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Irradiance and beam transmittance measurements off the west coast of the Americas
Get PDFMeasurements of total irradiance versus depth and beam transmission versus depth were made at
stations near shore along the west coast of the North and South American continents. The water types at
each station were optically classified according to the system of Jerlov (1976), thus providing additional
information for the description of the distribution of the world's ocean water types. In addition, the
parameter k/c, where k is the irradiance attenuation coefficient and c is the beam attenuation coefficient,
has been shown to be a useful parameter for determining the relative particle concentrations of ocean
water.Copyrighted by American Geophysical Union
Lagrangian characteristics of continental shelf flows forced by periodic wind stress
Get PDFThe coastal ocean may experience periods of fluctuating along-shelf wind direction, causing shifts between upwelling and downwelling conditions with responses that are not symmetric. We seek to understand these asymmetries and their implications on the Eulerian and Lagrangian flows. We use a two-dimensional (variations across-shelf and with depth; uniformity along-shelf) primitive equation numerical model to study shelf flows in the presence of periodic, zeromean wind stress forcing. The model bathymetry and initial stratification is typical of the broad, shallow shelf off Duck, NC during summer. After an initial transient adjustment, the response of the Eulerian fields is nearly periodic. Despite the symmetric wind stress forcing, there exist both mean Eulerian and Lagrangian flows. The mean Lagrangian displacement of parcels on the shelf depends both on their initial location and on the initial phase of the forcing. Eulerian mean velocities, in contrast, have almost no dependence on initial phase. In an experiment with sinusoidal wind stress forcing of maximum amplitude 0.1 N m⁻² and period of 6 days, the mean Lagrangian across-shelf displacements are largest in the surface and bottom boundary layers. Parcels that originate near the coast in the top 15 m experience complicated across-shelf and vertical motion that does not display a clear pattern. Offshore of this region in the top 10 m a rotating cell feature exists with offshore displacement near the surface and onshore displacement below. A mapping technique is used to help identify the qualitative characteristics of the Lagrangian motion and to clarify the long time nature of the parcel displacements. The complexity of the Lagrangian motion in a region near the coast and the existence of a clear boundary separating this region from a more regular surface cell feature offshore are quantified by a calculation from the map of the largest Lyapunov exponent
Very long period magnetotellurics at Tucson Observatory: implications for mantle conductivity
Get PDFIn a companion paper (Egbert et al., this issue) we describe the estimation of very long period (0.16 < T < 91 days) magnetotelluric (MT) impedances from 11 years of data collected at the Tucson geomagnetic observatory. Here we discuss the implications of these data for mantle conductivity. Using minimum norm (flattest and smoothest) inversions, we find simple one-dimensional models of electrical conductivity in the depth range 0-1500 km. We use forward modeling, a linearized resolution analysis, and constrained one-dimensional invertions to delineate the range of models which are consistent with the estimated impedances. Although the MT data have limited resolution, large-scale vertical averages of mantle conductivity are well constrained. We reach the following conclusions concerning mantle conductivity beneath Tucson: (1) The upper 200 km has a conductance of order 10⁴ Siemens (S). This anomalously high conductance may be concentrated in an aesthenospheric high conductivity layer, but the geometry of the conductive zone is not constrained. (2) Typical conductivities in the transition zone (400-700 km) are = 0.1-0.3 S m⁻¹. A step increase to reach this value at or near the 400 km olivine-spinet phase transition is consistent with, but not required by, the data. An upper mantle which is relative throughout (O.0S S m⁻¹ or less) is not allowed by the data. (3) Resolvable large-scale averages of conductivity increase from = 0.2 S m-⁻¹ to = 1.0 S m-⁻¹ between 600 and 900 km depth. A range of models, including those with step increases, and step decreases, at the 670 km seismic discontinuity are consistent with the data. (4) Between 900-1500 km, conductivity increases slowly. Average conductivities in this region are of the order of 1 S m⁻¹, to within a factor of 2 or 3. While limited zones of highly resistive mantle are consistent with the data, a lower mantle which is resistive throughout is not. Models in which conductivity is always above 5 S m⁻¹ below 1000 km can also be ruled out. In conjunction with improved laboratory estimates of electrical conductivities of mantle minerals at high temperatures and pressures, these constraints can provide important clues to the composition and physical state of the mantle
Nonlinear shear instabilities of alongshore currents over barred beaches
