An official website of the United States government
Abstract Hundreds of full-depth temperature and salinity profiles collected by Deepglider autonomous underwater vehicles (AUVs) in the North Atlantic reveal robust signals in eddy isopycnal vertical displacement and horizontal current throughout the entire water column. In separate glider missions southeast of Bermuda, subsurface-intensified cold, fresh coherent vortices were observed with velocities exceeding 20 cm s −1 at depths greater than 1000 m. With vertical resolution on the order of 20 m or less, these full-depth glider slant profiles newly permit estimation of scaled vertical wavenumber spectra from the barotropic through the 40th baroclinic mode. Geostrophic turbulence theory predictions of spectral slopes associated with the forward enstrophy cascade and proportional to inverse wavenumber cubed generally agree with glider-derived quasi-universal spectra of potential and kinetic energy found at a variety of locations distinguished by a wide range of mean surface eddy kinetic energy. Water-column average spectral estimates merge at high vertical mode number to established descriptions of internal wave spectra. Among glider mission sites, geographic and seasonal variability implicate bottom drag as a mechanism for dissipation, but also the need for more persistent sampling of the deep ocean. Significance Statement Relative to upper-ocean measurements of temperature, salinity, and velocity, deep ocean measurements (below 2000 m) are fewer in number and more difficult to collect. Deep measurements are needed, however, to explore the nature of deep ocean circulation contributing to the global redistribution of heat and to determine how upper-ocean behavior impacts or drives deep motions. Understanding of geographic and temporal variability in vertical structures of currents and eddies enables improved description of energy pathways in the ocean driven by turbulent interactions. In this study, we use newly developed autonomous underwater vehicles, capable of diving to the seafloor and back on a near daily basis, to collect high-resolution full ocean depth measurements at various locations in the North Atlantic. These measurements reveal connections between surface and deep motions, and importantly show their time evolution. Results of analyzing these vertical structures reveal the deep ocean to regularly “feel” events in the upper ocean and permit new comparisons to deep motions in climate models.
more »
« less
Wave Glider–Enhanced Vertical Seafloor Geodesy
We have developed a low-cost approach for accurately measuring short-term vertical motions of the seafloor and maintaining a continuous long-term record of seafloor pressure without the requirement for costly ship time. We equipped the University of Hawai‘i Liquid Robotics Wave Glider with an integrated acoustic telemetry package, a dual-frequency geodetic-grade global positioning system (GPS) receiver, meteorological pressure sensor, processing unit, and cellular communications. The Wave Glider interrogates high accuracy pressure sensors on the seafloor to retrieve their pressure and temperature data. We correct the seafloor pressure measurements using sea surface kinematic GPS location and atmospheric pressure data collected by the Wave Glider payload. By combining the concurrent seafloor and sea surface observations, we demonstrate the capability to provide timely, continuous, and high-accuracy estimation and monitoring of centimeter-scale vertical seafloor motions.
more »
« less
- Award ID(s):
- 1335693
- PAR ID:
- 10138452
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Atmospheric and Oceanic Technology
- Volume:
- 37
- Issue:
- 3
- ISSN:
- 0739-0572
- Page Range / eLocation ID:
- p. 417-427
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Due to the blockage of seawater, seafloor displacement cannot be directly measured by space geodesy. The combination of Global Navigation Satellite Systems‐acoustic ranging (GNSS‐A) has been used to overcome the electromagnetic barrier, so that a GNSS‐determined sea surface vessel's coordinates can be transformed to seafloor benchmarks in a global reference frame. Due to the high cost and science priorities, previous GNSS‐A studies mainly targeted relatively deep water and a minimum of three transponders were used to form an array, equivalent to a precision geodetic station. With recent developments in unmanned autonomous surface vessels, low cost GNSS‐A surveys are poised to become practical. Here we demonstrate that with a carefully designed surveying trajectory, Wave Glider‐based GNSS‐A surveying of a single transponder in shallow water can provide centimeter‐level accuracy on horizontal seafloor positioning, even if the sound speed model deviates from the actual value by a few meters per second. Results from a nine‐month experiment conducted at ∼54 m water depth show that the repeatability of the seafloor horizontal positioning is better than 2 cm. When conditions allow, the acoustic observations should be collected symmetrically about the transponder and data redundancies are recommended to reduce the error associated with time‐dependent variations in sound speed.more » « less
-
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−1over 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.more » « less
-
Abstract Data from an air–sea interaction tower are used to close the turbulent kinetic energy (TKE) budget in the wave-affected surface layer of the upper ocean. Under energetic wind forcing with active wave breaking, the dominant balance is between the dissipation rate of TKE and the downward convergence in vertical energy flux. The downward energy flux is driven by pressure work, and the TKE transport is upward, opposite to the downgradient assumption in most turbulence closure models. The sign and the relative magnitude of these energy fluxes are hypothesized to be driven by an interaction between the vertical velocity of Langmuir circulation (LC) and the kinetic energy and pressure of wave groups, which is the result of small-scale wave–current interaction. Consistent with previous modeling studies, the data suggest that the horizontal velocity anomaly associated with LC refracts wave energy away from downwelling regions and into upwelling regions, resulting in negative covariance between the vertical velocity of LC and the pressure anomaly associated with the wave groups. The asymmetry between downward pressure work and upward TKE flux is explained by the Bernoulli response of the sea surface, which results in groups of waves having a larger pressure anomaly than the corresponding kinetic energy anomaly, consistent with group-bound long-wave theory.more » « less
-
Abstract The main subject of this study is the low‐frequency (with the periods longer than 2 hr) wave processes in the coupled regional system of the Ross Ice Shelf (RIS), the Ross Sea and the atmosphere above them. We investigate possible causal relationships between the wave activity in the three media using a unique set of geophysical instruments: a hydrophone measuring pressure variations on the seafloor, a network of seismometers measuring vertical displacements of the RIS surface, and a Dynasonde system measuring wave characteristics at the ionospheric altitudes. We present an extension of the previously introduced theoretical model of the coupled resonance vibrations of the RIS that quantifies the connection between the ocean tide and the resonance vibrations of the RIS. The ocean tide is confirmed as the most significant source of excitation of the resonances. Analysis of average power spectra in year‐long data sets reveals multiple harmonics of the tide (eight) detected by the RIS seismometers while only three are detected by the seafloor sensor. This may represent a confirmation of the effect of resonance‐related broadband amplification predicted by the model. Several peaks in the spectrum of RIS vibrations have periods different from the periods of nearby tidal constituents and may be associated with broad‐scale resonance RIS vibrations. Resonances may play a role in maintaining the coupled atmosphere‐ocean wave activity. Our results reveal a statistically significant correlation between the spectra of the vertical displacements of the RIS and the spectra of the atmospheric waves.more » « less
