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Eric A. D'Asaro

Abstract

Modern surface drifters tracked by Argos are an attractive method for measuring the spatial structure of near-surface currents. This note discusses the accuracy to which velocity can be estimated from such data, assuming perfect drifters. The analysis concentrates on data from OCEAN STORMS centered at 47.5°N in the eastern North Pacific, a region of low mesoscale activity. The irregular, but nearly diurnally repeating, pattern of fixes leads to leakage between near-inertial (1.48 cpd) and subinertial (0.5 cpd) frequencies. Total spectral leakage for a naive spline interpolant of the fixes is about 2×10−3 in energy, or 5% in amplitude. Other interpolants can produce an order of magnitude more leakage. An algorithm that controls these errors is described. Only an inertial peak and frequencies well below 0.5 cpd can be resolved. The remaining noise can be described as the sum of a random fix error of 600 m rms and unresolved subinertial velocities with an rms displacement of about 550 m. The errors in the computed inertial and low-frequency velocities are 0.03 and 0.01 m s−1, respectively. These can be reduced with further time averaging. Significantly better estimates of velocities would require both more accurate and more frequent position fixes.

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Eric A. D'asaro

Abstract

The interaction of strong near-inertial frequency currents generated by a storm with preexisting subinertial frequency currents is investigated. For 10 days after the storm, the near-inertial currents remain in the mixed layer and the subinertial currents are steady, so their interaction is particularly simple. Linearized models predict that the frequency of the near-inertial currents should be shifted by ½ζ where ζ is the subinertial vorticity. This theory, combined with values of ζ estimated either from velocity measurements or from the vorticity equation, produces frequency shifts in the inertial currents at least five times larger than the confidence limits on the observations. Possible explanations include the concentration of ζ in narrow frontal zones and nonlinear wave-wave interactions.

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Eric A. D'asaro

Abstract

The evolution of near-inertial frequency currents is often thought to be controlled by the linear, inviscid equations of motion. This hypothesis is tested by simulating the near-inertial currents described in Part I using a two-dimensional, nearly inviscid, nonlinear layer model with realistic wind forcing and stratification. The β effect and mixing of momentum below the mixed layer during the storm are crucial to realistic modeling, whereas the nonlinear terms have only a minor effect. The model fails to simulate the observations in several ways. First, the mixed layer inertial currents decay more rapidly than predicted and propagate into the thermocline with a different pattern. Second, the shear at the base of the mixed layer decays much more rapidly than predicted. Third, mesoscale eddies modulate the evolution of the inertial currents much less than predicted. These differences are much larger than the errors in the observations and cannot be removed by reasonable variations of the forcing or stratification. The nearly linear and inviscid internal wave equations thus cannot accurately predict the observed evolution of the near-inertial currents; additional physical processes, perhaps nonlinear interactions with smaller-scale internal waves and/or fronts, are required in the equations.

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Eric A. D’Asaro

Abstract

Advances in low-power instrumentation and communications now often make energy storage the limiting factor for long-term autonomous oceanographic measurements. Recent advances in photovoltaic cells, with efficiencies now close to 30%, make solar power potentially viable even for vehicles such as floats that only surface intermittently. A simple application, the development of a solar-powered Argos recovery beacon, is described here to illustrate the technology. The 65-cm2 solar array, submersible to at least 750 dbar, powers an Argos beacon. Tests indicate that with minor improvements the beacon will run indefinitely at any latitude equatorward of about 50°. Scaling up this design to current operational profiling floats, each profile could easily be powered by a few hours of solar charging, a shorter time than is currently being used for Argos data communications.

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Eric A. D'Asaro

Abstract

A truly Lagrangian float would follow all three components of oceanic velocity on all timescales. Progress toward this goal is reviewed by analyzing the performance of nearly Lagrangian floats deployed in a variety of oceanic flows. Two new float types, described in this paper, are autonomous with durations of months, can alternate between Lagrangian and profiling modes, relay data via satellite, and can carry a variety of sensors. A novel hull design is light, strong, and has a compressibility close to that of seawater.

The key to making floats accurately Lagrangian is an improved understanding of the factors that control float buoyancy and motion. Several insights are presented here. Anodized aluminum gains weight in seawater due to reactions between its surface and seawater. At low pressure the buoyancy of floats with O-ring seals varies as if attached bubbles of air were being compressed. The volume of “air” decays exponentially with a decay scale of a few days from 10 to 30 cc at deployment to an asymptotic value that depends on pressure. The drag of floats moving slowly through a stratified ocean is dominated by internal wave generation and is thus linear, not quadratic. Internal wave drag acting on an isopycnal-seeking float will cause the float to be Lagrangian for frequencies greater than about N/30, where N is the buoyancy frequency.

These floats have proven most useful in measuring the turbulence in ocean boundary layers and other regions of strong turbulence where the ability of the floats to be Lagrangian on short timescales matches the short timescale of the processes and where the size of the turbulent eddies exceeds the size of the float. On longer timescales, the floats successfully operate as isopycnal followers. Because truly Lagrangian floats are highly sensitive to minor perturbations, extension of the frequency band over which the floats are Lagrangian will require careful control of float buoyancy and thus a detailed understanding of the float's equation of state.

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Eric A. D'Asaro

Abstract

The ability of neutrally buoyant, high-drag floats to measure the air–sea heat flux from within the turbulent oceanic boundary layer is investigated using float data from four different winter and fall float deployments. Two flux estimates can be made: Q 0A measures the vertical advection of heat, and Q 0D integrates the Lagrangian heating rate. Because the floats are only imperfectly Lagrangian, a key issue is diagnosing the ability of a given set of float data to make accurate flux measurements. A variety of diagnostics are explored and evaluated. Here Q 0A and Q 0D are compared to heat flux measurements computed using bulk formulas and shipboard measurements for one 2-week cruise. Quality controlled float-based fluxes agree with bulk fluxes to within 10 W m−2 for both positive and negative values. The differences are well within the expected statistical errors in the float measurements and the bias errors of the bulk measurements.