Get PDFThe nonlinear dynamics of finite amplitude shear instabilities of alongshore currents in the nearshore surf zone over barred beach topography are studied using numerical experiments. These experiments extend the recent study of Allen et al. [1996], which utilized plane beach (constant slope) topography by including shore-parallel sandbars. The model involves finite-difference solutions to the nonlinear shallow water equations for forced, dissipative, initial-value problems and employs periodic boundary conditions in the alongshore direction. Effects of dissipation are modeled by linear bottom friction. Forcing for the alongshore currents is specified using a model formulated by Thornton and Guza [1986] (T-G). Distinct classes of flows develop depending on the dimensionless parameter Q, the ratio of an advective to a frictional timescale. For Q greater than a critical value Qc the flows are linearly stable. For ∆Q = Qc - Q > 0 the flow is unstable. For small values of ∆Q, equilibrated shear waves develop that propagate alongshore at phase speeds and wavelengths that are in agreement with predictions from linear theory for the most unstable mode. At intermediate values of ∆Q, unsteady vortices form and exhibit nonlinear interactions as they propagate alongshore, occasionally merging, pairing, or being shed seaward of the sandbar. At the largest values of ∆Q examined, the resulting flow field resembles a turbulent shear flow. A net effect of the instabilities at large ∆Q is to distribute the time-averaged alongshore momentum from local maxima of the T-G forcing, located over the sandbar and near the shore, into the region of the trough. The across-shore structure of the time-averaged alongshore current is in substantially better qualitative agreement with observations than that given by a steady frictional balance with T-G forcing. The results point to the possible existence in the nearshore surf zone of an energetic eddy field associated with instabilities of the alongshore current.Copyrighted by American Geophysical Union
A dynamical attractor governs beach response to storms
Get PDFSandbars are ubiquitous, yet not well understood
beach features that change their position and shape in
response to changing wave conditions. We propose and test
a simple empirical model consisting of two coupled linear
differential equations that represents bar dynamics in terms
of wave forcing and two other state variables: (1) the mean
cross-shore bar position and (2) the alongshore variability
about that mean. Model coefficients are constrained by
fitting to a 2-month data set, and the modeled behavior is
examined with a stability analysis. The system is found to
be stable and, hence, predictable. Rates of change of the
bar position and its alongshore variability are found to be
significantly coupled, such that prediction of one variable
requires information about the other. The system response
time is slow compared to the storm wave cycle such that
the bar response continually orbits time-varying
equilibrium points in the state variable phase plane
Mesoscale physical and bio-optical structure of the Antarctic Polar Front near 170°W during austral spring
Get PDFAs part of the U.S. Joint Global Ocean Flux Study Southern Ocean
program, high-resolution surveys of the Antarctic Polar Front near 170øW were
conducted during October-November 1997 with a towed undulating system equipped
with conductivity-temperature-depth and bio-optical sensors. Transects along
170°W and two successive mapping surveys revealed zonal bands with sharp
meridional gradients in east-west velocity. The Polar Front (PF) was characterized
by a sea surface temperature drop from 1.6° to -1.6°C between 60.35° and 61.10°S,
with eastward velocities of 0.4-0.5m s¯1 in the core of the PF jet. Deep mixed
layers (> 200 m) were found within and north of the PF, but mixed layers shoaled
to 100-125m south of the PF to the edge of loose ice at 62.3°S. Highest mixed layer
chlorophyll concentration (0.35 mg m¯3) in late October along 170°W were to the
south of the PF and associated with cold, fresh water. A large meander of the PF
was observed with an alongfront wavelength of 175 km, a cross-front peak-to-peak
amplitude of 100 km, and an eastward phase propagation of 0.05-0.08m s¯1, all
of which are consistent with its formation via hydrodynamic instability of the PF
jet. Highest-phytoplankton biomass was located just poleward of the center of the
PF jet. A high-chlorophyll (up to 1.1 mg m¯3) 50 by 50 km region was found
downstream of the cyclonic bend associated with the meander. A survey 7.5 days
later revealed growth of this high biomass regions to that chlorophyll as in excess
of 0.8mg m¯3 over an 80 km cross front by (at least) 80 km along front region. High
biomass was observed to grow in place with respect to the meander rather than
being displaced far downstream as would be expected from advection. This pattern
is consistent with meander-driven upwelling of nutrients and/or trace metals, which
in turn stimulates phytoplankton growth. Detailed cross sections of the PF reveal
narrow 10-20 km wide bands or filaments of phytoplankton biomass that have
temperature/salinity properties distinct from surrounding water and are coherent
for at least 120 km alongfront.Copyrighted by American Geophysical Union
