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Daniel L. Rudnick and Jochen Klinke

Abstract

The development of the Underway Conductivity–Temperature–Depth (UCTD) instrument is motivated by the desire for inexpensive profiles of temperature and salinity from underway vessels, including volunteer observing ships (VOSs) and research vessels. The UCTD operates under the same principle as an expendable probe. By spooling tether line both the probe and a winch aboard ship, the velocity of the line through the water is zero, the line drag is negligible, and the probe can get arbitrarily deep. Recovery is accomplished by reeling the line back in. Recovering the UCTD has some advantages: 1) the cost per profile decreases with increasing use, 2) sensors can be calibrated postdeployment, 3) the UCTD carries a pressure sensor so depth is measured directly, and 4) no hazardous materials are left behind. The design goal for the UCTD was to obtain profiles deeper than 100 m at 20 kt (typical of a VOS). This goal has been surpassed, as it is able to profile to over 150 m at 20 kt and to over 400 m at 10 kt. The first operational use of the UCTD occurred during a May–June 2004 cruise, the purpose of which was to examine the effect of internal waves and spice on long-range acoustic propagation. Over 160 UCTD casts were completed, resulting in a hydrographic section with resolutions of 10 km horizontally and 5 m vertically.

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Martha C. Schönau and Daniel L. Rudnick

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Autonomous underwater Spray gliders made repeat transects of the Mindanao Current (MC), a low-latitude western boundary current in the western tropical North Pacific Ocean, from September 2009 to October 2013. In the thermocline (<26 kg m−3), the MC has a maximum velocity core of −0.95 m s−1, weakening with distance offshore until it intersects with the intermittent Mindanao Eddy (ME) at 129.25°E. In the subthermocline (>26 kg m−3), a persistent Mindanao Undercurrent (MUC), with a velocity core of 0.2 m s−1 and mean net transport, flows poleward. Mean transport and standard deviation integrated from the coast to 130°E is −19 ± 3.1 Sv (1 Sv ≡ 106 m3 s−1) in the thermocline and −3 ± 12 Sv in the subthermocline. Subthermocline transport has an inverse linear relationship with the Niño-3.4 index and is the primary influence of total transport variability. Interannual anomalies during El Niño are greater than the annual cycle for sea surface salinity and thermocline depth. Water masses transported by the MC/MUC are identified by subsurface salinity extrema and are on isopycnals that have increased finescale salinity variance (spice variance) from eddy stirring. The MC/MUC spice variance is smaller in the thermocline and greater in the subthermocline when compared to the North Equatorial Current and its undercurrents.

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Craig M. Lee and Daniel L. Rudnick

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Moored observations of atmospheric variables and upper-ocean temperatures from the Long-Term Upper-Ocean Study (LOTUS) and the Frontal Air-Sea Interaction Experiment (FASINEX) are used to examine the upper-ocean response to surface heating. FASINEX took place between January and June 1986 at 27°N, 7°M while LOTUS took place between May and October 1982 at 34°N, 7°W. The frequency-domain transfer function between rate of change of heat and the net surface heat flux is consistent with a one-dimensional heat balance between heating and convergence of vertical turbulent heat flux at timescales longer than the inertial. The observations satisfy the vertically integrated one-dimensional heat equation and indicate that the response to surface heating has been successfully isolated. Within the internal waveband, upward phase propagation in the response is inconsistent with a one-dimensional balance and the vertically integrated heat balance fails. The internal waveband response is explained as a balance between rate of change of heat, mixing, and vertical advection. A simple model, which admits internal waves forced by an oscillatory surface buoyancy flux, illustrates the competition between these three terms. Stratification modulates the depth to which surface heating is mixed. The estimated eddy diffusivity may be considered a linear function of frequency where the scaling constant reflects the mixed layer depth.

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Daniel L. Rudnick and Russ E. Davis

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A unified theory of frontogenesis in mixed layers is proposed. Analytical solutions of the nonlinear mass and density balances in a mixed layer are found in the Lagrangian frame for a wide class of entrainment parameterizations. These solutions show mixed-layer depth and density to be functions of particle position and separation, where mixing is represented by an integral over time following a particle. Thus, given a knowledge of only the crossfront velocity field, the likelihood of frontal formation can be predicted. Both surface convergence, and divergence in conjunction with vertical mixing can be frontogenetic. Three different types of fronts, distinguished by their associated crossfront velocity fields, are discussed. Large-scale fronts, such as the subtropical front, are described by a convergent surface flow acting upon an initial horizontal density gradient. The powerful divergence caused by an offshore Ekman flow forced to zero at a coast causes upwelling fronts. The divergence associated with a shoaling barotropic flow forms tidal mixing fronts, which may be sharpened by convergence during an outgoing tide.

