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Charles C. Eriksen

Abstract

Two 3.5 month time series records of upper-ocean current and density profiles collected in opposite seasons as part of the LOTUS (Long-Term Upper-Ocean Study) project at 34°N, 70°W indicate substantial variation in the shape of horizontal current spectra in the internal wave frequency range. The near-inertial peak in these records shifts by as much as 10% in frequency and varies by a factor of 5 in height across the depth range of the seasonal pycnocline. Near-inertial currents are weaker and less strongly circularly polarized in the spring-summer than in the fall-winter record. Spectral frequency and vertical wavenumber dependences differ from those predicted by the Garrett-Munk internal wave model. Energy fluxes are vertically asymmetric and internal wave energy varies linearly with rms subinertial current speed.

Several aspects of the observed variability can be understood through two simple models. Depth dependence of near-inertial motions can be explained as a characteristic of linear response to forcing by random moving wind systems. Temporal variations in the shape, level, and polarization of current spectra can be explained as modulation of the deep-ocean internal wave spectrum through conservation of wave action flux as low-frequency, large vertical scale, weakly horizontally sheared currents are encountered.

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Charles C. Eriksen

Abstract

A linear internal wave model for reflection off a sloping bottom applied to a field of horizontally isotropic waves typical of the deep ocean leads to a strongly perturbed frequency-vertical wavenumber energy spectrum. The spectrum is dominated by a nonintegrable singularity at the internal wave critical frequency characteristic of the environment and bottom slope. An observational requirement that the internal wave spectrum near the bottom relax to the open deep-ocean level and shape within a few hundred meters vertically implies a flux imbalance normal to the boundary. The flux that must be redistributed over the internal wave spectrum, or lost from it, amounts to O(10−2 W m−2), larger than for most other energy transfer mechanisms estimated for internal waves. A small fraction of this flux imbalance applied to mixing can account for a basin-averaged effective vertical diffusivity of 10−4 m2 s−1. Bottom reflection represents not only a likely and powerful sink for internal wave energy, but a mechanism that may be important to the oceanic general circulation through its contribution to mixing.

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Charles C. Eriksen

Abstract

The response of the ocean at low latitude to idealized westerly wind bursts can be described as a wave wake composed of equatorial gravity and Rossby-gravity modes. The excited waves are those with phase speeds that match the zonal translation speed of a wind burst, typically 10 m s−1. These modes sum to produce oscillations near the local inertial frequency at each latitude, analogous to near-inertial internal gravity waves generated by moving storms at midlatitude. Linear theory predicts that typical wind burst amplitudes (stresses of 0.1 Pa) will generate substantial current oscillations [O (1 m s−1)] in the upper ocean. Response is initially confined to the region directly beneath a wind burst, after which the wake descends and refracts equatorward as a propagating beam. Waves are of sufficient amplitude to dominate shear and vertical strain in the upper ocean. Phase differences between oscillations at neighboring latitudes induce motion in the meridional-vertical plane at ever-decreasing meridional scales. Mixing associated with predicted low Richardson numbers is expected to check development of nonlinearity from vertical and meridional advection by the waves.

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Charles C. Eriksen

Abstract

Two-year time series of current and temperature collected in the deep equatorial central Pacific 0cean as part of the Pacific Equatorial Ocean Dynamics (PEQUOD) project indicate that motions with vertical scales comparable to those of low baroclinic modes dominate motions from annual to fortnightly frequencies. Properties of equatorial waves are consistent with many aspects of the observed spectra and coherences. In particular, the hypothesis of a spectrum of long Rossby and Kelvin waves at periods longer than about 40 days and Rossby, mixed Rossby-gravity, and Kelvin waves at periods between about 14 and 40 days is consistent with the observations. Neither vertically propagating rays nor randomly phased baroclinic modes can explain coherence phases uniformly, but forced phase-locked baroclinic modes provide a possible explanation. Sea level in the central Pacific is coherent with deep motions at periods of months or shorter. In particular, the quasi-annual sea level signal associated with the 1982–83 El Niño event is not coherent with deep current or temperature.

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Charles C. Eriksen

Abstract

A two-year array of sea-level and deep current and temperature measurements made in the Gilbert Group, Republic of Kiribati, is used to verify the hypothesis that equatorial gravity waves in baroclinic modes are responsible for sea-level spectral peaks at 2–7 day periods corresponding to vanishing zonal wavenumber or zonal energy flux. Sea level and deep temperature are significantly coherent at these special periods, both in the same and at differing geographical location, with phases which can be rationalized from linear theory. Zonal wavenumber-frequency spectral estimates indicate that, at least in the lowest baroclinic, second meridional mode, energy is concentrated at the wavenumber of vanishing zonal energy flux. At longer periods (7–40 days), observed wavenumbers are eastward and increase monotonically with frequency. These fluctuations are interpreted as lowest-baroclinic-mode Kelvin waves travelling 20–30% faster than linear theory predicts because of nonlinearities.

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Charles C. Eriksen

Abstract

Vertical profiles of current and density made within 5° latitude of the equator along longitudes 168 and 179°E (the vicinity of the Gilbert Islands) reveal multiple deep current reversals which are confined to the equator. These current jets have amplitudes of roughly 5–20 cm s−1, vertical scales of order hundreds of meters, and time scales longer than the one month of measurements during the cruise.

Spectral analysis of profiles indicates that 1) meridional trapping varies roughly as the square root of the vertical scale, 2) both east and north currents are coherent over meridional separations of 0°45′ within roughly 1°30′ of the equator; and 3) zonal current lags vertical displacement by π/4 in depth within 0°45′ of the equator. Zonal coherence scales are smaller than the smallest separation of 3°30′ along the equator. Kinetic energy decreases near the island chain.

