Observations of upper ocean sound-speed structures in the North Pacific and their effects on long-range acoustic propagation at low and mid-frequencies

The article of record as published may be found at http://dx.doi.org/10.1121/10.0002174Three 1000-km long, high resolution conductivity, temperature, depth sections in the North Pacific Ocean obtained by the ship towed vehicle SeaSoar are analyzed to quantify 2005 March/April upper-ocean sound-speed structure and determine the effects on low to mid-frequency transmission loss (TL) through numerical simulation. The observations reveal a variable mixed layer acoustic duct (MLAD) with a mean sonic layer depth of 91-m, and an even higher variability, 80-m-average-thickness transition layer connecting the mixed layer (ML) with the main ther- mocline. The sound-speed structure is hypothesized to be associated with thermohaline processes such as air-sea fluxes, eddies, submesoscale, fronts, internal waves, turbulence, and spice, but the analysis does not isolate these factors. Upper-ocean variability is quantified using observables of layer depth, ML gradient, and sound speed to compute low order moments, probability density functions, horizontal wavenumber spectra, and empirical orthogo- nal function decomposition. Coupled mode acoustic propagation simulations at 400 and 1000 Hz were carried out using the sound-speed observations from the upper 400-m appended to climatology, which reveal propagation phys- ics associated with diffraction, random media effects, and deterministic feature scattering. Statistics of TL reveal important energy transfers between the MLAD and the deep sound channel.This work was supported by the Office of Naval Research (ONR) code 32 Ocean Acoustics section, as well as support from the ONR Task Force Ocean initiative.This work was supported by the Office of Naval Research (ONR) code 32 Ocean Acoustics section, as well as support from the ONR Task Force Ocean initiative

Potential vorticity structure in the North Atlantic western boundary current from underwater glider observations

Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 327–348, doi:10.1175/JPO-D-15-0112.1.Potential vorticity structure in two segments of the North Atlantic’s western boundary current is examined using concurrent, high-resolution measurements of hydrography and velocity from gliders. Spray gliders occupied 40 transects across the Loop Current in the Gulf of Mexico and 11 transects across the Gulf Stream downstream of Cape Hatteras. Cross-stream distributions of the Ertel potential vorticity and its components are calculated for each transect under the assumptions that all flow is in the direction of measured vertically averaged currents and that the flow is geostrophic. Mean cross-stream distributions of hydrographic properties, potential vorticity, and alongstream velocity are calculated for both the Loop Current and the detached Gulf Stream in both depth and density coordinates. Differences between these mean transects highlight the downstream changes in western boundary current structure. As the current increases its transport downstream, upper-layer potential vorticity is generally reduced because of the combined effects of increased anticyclonic relative vorticity, reduced stratification, and increased cross-stream density gradients. The only exception is within the 20-km-wide cyclonic flank of the Gulf Stream, where intense cyclonic relative vorticity results in more positive potential vorticity than in the Loop Current. Cross-stream gradients of mean potential vorticity satisfy necessary conditions for both barotropic and baroclinic instability within the western boundary current. Instances of very low or negative potential vorticity, which predispose the flow to various overturning instabilities, are observed in individual transects across both the Loop Current and the Gulf Stream.Glider operations in the Gulf Stream were supported by the National Science Foundation under Grant OCE-0220769. Glider operations in the Gulf of Mexico were supported by BP. R.E.T. was supported by the Penzance Endowed Fund in Support of Assistant Scientists and the Independent Research and Development Program at WHOI.2016-07-0

Bottom water variability in the Samoa Passage

The Samoa Passage (near 10S, 170W) is the channel through which the coldest, saltiest, densest bottom water approaches the North Pacific Ocean from its southern source. Over the past 25 years, three hydrographic sections have been made across the passage. A section occupied in 1968 shows little sign of modified North Atlantic Deep Water (NADW) within the northward flowing Lower Circumpolar Water (LCPW). In contrast, a section occupied in 1987 shows a strong negative curvature in -S (potential temperature-salinity) and a local maximum in salinity characteristic of NADW. A third section occupied in 1992 reveals a marginal NADW signature. The three sections are objectively mapped and very fine-scale bivariate areal -S censuses are made for a quantitative comparison of differences in water-mass structure. The strength of the NADW signature could fluctuate over a wide range of time-scales. However, these data are consistent with decadal variability, with no NADW signal in the passage in 1968, a strong signal in 1987, and a weak one in 1992. The geostrophic volume transport through the passage is 1.0 ± 0.2, 5.6 ± 1.3, and 4.8 ± 0.6 × 106 m3•s−1 below a zero-velocity surface (ZVS) of = 1.2°C for the 1968, 1987, and 1992 sections respectively. The transport estimates, made for comparison with those from velocity data presently being collected by a current meter array in the passage, are sensitive to variations in the choice of ZVS

Near-inertial kinetic energy budget of the mixed layer and shear evolution in the transition layer in the Arabian Sea during the monsoons