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T. M. Shaun Johnston and Daniel L. Rudnick

Abstract

The transition layer is the poorly understood interface between the stratified, weakly turbulent interior and the strongly turbulent surface mixed layer. The transition layer displays elevated thermohaline variance compared to the interior and maxima in current shear, vertical stratification, and potential vorticity. A database of 91 916 km or 25 426 vertical profiles of temperature and salinity from SeaSoar, a towed vehicle, is used to define the transition layer thickness. Acoustic Doppler current measurements are also used, when available. Statistics of the transition layer thickness are compared for 232 straight SeaSoar sections, which range in length from 65 to 1129 km with typical horizontal resolution of ∼4 km and vertical resolution of 8 m. Transition layer thicknesses are calculated in three groups from 1) vertical displacements of the mixed layer base and of interior isopycnals into the mixed layer; 2) the depths below the mixed layer depth of peaks in shear, stratification, and potential vorticity and their widths; and 3) the depths below or above the mixed layer depth of extrema in thermohaline variance, density ratio, and isopycnal slope. From each SeaSoar section, the authors compile either a single value or a median value for each of the above measures. Each definition yields a median transition layer thickness from 8 to 24 m below the mixed layer depth. The only exception is the median depth of the maximum isopycnal slope, which is 37 m above the mixed layer base, but its mode is 15–25 m above the mixed layer base. Although the depths of the stratification, shear, and potential vorticity peaks below the mixed layer are not correlated with the mixed layer depth, the widths of the shear and potential vorticity peaks are. Transition layer thicknesses from displacements and the full width at half maximum of the shear and potential vorticity peak give transition layer thicknesses from 0.11× to 0.22× the mean depth of the mixed layer. From individual profiles, the depth of the shear peak below the stratification peak has a median value of 6 m, which shows that momentum fluxes penetrate farther than buoyancy fluxes. A typical horizontal scale of 5–10 km for the transition layer comes from the product of the isopycnal slope and a transition layer thickness suggesting the importance of submesoscale processes in forming the transition layer. Two possible parameterizations for transition layer thickness are 1) a constant of 11–24 m below the mixed layer depth as found for the shear, stratification, potential vorticity, and thermohaline variance maxima and the density ratio extrema; and 2) a linear function of mixed layer depth as found for isopycnal displacements and the widths of the shear and potential vorticity peaks.

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Daniel L. Rudnick and Robert A. Weller

Abstract

The superinertial and near-inertial wind-driven flow in the western North Atlantic is examined using data from two recent experiments. The Frontal Air–Sea Interaction Experiment (FASINEX) took place at 27°N, 70°W during 1986. The Long-Term Upper-Ocean Study (LOTUS) took place at 34°N, 70°W during 1982. Each experiment included moored measurements of meteorological variables that allowed estimation of the wind stress and oceanic currents. The directly wind-driven flow is isolated from other sources of variability, such as internal waves and mooring motion, using a transfer function between geocentric acceleration and wind stress. The transfer function is examined in rotary spectral bands bounded by periods of 36 and 12 h, and 12 and 2 h. For surface-moored observations, wind-driven mooring motion is found to cause a response that extends at least to 1000 m (much deeper than the frictional layer of direct wind forcing). Once this artifact is removed, the directly wind-driven flow is identified. This response is found to rotate to the left (right) for clockwise (counterclockwise) rotating superinertial wind stress, in agreement with the solution of the time-dependent Ekman spiral. When vertically integrated the Ekman transport relation is satisfied, indicating that all of the wind-driven flow has been isolated.

