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Eleanor Frajka-Williams, Peter B. Rhines, and Charles C. Eriksen

Abstract

Deep convection—the process by which surface waters are mixed down to 1000 m or deeper—forms the primary downwelling of the meridional overturning circulation in the Northern Hemisphere. High-resolution hydrographic measurements from Seagliders indicate that during deep convection—though water is well mixed vertically—there is substantial horizontal variation in density over short distances (tens of kilometers). This horizontal density variability present in winter (January–February) contains sufficient buoyancy to restratify the convecting region to observed levels 2.5 months later, as estimated from Argo floating platforms. These results highlight the importance of small-scale heterogeneities in the ocean that are typically poorly represented in climate models, potentially contributing to the difficulty climate models have in representing deep convection.

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Noel A. Pelland, Charles C. Eriksen, and Craig M. Lee

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In the California Current System, subthermocline, lenslike anticyclonic eddies generated within the California Undercurrent (CU) are one mechanism for lateral transport of the warm, saline waters of the CU. Garfield et al. established the name “Cuddies” for eddies of this type and hypothesized that they account for a significant fraction of the offshore transport of CU water. This study presents observations of subthermocline eddies collected from a time series of Seaglider surveys in the northern California Current System. Gliders made 46 crossings of subthermocline anticyclones and 17 crossings of subthermocline cyclones over 5.5 yr. Close inspection grouped these into 20 distinct anticyclones and 10 distinct cyclones. Water properties at the core of anticyclonic eddies were similar to those in the core of the CU over the continental slope; these anticyclones are examples of Cuddies. Anticyclonic (cyclonic) eddies had average radii of 20.4 (20.6) km, peak azimuthal current speeds of 0.25 (0.23) m s−1, and average core anomalies of potential vorticity 65% below (125% above) ambient values. Anticyclones contained an order of magnitude greater available heat and salt anomaly relative to background conditions than cyclones on average. Circumstantial evidence of eddy decay through lateral intrusions was found although this was not observed consistently. Observed eddy properties and the geometry of flow over the continental slope were consistent with eddy formation due to frictional torque acting on the CU. Loss of heat and salt from the CU due to subthermocline eddies is estimated to account for 44% of the freshening and cooling of the CU as it flows poleward.

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Hongbo Qi, Roland A. De Szoeke, Clayton A. Paulson, and Charles C. Eriksen

Abstract

Current meter data from two sites were analyzed for near-inertial motions generated by storm during the ten-month period of the Ocean Storms Experiment in the northeast Pacific Ocean. The most striking feature of the inertial wave response to storms was the almost instantaneous generation of waves in the mixed layer, followed by a gradual propagation into the thermocline that often lasted many days after the initiation of the storm. The propagation of near-inertial waves generated by three storms in October, January, and March was studied by using group propagation theory based on the WKB approximation. It was found that wave frequencies were slightly superinertial, with inertial shifts 1%–3% in October and March and around 1% in January. The phase of near-inertial currents propagated upward below the mixed layer, confirming the downward radiation of energy by these waves. The average downward energy flux during the storm periods was between 0.5 and 2.8 mW m−2. The vertical wavelengths indicated by the vertical phase differences ranged from 150 to 1500 m. The vertical group velocity was estimated from the arrival times of the groups at successive depths. Using this in the dispersion relation, horizontal wavelengths ranging from 140 to 410 km were obtained. A relation between density and velocity that gives the horizontal directionality of internal waves was derived. During the storm periods examined, the propagation directions of near-inertial waves mainly lay between northeast and south, indicating sources west of moorings. The directions tended to rotate clockwise with increasing depth, consistent with the expected effect of the earth's curvature. The estimated horizontal wavelength and propagation direction were consistent with the horizontal phase difference between inertial currents at the two sites.

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Noel A. Pelland, James S. Bennett, Jacob M. Steinberg, and Charles C. Eriksen

Abstract

Automated feature tracking and vehicle navigation have the potential to facilitate autonomous surveys of ocean eddies by increasing sampling quality and/or decreasing operator workload. During an observational campaign in late 2013 and early 2014, methods for automated tracking were used to direct multiple ocean gliders during persistent surveys of a California Undercurrent eddy in Washington and British Columbia, Canada, coastal waters over a 3-month period. Glider observations of depth-averaged currents in the ocean’s upper kilometer and vertical separation of selected isopycnals were assimilated into a simple model describing eddy position, size, strength, and background flows using an extended Kalman filter. Though differing in detail from observations, results show the assumed eddy structure was sufficient to describe its essential characteristics and stably estimate eddy position through time. Forecast eddy positions and currents were used to select targets automatically to guide multiple gliders along transects through the eddy center as it translated. Transects performed under automated navigation had comparable or better straightness and distance from the eddy center when compared to navigation based on manual interpretation of the eddy scale and position. The tracking results were relatively insensitive to model choices at times when the eddy was well sampled, but they were more sensitive during sampling gaps and redundancies or rapid eddy translation. Overall, the results provide evidence that automated tracking and navigation are feasible with potential for widespread application in autonomous eddy surveys.