Current-meter records taken over nearly two years near the islands suggest strong quasi-annual variability in deep currents near the equator, which is not present at latitudes of a few degrees. Mean currents are much smaller than fluctuations.

Simple sums of linear free equatorial Rossby, Kelvin and mixed Rossby-gravity waves are consistent with the observed equatorial intensification. Rossby modes with periods of at least one year must be included. The jets are confined too broadly to be explained by Kelvin or mixed Rossby-gravity modes alone, but these waves appear to dominate motions within 0°45′ of the equator. The model spectra also suggest more energy in long than short Rossby wards. Coherences calculated from the models are consistent with those observed.

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Craig M. Lee and Charles C. Eriksen

Abstract

The upper-ocean to forcing by pressure gradients and wind stress is examined using observations from the Frontal Air–Sea Interaction Experiment. A moored way acquired time series of winds. upper-ocean currents, temperatures, and salinities between winter and late spring in a region of the Sargasso Sea known for the presence of upper-ocean fronts. These fronts have timescales of 10 days and dominate current variance, while winds varied with the 4-day timescale of passing weather systems. Employing a frequency domain regression model. it is found that gestrophy accounts for most of the low-frequency (>100 h)current variance in the seasonal pycnoclino, but wind-forced shear becomes important nearer the surface. In particular, currents oriented in the typical NE–SW alonfront direction display geostrophic balance, while those perpendicular to them do not.

Wind forcing can produce geostrophic currents indirectly through Ekman pumping. and knowledge of the geostrophic shear is required to distinguish between this and currents driven directly by the wind through turbulent shear stress. Previous investigations rely on the assumption that no wind-driven stress penetrates below the mixed layer to remove the wind-coherent geostrophic flow. Baroclinic pressure gradients are calculated using estimates of density across the moored array. A linear regression model uses pressure gradients record to explicitly remove the geostrophic shear and isolate the directly wind-driven acceleration at timescales longer than 10 days. The resulting response satisfies the Ekman transport relation, penetrates well into the stratified fluid spirals to the right, and decays with depth.

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Jacob M. Steinberg and Charles C. Eriksen

Abstract

Idealized simulations of autonomous underwater glider sampling along sawtooth vertical–horizontal paths are carried out in two high-resolution ocean numerical models to explore the accuracy of isopycnal vertical displacement and geostrophic velocity profile estimates. The effects of glider flight speed, sampling pattern geometry, and measurement noise on velocity profile accuracy are explored to interpret recent full-ocean-depth Deepglider observations and provide sampling recommendations for glider missions. The average magnitude of velocity error profiles, defined as the difference between simulated glider-sampled geostrophic velocity profile estimates and model velocity profiles averaged over the spatial and temporal extent of corresponding simulated glider paths, is less than 0.02 m s−1 over most of the water column. This accuracy and the accuracy of glider geostrophic shear profile estimates are dependent on the ratio of mesoscale eddy to internal wave velocity amplitude. Projection of normal modes onto full-depth vertical profiles of model and simulated glider isopycnal vertical displacement and geostrophic velocity demonstrates that gliders are capable of resolving barotropic and baroclinic structure through at least the eighth baroclinic mode.

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Hjálmar Hátún, Charles C. Eriksen, and Peter B. Rhines

Abstract

Intense, buoyant anticyclonic eddies spawned from the west Greenland boundary current were observed with high-resolution autonomous Seaglider hydrography and satellite altimetry as they entered the Labrador Sea interior. Surveys of their internal structure establish the transport of both low-salinity water in the upper ocean and warm, saline Irminger water at depth. The observed eddies can contribute significantly to the rapid restratification of the Labrador Sea interior following wintertime deep convection. These eddies have saline cores between 200 and 1000 m, low-salinity cores above 200 m, and a velocity field that penetrates to at least 1000 m, with 0–1000-m average speeds exceeding 40 cm s−1. Their trajectory, together with earlier estimates of the gyre circulation, suggests why the observed region of deep convection is so small and does not occur where wintertime cooling by the atmosphere is most intense. The cyclostrophic surface velocity field of the anticylones from satellite altimetry matched well with in situ dynamic height baroclinic velocity calculations.

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

Abstract

A California Undercurrent eddy (Cuddy) was repeatedly surveyed using multiple Seagliders for over three months. Found and tracked off of the Washington–Vancouver Island coasts, this Cuddy traveled over 400 km, remaining between the 1000- and 2000-m isobaths, as it was swept along in poleward flow of the California Current System. Three Seagliders made repeat bisecting transects of the Cuddy core capturing its detailed three-dimensional structure in time. Its evolution was analyzed through comparison of 11 independent Cuddy “snapshots.” A two dimensional Gaussian model fit to the geopotential anomaly field for each snapshot allowed computation of dynamic fields inaccessible in Seaglider profiles alone. Results indicate that the Cuddy decayed as its core waters became less isolated over time: Cuddy total mechanical energy (kinetic + potential), salt content, and the magnitude of the core potential vorticity anomaly decreased. Core spice and dissolved oxygen variance increased tenfold, and thermohaline fine structure, suggestive of lateral intrusions, was observed progressively closer to the eddy core. The estimated gradient-wind balanced velocity field similarly weakened as the Rossby number decreased to 0.32 from an initial value of 0.48. The observed changes in eddy properties occurred as the Cuddy was exposed to changes in the background stratification and Coriolis parameter as it translated alongshore. Idealized modeling of eddy adjustment indicates that both erosion and changing background conditions are required to explain the observed eddy changes. Adjustment in response to both effects simultaneously leads to changes in eddy properties qualitatively consistent with those observed.

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