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Joseph P. Martin and Daniel L. Rudnick

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The Hawaiian Ridge is one of the most energetic generators of internal tides in the pelagic ocean. The density and current structure of the upper ocean at the Hawaiian Ridge were observed using SeaSoar and Doppler sonar during a survey extending from Oahu to Brooks Banks and up to 200 km from the ridge peak. Survey observations are used to quantify spatial changes in internal-wave-induced turbulent dissipation and mixing. The turbulent dissipation rate of kinetic energy ε and diapycnal eddy diffusivity Kρ are inferred from an established parameterization using internal wave shear as input. At the Kauai Channel (KC) and French Frigate Shoals/Brooks Banks sites, ε and Kρ decay away from the ridge with maxima exceeding minima by 5 times. At both sites, average Kρ is everywhere greater than the canonical open-ocean value of 10−5 m2 s−1. Along the ridge, ε and Kρ vary by up to 100 times and are largest at sites of largest numerical model internal tide energy density. In the eastern KC, Kρ > 10−3 m2 s−1 is typical in a patch more than 200 m thick located above the path of an M 2 internal tide ray. An upper limit on the dissipation rate from M 2 internal tides to turbulence within 50 km of the Hawaiian Ridge is roughly estimated to be in the range of 4–9 GW. At KC, the depth-integrated internal wave energy density and dissipation rate are positively correlated. Potential density inversions occur near the main ridge axis at significant topographic features. Average Kρ is larger inside inversions.

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Kristin L. Zeiden, Daniel L. Rudnick, and Jennifer A. MacKinnon

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In this study, a 2-yr time series of velocity profiles to 1000 m from meridional glider surveys is used to characterize the wake in the lee of a large island in the western tropical North Pacific Ocean, Palau. Surveys were completed along sections to the east and west of the island to capture both upstream and downstream conditions. Objectively mapped in time and space, mean sections of velocity show the incident westward North Equatorial Current accelerating around the island of Palau, increasing from 0.1 to 0.2 m s−1 at the surface. Downstream of the island, elevated velocity variability and return flow in the lee are indicative of boundary layer separation. Isolating for periods of depth-average westward flow reveals a length scale in the wake that reflects local details of the topography. Eastward flow is shown to produce an asymmetric wake. Depth-average velocity time series indicate that energetic events (on time scales from weeks to months) are prevalent. These events are associated with mean vorticity values in the wake up to 0.3f near the surface and with instantaneous values that can exceed f (the local Coriolis frequency) during periods of sustained, anomalously strong westward flow. Thus, ageostrophic effects become important to first order.

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Bo Qiu, Shuiming Chen, Daniel L. Rudnick, and Yuji Kashino

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Subthermocline western boundary circulation along the low-latitude North Pacific Ocean (2°–25°N) is investigated by using profiling float and historical CTD/expendable CTD (XCTD) data and by analyzing an eddy-resolving global OGCM output. In contrast to the existing paradigm depicting it as a reversed pattern of the wind-driven circulation above the ventilated thermocline (i.e., depth < 26.8 σ θ), the subthermocline western boundary circulation is found to comprise two components governed by distinct dynamical processes. For meridional scales shorter than 400 km, the boundary flows along the Philippine coast exhibit convergent patterns near 7°, 10°, 13°, and 18°N, respectively. These short-scale boundary flows are driven by the subthermocline eastward zonal jets that exist coherently across the interior North Pacific basin and are generated by the triad instability of wind-forced annual baroclinic Rossby waves. For meridional scales longer than 400 km, a time-mean Mindanao Undercurrent (MUC) is observed from 6° to 13°N together with another northward-flowing boundary flow beneath the Kuroshio from 16° to 24°N. Rather than remote eddy forcing from the interior Pacific Ocean, both of these broad-scale subthermocline boundary flows are induced by baroclinic instability of the overlying wind-driven western boundary currents, the Mindanao Current, and Kuroshio.

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Andrey Y. Shcherbina, Daniel L. Rudnick, and Lynne D. Talley

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The feasibility of ice-draft profiling using an upward-looking bottom-mounted acoustic Doppler current profiler (ADCP) is demonstrated. Ice draft is determined as the difference between the instrument depth, derived from high-accuracy pressure data, and the distance to the lower ice surface, determined by the ADCP echo travel time. Algorithms for the surface range estimate from the water-track echo intensity profiles, data quality control, and correction procedures have been developed. Sources of error in using an ADCP as an ice profiler were investigated using the models of sound signal propagation and reflection. The effects of atmospheric pressure changes, sound speed variation, finite instrument beamwidth, hardware signal processing, instrument tilt, beam misalignment, and vertical sensor offset are quantified. The developed algorithms are tested using the data from the winter-long ADCP deployment on the northwestern shelf of the Okhotsk Sea.

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