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Eleanor Frajka-Williams, Charles C. Eriksen, Peter B. Rhines, and Ramsey R. Harcourt

Abstract

Vertical velocities in the world’s oceans are typically small, less than 1 cm s−1, posing a significant challenge for observational techniques. Seaglider, an autonomous profiling instrument, can be used to estimate vertical water velocity in the ocean. Using a Seaglider’s flight model and pressure observations, vertical water velocities are estimated along glider trajectories in the Labrador Sea before, during, and after deep convection. Results indicate that vertical velocities in the stratified ocean agree with the theoretical Wentzel–Kramers–Brillouin (WKB) scaling of w; and in the turbulent mixed layer, scale with buoyancy, and wind forcing. It is estimated that accuracy is to within 0.5 cm s−1. Because of uncertainties in the flight model, velocities are poor near the surface and deep apogees, and during extended roll maneuvers. Some of this may be improved by using a dynamic flight model permitting acceleration and by better constraining flight parameters through pilot choices during the mission.

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James S. Bennett, Frederick R. Stahr, Charles C. Eriksen, Martin C. Renken, Wendy E. Snyder, and Lora J. Van Uffelen

Abstract

Seagliders are buoyancy-driven autonomous underwater vehicles whose subsurface position estimates are typically derived from velocities inferred using a flight model. We present a method for computing velocities and positions during the different phases typically encountered during a dive–climb profile based on a buoyancy-driven flight model. We compare these predictions to observations gathered from a Seaglider deployment on the acoustic tracking range in Dabob Bay (200 m depth, mean vehicle speeds ~30 cm s−1), permitting us to bound the position accuracy estimates and understand sources of various errors. We improve position accuracy estimates during long vehicle accelerations by numerically integrating the flight model’s fundamental momentum-balance equations. Overall, based on an automated estimation of flight-model parameters, we confirm previous work that predicted vehicle velocities in the dominant dive and climb phases are accurate to <1 cm s−1, which bounds the accumulated position error in time. However, in this energetic tidal basin, position error also accumulates due to unresolved depth-dependent flow superimposed upon an inferred depth-averaged current.

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Eric A. D'Asaro, Charles C. Eriksen, Murray D. Levine, Clayton A. paulson, Peter Niiler, and Pim Van Meurs

Abstract

A strong, isolated October storm generated 0.35–0.7 m s−1 inertia] frequency currents in the 40-m deep mixed layer of a 300 km×300 km region of the northeast Pacific Ocean. The authors describe the evolution of these currents and the background flow in which they evolve for nearly a month following the storm. Instruments included CTD profilers, 36 surface drifters, an array of 7 moorings, and air-deployed velocity profilers. The authors then test whether the theory of linear internal waves propagating in a homogeneous ocean can explain the observed evolution of the inertial frequency currents.

The subinertial frequency flow is weak, with typical currents of 5 cm s−1, and steady over the period of interest. The storm generates inertial frequency currents in and somewhat below the mixed layer with a horizontal scale much larger than the Rossby radius of deformation, reflecting the large-scale and rapid translation speed of the storm. This scale is too large for significant linear propagation of the inertial currents to occur. It steadily decreases owing to the latitudinal variation in f, that is, β, until after about 10 days it becomes sufficiently small for wave propagation to occur. Inertial energy then spreads downward from the mixed layer, decreasing the mixed layer inertial energy and increasing the inertial energy below the mixed layer. A strong maximum in inertial energy is formed at 100 m ("the Beam"). By 21 days after the storm. both mixed layer inertial energy and inertial frequency shear maximum just below the mixed layer have been reduced to background levels. The total depth-average inertial energy decreases by about 40% during this period.

Linear internal wave theory can only partially explain the observed evolution of the inertial frequency currents. The decrease in horizontal wavelength is accurately predicted as due to the β effect. The decrease in depth-average inertial energy is explained by southward propagation of the lowest few modes. The superinertial frequency and clockwise rotation of phase with depth are qualitatively consistent with linear theory. However, linear theory underpredicts the initial rate at which inertial energy is lost from the mixed layer by 20%–50% and cannot explain the decrease of mixed layer energy and shear to background levels in 21 days.

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