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 6492–6507, doi:10.1002/2014JC010198.We present the horizontal kinetic energy (KE) balance of near-inertial currents in the mixed layer and explain shear evolution in the transition layer using observations from a mooring at 15.26° N in the Arabian Sea during the southwest monsoon. The highly sheared and stratified transition layer at the mixed-layer base varies between 5 m and 35 m and correlates negatively with the wind stress. Results from the mixed layer near-inertial KE (NIKE) balance suggest that wind energy at times can energize the transition layer and at other times is fully utilized within the mixed layer. A simple two layer model is utilized to study the shear evolution in the transition layer and shown to match well with observations. The shear production in this model arises from alignment of wind stress and shear. Although the winds are unidirectional during the monsoon, the shear in the transition layer is predominantly near-inertial. The near-inertial shear bursts in the observations show the same phasing and magnitude at near-inertial frequencies as the wind-shear alignment term.NASA Grant Number: NNX12AD47G, NSF Grant Number: 0928138, ONR Grant Numbers: N00014-11-1-0429 and N00014-10-1-0273, NSF Grant Number: OCE-07455082016-03-2

Intensive surveys of the Azores Front: 1. Tracers and dynamics

Abstract. The hypothesis that fronts are sites of active subduction is examined using density, temperature, salinity, and horizontal velocity data from a trio of surveys of the Azores Front done in May 1991 and March 1992. These surveys were made using a SeaSoar equipped with a conductivity-temperature-depth profiler and a shipboard acoustic Doppler current profiler. The potential density and potential vorticity indicate that dense water from the north side of the front may be sliding down beneath the surface outcrop. This apparently subducting isopycnal has a great deal of temperature and salinity variability. Horizontal velocity is nearly parallel to isopycnals, indicating that the time rate of change and vertical advection must be small. The thermal wind balance is observed to be valid, especially in the region of the largest horizontal density gradients. Shear at the base of the mixed layer is likely due to near-inertial motions. The potential vorticity is dominated by the planetary vorticity, except at the front, where vertical shears (the tilting term) become large. The tilting term acts to reduce the magnitude of the potential vorticity at the front, in agreement with simple theoretical models. The magnitude of the tilting term is similar to the total vorticity in the seasonal thermocline

Absolute velocity estimates from autonomous underwater gliders equipped with Doppler current profilers

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Atmospheric and Oceanic Technology 34 (2017): 309-333, doi:10.1175/JTECH-D-16-0156.1.Doppler current profilers on autonomous underwater gliders measure water velocity relative to the moving glider over vertical ranges of O(10) m. Measurements obtained with 1-MHz Nortek acoustic Doppler dual current profilers (AD2CPs) on Spray gliders deployed off Southern California, west of the Galápagos Archipelago, and in the Gulf Stream are used to demonstrate methods of estimating absolute horizontal velocities in the upper 1000 m of the ocean. Relative velocity measurements nearest to a glider are used to infer dive-dependent flight parameters, which are then used to correct estimates of absolute vertically averaged currents to account for the accumulation of biofouling during months-long glider missions. The inverse method for combining Doppler profiler measurements of relative velocity with absolute references to estimate profiles of absolute horizontal velocity is reviewed and expanded to include additional constraints on the velocity solutions. Errors arising from both instrumental bias and decreased abundance of acoustic scatterers at depth are considered. Though demonstrated with measurements from a particular combination of platform and instrument, these techniques should be applicable to other combinations of gliders and Doppler current profilers.Spray glider missions were supported by the National Science Foundation (OCE-1232971, OCE-1233282), the National Oceanic and Atmospheric Administration (NA10OAR4320156, NA15OAR4320071), Eastman Chemical Company, the Oceans and Climate Change Institute at WHOI, and the W. Van Alan Clark Jr. Chair for Excellence in Oceanography at WHOI. RET acknowledges additional support for analysis and publication from the National Science Foundation (OCE-1633911).2017-07-3

On the predictability of sea surface height around Palau

Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(11), (2020): 3267–3294, https://doi.org/10.1175/JPO-D-19-0310.1.As part of the Flow Encountering Abrupt Topography (FLEAT) program, an array of pressure-sensor equipped inverted echo sounders (PIESs) was deployed north of Palau where the westward-flowing North Equatorial Current encounters the southern end of the Kyushu–Palau Ridge in the tropical North Pacific. Capitalizing on concurrent observations from satellite altimetry, FLEAT Spray gliders, and shipboard hydrography, the PIESs’ 10-month duration hourly bottom pressure p and round-trip acoustic travel time τ records are used to examine the magnitude and predictability of sea level and pycnocline depth changes and to track signal propagations through the array. Sea level and pycnocline depth are found to vary in response to a range of ocean processes, with their magnitude and predictability strongly process dependent. Signals characterized here comprise the barotropic tides, semidiurnal and diurnal internal tides, southeastward-propagating superinertial waves, westward-propagating mesoscale eddies, and a strong signature of sea level increase and pycnocline deepening associated with the region’s relaxation from El Niño to La Niña conditions. The presence of a broad band of superinertial waves just above the inertial frequency was unexpected and the FLEAT observations and output from a numerical model suggest that these waves detected near Palau are forced by remote winds east of the Philippines. The PIES-based estimates of pycnocline displacement are found to have large uncertainties relative to overall variability in pycnocline depth, as localized deep current variations arising from interactions of the large-scale currents with the abrupt topography around Palau have significant travel time variability.Support for this research was provided by Office of Naval Research Grants N00014-16-1-2668, N00014-18-1-2406, N00014-15-1-2488, and N00014-15-1-2622. R.C.M. was additionally supported by the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Weston Howland Jr. Postdoctoral Scholarship

