaiian islands of Oahu and Kauai. The energy conversion from the barotropic to baroclinic tide measured near the ridge crest varies by a factor of 2 over the 6-month mooring deployment (0.5–1.1 W m−2). The energy flux measured just off the ridge undergoes a similar modulation as the ridge conversion. The energy conversion varies largely because of changes in the phase of the perturbation pressure, suggesting variable work done on remotely generated internal tides. During the mooring deployment, low-frequency current and stratification fluctuations occur on and off the ridge. Model simulations suggest that these variations are due to two mesoscale eddies that passed through the region. The impact of these eddies on low-mode internal tide propagation over the ridge crest is considered. It appears that eddy-related changes in stratif...

Tidal Mixing Events on the Deep Flanks of Kaena Ridge, Hawaii

Abstract A 3-month mooring deployment (August–November 2002) was made in 2425-m depth, on the south flank of Kaena Ridge, Hawaii, to examine tidal variations within 200 m of the steeply sloping bottom. Horizontal currents and vertical displacements, inferred from temperature fluctuations, are dominated by the semidiurnal internal tide with amplitudes of ≥ 0.1 m s−1 and ∼100 m, respectively. A series of temperature sensors detected tidally driven overturns with vertical scales of ∼100 m. A Thorpe scale analysis of the overturns yields a time-averaged dissipation near the bottom of 1.2 × 10−8 W kg−1, 10–100 times that at similar depths in the ocean interior 50 km from the ridge. Dissipation events much larger than the overall mean (up to 10−6 W kg−1) occur predominantly during two phases of the semidiurnal tide: 1) at peak downslope flows when the tidal stratification is minimum (N = 5 × 10−4 s−1) and 2) at the flow reversal from downslope to upslope flow when the tidal stratification is ordinarily increasi...

On a Simple Empirical Parameterization of Topography-Catalyzed Diapycnal Mixing in the Abyssal Ocean

Abstract The global spatial distribution of the turbulent diapycnal diffusivity in the abyssal ocean is reexamined in light of the growing body of microstructure data revealing bottom-intensified turbulent mixing in regions of rough topography. A direct and nontrivial implication of the observed intensification is that the diapycnal diffusivity Kρ, is depth dependent and patchily distributed horizontally across the world’s oceans. Theoretical and observational studies show that bottom-intensified mixing is dependent upon a variety of energy sources and processes whose contributions to mixing are sufficiently complex that their physical parameterization is premature; only rudimentary parameterizations of tidally induced mixing have been attempted, although the tides likely provide no more than half of the mechanical energy available for diapycnal mixing in the abyssal ocean. Here, an empirical (and still rudimentary) parameterization of the spatially variable mean diffusivity Kρ based on a large collection...

Mean zonal momentum balance in the upper and central equatorial Pacific Ocean

We examine the mean zonal momentum balance in the tropical mid-Pacific using a year of acoustic Doppler current profiler velocities and conductivity-temperature-depth profiler densities from the Hawaii-to-Tahiti Shuttle Experiment. All significant contributions from the mean, annual cycle, and higher-frequency flow fields are determined with the exception of the vertical stresses. We find that even neglecting vertical stresses, the zonal momentum equation is in rough balance at 90–117-m depth at all latitudes from 4°S to 10°N. While the formal error bars are large, this rough balance is reproducible over four to five independent latitudes and so is probably real. The balance at 90-m depth is geostrophic to within 5° of the equator. Closer to the equator, meridional mean convergence and meridional eddy stresses contribute important forces to balance the mean pressure gradient. Nearer the surface, the zonal momentum equation is dominated by eastward pressure gradients near the equator and eastward Coriolis forces from a strong, northward Ekman flow poleward of 2°N. In the vertical integral these forces roughly balance the surface wind stress; thus vertical stresses suffice to close our momentum budget. We conclude that on average vertical stresses arising from the wind forcing do not penetrate deeper than 90 m into the tropical ocean. This contradicts an earlier study of the equatorial zonal momentum budget but is consistent with turbulent dissipation measurements on the equator. Previous findings of stronger, deeper dissipation on the equator are probably due to the stronger, deeper mean shear there rather than to a locally altered stress profile. Vertical turbulent viscosities derived from our observations agree with previous observations on the equator but contradict the conventional, Richardson number parameterization off the equator.