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Douglas S. Luther

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.

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Douglas S. Luther and Carl Wunsch

Abstract

New co-amplitude and cotidal charts of the central Pacific Ocean have been constructed for constituents M2, S2, N2, K1 and O1. The charts exhibit some significant differences from previous attempts. Admittance curves, calculated where possible, do not show any rapidly varying characteristics in contrast to the North Atlantic.

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Thomas Decloedt and Douglas S. Luther

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 of microstructure data from several oceanic regions, is provided. The parameterization, called the roughness diffusivity model (RDM), depends only on seafloor roughness and height above bottom and has the advantage of tacitly including a broad range of mixing processes catalyzed by the roughness or acuteness of the bottom topography. The study focuses in particular on the vertical structure of Kρ and shows that exponential decay, prominent in current diapycnal mixing parameterizations, does not provide an adequate representation of the mean vertical profile. Instead, an inverse square law decay with a scale height and maximum near-boundary value depending on topographic roughness is shown to provide a more realistic vertical structure. Resulting basin-averaged diffusivities based on the RDM, which increase from ∼3 × 10−5 m2 s−1 at 1-km depth to ∼1.5 × 10−4 m2 s−1 at 4 km, are roughly consistent with spatial averages derived from hydrographic data inversions, supporting the contention that strong, localized mixing plays a major role in maintaining the observed abyssal stratification. The power required to sustain the stratification in the abyssal ocean (defined as 40°S–48°N, 1–4-km depth) is shown to be sensitive to the spatial distribution of the mixing. The power consumption in this domain, given the parameterized bottom-intensified and horizontally heterogeneous diffusivity structure in the RDM, is estimated as approximately 0.37 TW (TW = 1012 W), considerably less than the canonical value of ∼2 TW estimated under the assumption of a uniform diffusivity of ∼10−4 m2 s−1 in the abyssal ocean.

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Christopher S. Meinen and Douglas S. Luther

Abstract

In the presence of a strong current, such as the Gulf Stream or the North Atlantic Current, current meter moorings are known to “blow over” due to drag from the moving water. This dipping of the current meters, which has been documented to exceed 500 m in some cases, can significantly affect estimates of fluxes on level surfaces. Pressure measurements made by sensors collocated along the mooring near each current meter are commonly used to correct for this mooring motion. Data from a current meter mooring near 42°N, 45°W are used to demonstrate that, in cases where there is a failure of the pressure sensors, measurements from an inverted echo sounder near the current meter mooring can be combined with the mooring temperature records and historical hydrography to produce “synthetic” pressure records for current meters within the main thermocline depth range. Pressures at other current meters on the mooring can then be determined using mooring design parameters. This technique allows corrections for mooring motion when they would otherwise be impossible due to the loss of the directly measured pressure records. Comparison to directly measured pressures in the main thermocline from a mooring near the North Atlantic Current demonstrates that this technique can determine synthetic pressure records to within a root-mean-square difference of about 46 dbar for an instrument with observed mooring motion related pressure dips of 200–500 dbar. The technique is also applied to a number of other current meters in the North Atlantic Current region as well as instruments that were moored in the Subantarctic Front near 143°E to demonstrate where the technique will and will not work.

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Douglas S. Luther and Eric S. Johnson

Abstract

Eddy energetics in the central equatorial Pacific Ocean is examined using Acoustic Doppler Current Profiler velocities and CTD densities collected during the Hawaii-to-Tahiti Shuttle Experiment, in 1979–80. Three distinct sources of eddy energy are identified with varying degrees of statistical reliability, and are interpreted as evidence for three separate instabilities of the mean flow field. An instability at and just north of the equator occurs primarily in boreal summer and fall. It arises from the cyclonic shear between the Equatorial Undercurrent and the South Equatorial Current (SEC) north of the equator. The instability is present only when and where both currents are well developed, and there is little involvement of the shear between the SEC and the North Equatorial Countercurrent (NECC). The instability is characterized by local maxima in zonal and meridional eddy velocity variance, strong U*V* Reynolds stress, and large mean flow to eddy kinetic energy conversion. Despite seasonal variability of the eddy kinetic energy production, no annual cycle energy is converted to eddy energy. A second instability occurs at the equatorial front at 3°N to 6°N, primarily during boreal winter. The instability is identified by large mean-to-eddy potential energy conversion. Finally, a third instability is evidenced by strong downgradient (northward) eddy heat flux and large mean flow to eddy potential energy conversion, in the thermocline of the NECC during boreal spring. Both features are confined below 60 m at 5°N–9°N. While the eddies gain potential energy from these last two instabilities, they are losing kinetic energy to the mean flow at a somewhat slower rate.

Nonlinear advection appears to be unimportant in the total eddy energy balance, but the meridional diffusion of eddy energy represented by the meridional divergence of eddy pressure work is large and significant The latter redistributes eddy energy into (not out of) the region of the barotropic instability just north of the equator.

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Alan D. Chave, Douglas S. Luther, and Christopher S. Meinen

Abstract

Interactions between motional electric fields and lateral gradients in electrical conductivity (e.g., seafloor topography) produce boundary electric charges and galvanic (i.e., noninductive) secondary electric fields that result in frequency-independent changes in the electric field direction and amplitude that are specific to a single location. In this paper, the theory of galvanic distortion of the motional electric field is developed from first principles and a procedure to correct for it is then derived. The algorithm is based on estimation of intersite transfer tensors for the horizontal electric fields at the high frequencies where external (ionospheric and magnetospheric) sources, not oceanic motionally induced electric fields, dominate. A decomposition of each measured tensor is derived that expresses it as the product of a set of distortion tensors and the underlying, undistorted transfer tensor. The algorithm may be applied simultaneously to a set of sites and assessed statistically, yielding the undistorted electric field uniquely at each site except for a single site-dependent multiplicative scalar, which must be obtained from other data. Because the distortion is frequency independent, the same tensors may be used to undistort the low-frequency, motional induction components that are of interest in oceanography. This procedure is illustrated using an electric field dataset collected in the Southern Ocean in 1995–97, which is significantly distorted by galvanic processes.