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Eric A. D'Asaro

Abstract

Time series of wind stress computed from long-term meteorological buoy data off North America are used to examine the forcing of surface inertial currents by the wind. A simple damped slab model of the mixed layer is used to compute 〈Π(H)〉, the average flux of energy from the wind to mixed layer inertial currents in a mixed layer of fixed depth H. The forcing of mixed layer inertial motions is highly intermittent. Most of the forcing occurs during the winter months, with a few dozen events accounting for typically over half the total energy flux. Major forcing events are usually associated with translating cold fronts or small lows with scales of about 100 km. The larger, synoptic scale features have little energy at the inertial frequency and thus result in only weak forcing of inertial currents. A strong seasonal signal exists in the inertial forcing. At OWS-P (50°N, 145°W), 〈Π(50 m)〉 is largest from October to February and is a factor of 12 above the June and July values. If seasonally varying mixed layer depths are used, 〈Π(H)〉 is largest in October, due to the combination of a shallow mixed layer and strong forcing. The forcing of inertial motions varies with location, although comprehensive geographical coverage is not obtained here. In these data the 1~ wintertime average 〈Π(50 m)〉 is found off the east coast of North America at about 35°N. The smallest value, in the Gulf of Mexico, is four times less. Although a strong correlation between 〈Π(50 m)〉 and 〈u *3〉 the parameterized flux of energy to mixed layer turbulence, is found at OWS-P, this correlation does not hold at all other locations. This suggests that climatological models that attempt to parameterize 〈Π〉 in terms of 〈u *3〉 will need to be tuned to local conditions.

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Eric A. D'Asaro

Abstract

Three neutrally buoyant floats were air deployed ahead of Hurricane Dennis on 28 August 1999. These floats were designed to accurately follow three-dimensional water trajectories and measure pressure (i.e., their own depth) and temperature. The hurricane eye passed between two of the floats; both measured the properties of the ocean boundary layer beneath sustained 30 m s−1 winds. The floats repeatedly moved through a mixed layer 30–70 m deep at average vertical speeds of 0.03–0.06 m s−1. The speed was roughly proportional to the friction velocity. Mixed layer temperature cooled about 2.8° and 0.75°C at the floats on the east and west sides of the northward-going storm, respectively. Much of the cooling occurred before the eye passage. The remaining terms in the horizontally averaged mixed layer heat budget, the vertical velocity–temperature covariance and the Lagrangian heating rate, were computed from the float data. Surface heat fluxes accounted for only a small part of the cooling. Most of the cooling was due to entrainment of colder water from below and, on the right-hand (east) side only, horizontal advection and mixing with colder water. The larger entrainment flux on this side of the hurricane was presumably due to the much larger inertial currents and shear. Although these floats can make detailed measurements of the heat transfer mechanisms in the ocean boundary layer under these severe conditions, accurate measurements of heat flux will require clusters of many floats to reduce the statistical error.

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Eric A. D'Asaro

Abstract

Upper ocean currents and temperature in the northeastern Pacific were measured during a 14-day period in November 1980 as part of STREX. Velocities in the upper ocean are dominated by near-inertial frequency oscillations. Thew oscillations are modified by wind stress variations associated with the passage of a strong cold front. The change in the inertial currents both in the mixed layer and below is qualitatively consistent with linear internal wave dynamics if turbulent stresses during the storm are assumed to extend 10–20 m below the mixed layer.

The ratio of mean squared buoyancy frequency N 2 and mean squared shear S 2 computed over a 10 m interval defines an average Richardson number R 0=N 2/S 2; R 0 is approximately 2.5 except in regions of high inertial shear. In particular, values as low as 0.7 are obtained in a 20 m thick region immediately below the base of the mixed layer. The data are consistent with a model of the oecanic shear field consisting of a background shear, corresponding to a value of R 0=2.5, plus a variable inertial frequency shear field. Variations in R 0, and by implication the rate of mixing, are due primarily to variations in the inertial frequency shear.

The mixed layer deepened 3–5 m during the 15 November storm. The temperature profiles suggest that mixing due to the storm extended roughly 5 m below the mixed layer. The mixed layer model of Niiler successfully models the observed response of the mixed layer. The amount of deepening is sensitive to the preexisting inertial currents during the storm passage. Using this model the amount of deepening could have been up to 80% greater than observed, if the storm had occurred earlier in the measurements, when the preexisting inertial currents were larger.

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Eric A. D'Asaro

Abstract

Vertical velocities in the ocean boundary layer were measured for two weeks at an open ocean, wintertime site using neutrally buoyant floats. Simultaneous measurements of the surface meteorology and surface waves showed a large variability in both wind and wave properties and only weak correlations between them. Buoyancy forcing was weak. The mean square vertical velocity in the boundary layer σ2w measured from the vertical motion of the floats was proportional to the squared friction velocity u2 estimated from shipboard meteorological measurements using bulk formulas. Thus σ2w = A u2 (the rate of momentum transport from the atmosphere to the ocean is ρ u2, where ρ is the density of the water). The deviations from this relation can be attributed entirely to statistical variation and measurement error. The measured values of σ2w were corrected for measurement biases and the nonturbulent contributions of internal waves. The value of the turbulent part of A is 1.75–2 times that measured in laboratory and oceanic solid-wall turbulent boundary layers driven by shear alone. Although surface waves undoubtedly play a large role in the physics of the oceanic boundary layer, their effects on vertical velocity variance are remarkably well parameterized by wind stress in this data.

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