Bifurcation and upwelling of the equatorial undercurrent west of the Galapagos Archipelago

Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(4), (2020): 887-905, doi:10.1175/JPO-D-19-0110.1.The Equatorial Undercurrent (EUC) encounters the Galápagos Archipelago on the equator as it flows eastward across the Pacific. The impact of the Galápagos Archipelago on the EUC in the eastern equatorial Pacific remains largely unknown. In this study, the path of the EUC as it reaches the Galápagos Archipelago is measured directly using high-resolution observations obtained by autonomous underwater gliders. Gliders were deployed along three lines that define a closed region with the Galápagos Archipelago as the eastern boundary and 93°W from 2°S to 2°N as the western boundary. Twelve transects were simultaneously occupied along the three lines during 52 days in April–May 2016. Analysis of individual glider transects and average sections along each line show that the EUC splits around the Galápagos Archipelago. Velocity normal to the transects is used to estimate net horizontal volume transport into the volume. Downward integration of the net horizontal transport profile provides an estimate of the time- and areal-averaged vertical velocity profile over the 52-day time period. Local maxima in vertical velocity occur at depths of 25 and 280 m with magnitudes of (1.7 ± 0.6) × 10−5 m s−1 and (8.0 ± 1.6) × 10−5 m s−1, respectively. Volume transport as a function of salinity indicates that water crossing 93°W south (north) of 0.4°S tends to flow around the south (north) side of the Galápagos Archipelago. Comparisons are made between previous observational and modeling studies with differences attributed to effects of the strong 2015/16 El Niño event, the annual cycle of local winds, and varying longitudes between studies of the equatorial Pacific.This work was supported by National Science Foundation (Grants OCE-1232971 and OCE-1233282) and the NASA Earth and Space Science Fellowship Program (Grant 80NSSC17K0443)

Evidence for a Black Hole and Accretion Disk in the LINER NGC 4203

We present spectroscopic observations from the Hubble Space Telescope that reveal for the first time the presence of a broad pedestal of Balmer-line emission in the LINER galaxy NGC 4203. The emission-line profile is suggestive of a relativistic accretion disk, and is reminiscent of double-peaked transient Balmer emission observed in a handful of other LINERs. The very broad line emission thus constitutes clear qualitative evidence for a black hole, and spatially resolved narrow-line emission in NGC 4203 can be used to constrain its mass, with M_BH less than 6 x 10^6 solar masses at 99.7% confidence. This value implies a ratio of black-hole mass to bulge mass of less than approximately 7 x 10^-4 in NGC 4203, which is less by a factor of ~3 - 9 than the mean ratio obtained for other galaxies. The availability of an independent constraint on central black-hole mass makes NGC4203 an important testbed for probing the physics of weak active galactic nuclei. Assuming M_BH near the detection limit, the ratio of observed luminosity to the Eddington luminosity is approximately 10^-4. This value is consistent with advection-dominated accretion, and hence with scenarios in which an ion torus irradiates an outer accretion disk that produces the observed double-peaked line emission. Follow-up observations will make it possible to improve the black-hole mass estimate and study variability in the nuclear emission.Comment: 10 pages (LaTeX, AASTeX v4.0), 2 postscript figures, accepted for publication in The Astrophysical Journal Letter

Propagation of sound through a spicy ocean, the SOFAR overture

Using a closely sampled 1000-km hydrographic section in the eastern North Pacific, the sound-speed finestructure is separated into two component fields: (i) isopyncal tilt dominated by internal waves (the traditional view) and (ii) "spicy" (cold-fresh to hot-salty) millifronts associated with upper ocean stirring. Numerical transmission experiments show significant scatter within the mixed layer from the spicy fronts. This scattered energy arrives near the start of the SOFAR sequence, and is superimposed on a triplication of the channel dispersion at the transition from reflected to upper ocean refracted energy. (This SOFAR overture is totally different from the finale which has been prominent for over 50 years.) The critical dependence of the overture on mixed layer processes suggests a scheme for acoustically monitoring the upper oceans at surface-conjugate depths (3 to 5 km), offering some advantages over in situ monitoring. (C) 2004 Acoustical Society of America