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Arthur J. Miller, Douglas S. Luther, and Myrl C. Hendershott

Abstract

The fortnightly and monthly tides are discussed in the light of recent sea level observations and numerical modeling results. Within the tide gauge network of the low-latitude Pacific, the fortnightly tide is shown to possess a large-scale phase lag of roughly 10–40 degrees. Although the nonequilibrium part of the fortnightly tide is traditionally thought to be dominated by Rossby wave dynamics, it is shown, via global shallow-water modeling studies, that this large-scale phase lag is explicable in terms of remotely forced gravity waves whose origin is mainly in the Arctic Ocean. Although future observations outside the low-latitude region of the Pacific may eventually reveal Rossby wave excitation, the fortnightly tidal signal in the tide gauge network at hand appears to reveal at most only weak excitation of Rossby waves compared to the phase lag due to remotely forced gravity waves. The observed monthly tide appears to be only slightly closer to equilibrium than the fortnightly tide. The reason for this remains unclear since the monthly tide is less affected by the remotely forced gravity waves than the fortnightly tide.

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Alan D. Chave, Douglas S. Luther, and Jean H. Filloux

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Five current meter moorings and four horizontal electric field records spanning June 1990–February 1992 are used to describe the mean structure and variability of the vertically averaged velocity field and volume transport extending 425 km east of Abaco, The Bahamas, at 26.5°N. Examination of zonal and meridional velocity sections shows that, while meandering may explain part of the variability, there is substantial evidence for pulsation of the core DWBC velocity in the record. Intermittently strong northward flow is observed 225 km east of Abaco that is significantly coherent and out of phase with the currents closer to the boundary at periods of 50–100 d and longer, suggesting recirculation. This is never observed 315 km offshore. At 380 km offshore and extending east at least 60 km, a strong, dominantly southward flow is observed that is coherent with both that near Abaco (in phase) and in the intervening recirculation zone (out of phase) at long periods. The net mean transport (over 17 months) from Abaco to 325 km offshore, spanning the recirculation region, is −17 Sv (Sv ≡ 106 m3 s−1). The transport exhibits robust annual variability, and correction for the bias from the annual cycle in the 17-month dataset reduces the net mean transport at 325 km to −13.6 Sv. Allowing for a northward Antilles Current transport of 5.1 Sv yields a mean southward DWBC transport corrected for local recirculation of about 18.5 Sv, in approximate agreement with the thermohaline input from the northern North Atlantic. Comparison of transport time series from the Florida Current with that extending 125 km east of Abaco demonstrates significant and out of phase coherence over the period range 100–250 d. The coherence decreases as the Abaco transport is integrated farther to the east.

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Gerard J. McNally, Douglas S. Luther, and Warren B. White

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Previous analyses of satellite-tracked drifting buoy data (30 m drogue depth) and Fleet Numerical Ocean Center (FNOC) surface wind stress in the midlatitude North Pacific during autumn/winter have shown near-surface current vectors 25° to the right of the surface wind stress vector (i.e., approximately parallel to sea level air pressure isobars). In the present study, the complex coherence between time series of the two vectors, near-surface current and surface wind stress, is examined using the vector cross-spectral analysis technique developed by Mooers, yielding the frequency response of surface current to wind stress from the inertial frequency down to one cycle per 16 days. The analysis shows that during summer the near-surface a few days. In this season, the rotary spectrum of the near-surface current is dominated by anticyclonic motion, with periods of approximately 8 to 32 days, that is not locally wind-forced. In contrast, during autumn/winter, the two vectors are highly coherent over the subinertial frequency range corresponding to periods of 1 to 16 days. The phase estimates provided by vector cross-spectral analysis yield information on both the mean spatial angle and mean temporal phase difference between the major axes of the two ellipses described by the vector motions. Over the same subinertial frequency range where the coherence amplitude is significant, the average spatial angle between the major axis of the wind stress and the major axis of the near-surface current varies from 75° at the near-inertial frequencies to 15° at the low frequencies. The sign of the spatial angle is such that the near-surface current vector is always directed to the right of the surface wind stress vector. The temporal phase differences between the vectors show that the near-surface current vector lags the surface wind stress vector by 20° to 30° at near-inertial frequencies, diminishing to zero degrees with decreasing frequency. These phase lags correspond to temporal lags of up to four hours.

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David Halpern, Robert A. Knox, and Douglas S. Luther

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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. The penetration depth of the oscillation was greatest at 110° and least at 140°W. At the equator the motion was essentially rectilinear, and at the off-equatorial sites the current vector rotated anticyclonically. Estimates of westward phase speeds were 81 and 93 cm s−1, the longitudinal wavelengths along the equator were about 1320 and 1600 km. The amplitude was seasonally and El Niño modulated, with the waves absent during March to May and during the onset of the 1982–83 El Niño when on both occasions the westward flowing South Equatorial Current at the equator was substantially reduced. Even during intervals of intense wave activity, the mean Richardson number was lowered by only about 15%, but remained above ¼. There was no evidence that the 0.05 cpd waves were directly generated by the local surface wind.

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