Aliased tidal errors in TOPEX/POSEIDON sea surface height data
Get PDFAlias periods and wavelengths for the M2, S2, N2, K1, O1, and P1 tidal constituents are calculated for TOPEX/POSEIDON. Alias wavelengths calculated in previous studies are shown to be in error, and a correct method is presented. With the exception of the K1 constituent, all of these tidal aliases for TOPEX/POSEIDON have periods shorter than 90 days and are unlikely to be confounded with long-period sea surface height signals associated with real ocean processes. In particular, the correspondence between the periods and wavelengths of the M: alias and annual baroclinic Rossby waves that plagued Geosat sea surface height data is avoided. The potential for aliasing residual tidal errors in smoothed estimates of sea surface height is calculated for the six tidal constituents. The potential for aliasing the lunar tidal constituents M2, N2, and O1 fluctuates with latitude and is different for estimates made at the crossovers of ascending and descending ground tracks than for estimates at points midway between crossovers. The potential for aliasing the solar tidal constituents S2, K1, and P1 varies smoothly with latitude. S2 is strongly aliased for latitudes within 50 degrees of the equator, while K1 and P1 are only weakly aliased in that range. A weighted least squares method for estimating and removing residual tidal errors from TOPEX/POSEIDON sea surface height data is presented. A clear understanding of the nature of aliased tidal error in TOPEX/POSEIDON data aids the unambiguous identification of real propagating sea surface height signals. Unequivocal evidence of annual period, westward propagating waves in the North Atlantic is presented
Surface stress in offshore flow and quasi-frictional decoupling
Get PDFAircraft data collected at approximately 15 m above the sea surface in the coastal zone are analyzed to examine the spatial distribution of surface stress. Advection of stronger turbulence from land dominates the near-surface turbulence for the first few kilometers offshore. With offshore flow of warm air over cold water, strong stratification leads to very small surface stress. Because the stability restricts the momentum transfer to the waves, the aerodynamic surface roughness decreases to very small values, which in turn decreases atmospheric mixing. The redevelopment of the boundary layer farther downstream is examined. Computation of fluxes from observations for stable cases is difficult due to a variety of errors including large random flux errors, possible instrumental loss of small-scale flux, difference between the surface flux and that at the observational level, and inadvertent capture of mesoscale motions in the computed turbulent fluctuations. Although the errors appear to be substantial, the aircraft momentum fluxes compare favorably with those from sonic anemometers on two buoys and a tower at the end of a 570-m pier, even with near collapse of the turbulenc
Data report : major-element chemistry of Hole 896a glass
Get PDFOcean Drilling Program Hole 896A (1°13.01'N, 83°43.39'W) is
in 3440 m of water east of the Galapagos Platform in the equatorial
eastern Pacific Ocean. At this site in 5.9 Ma crust, basement rocks
were recovered over the depth range of 195.1 meters below seafloor
(mbsf) to 469 mbsf. These rocks are mostly pillow lavas, but massive
flows, breccias and dikes are also present. Many of the lavas and
flows have quenched glass on their exteriors, and breccias commonly
contain glass. Glass was analyzed by electron microprobe in six laboratories
as part of shore-based studies of the rocks. These analyses
are collected here so that all the chemical analyses are available in
one publication. The analyses are interpreted in individual publications
elsewhere in this volume
Near-offset vertical seismic experiments during leg 204
Get PDFThree successful vertical seismic profiles (VSPs) were acquired during
Ocean Drilling Program (ODP) Leg 204 at South Hydrate Ridge. The
data confirm earlier results from ocean bottom seismometer data and
analysis of moveout from common midpoint reflection data that the
average velocity between the seafloor and the bottom-simulating reflector
(BSR) is <1600 m/s throughout the region and is lowest near the
summit, where the amount of hydrate is greatest. This result supports
the conclusions that free gas and hydrate coexist beneath the summit
and that the average amount of gas hydrate present elsewhere is low.
The data also indicate that low-velocity zones (LVZs) resulting from free
gas beneath the BSR must be thin and stratigraphically controlled. The
only LVZ resolvable from traveltime analysis of the VSP data is associated
with Horizon A, which has been interpreted to be the primary conduit
transporting free gas to vents at the summit of South Hydrate
Ridge. Thin LVZs associated with Horizons B and B', however, are indicated
by sonic logs as well as by strong negative polarity reflections in
the multichannel seismic data. This limited distribution of sub-BSR free
gas contrasts with previous results at North Hydrate Ridge (Leg 146)
and Blake Ridge (Leg 164), which indicate the presence of free gas zones
several hundred meters thick that result in distinct LVZs in the VSP data
from those earlier ODP legs