Search Results

You are looking at 1 - 10 of 27 items for

  • Author or Editor: W. Brechner Owens x
  • All content x
Clear All Modify Search
W. Brechner Owens

Abstract

Eulerian potential vorticity and heat balances calculated for a multi-layered quasi-geostrophic model are shown to be consistent with those inferred from the MODE data. Above the thermocline the balances are the result of the turbulent cascade of relative vorticity which tends to separate the first moments of the enstrophy and energy spectra. The vorticity balance is dominated by the passive advection of the small-scale contributions to the relative vorticity by the energy-containing scales. By either transferring to the Lagrangian frame or focusing on the energy-containing scales this advective effect is suppressed and one observes a nearly linear response by relative vorticity and vertical vortex stretching to changes in planetary vorticity.

In the bottom layer there are topographic Rossby waves superposed on a time-mean flow along f/H contours. Both of these effects “detune” the bottom layers from those above the thermocline to inhibit the cascade to barotropic flow which otherwise occurs in quasi-geostrophic turbulence. The intermediate layers have a balance which is a mixture of those described above.

Full access
W. Brechner Owens

Abstract

Trajectories of 17 SOFAR floats ballasted to nominal depth of 700 and 2000 m and tracked for 15 months have been analyzed to produce synoptic and statistical descriptions of the Gulf Stream and subtropical gyre. SOFAR floats launched in and near the Gulf Stream along with positions of the surface Gulf Stream and satellite tracked buoys demonstrate strong baroclinic shear. Nevertheless, the horizontal patterns of the Gulf Stream meanders and Gulf Stream rings are the same at the surface and at 700 and 2000 m depth. Above the thermocline, particle speeds greater than the propagation speed of the meanders cause parcels of water to move with the flow pattern. At greater depths, the velocities are at most equal to the propagation speed and the deep trajectories are free to pass through the Gulf Stream or ring.

Strong temporal variability is found near the tail of the Grand Banks. Instances of flow both southward into a narrow Gulf Stream recirculation and northward into the North Atlantic Current can be seen.

Statistical averages along 55°W are consistent with other measurements at that longitude. Eddy kinetic energy varies by over two orders of magnitude from the Gulf Stream to mid-gyre. A mean Gulf Stream, flowing eastward at all depths, can be seen. In the interior, the zonal velocity has a short meridional scale with bands of flow in either direction.

Full access
W. Brechner Owens

Abstract

The vertical structure of eddy variability on the southern edge of the Gulf Stream recirculation is presented as a number of different forms of empirical orthogonal modes. The velocity dot-product modes show more barotropic eddy variability than the MODE experiment which was 300 km to the south. Temperature modes are consistent with the velocity modes and indicate a rapid decrease in energy with increasing model index.

The empirical modes describing the variability within the lowest frequency bands that the measurements resolved (covering periods from 580 to 32 days) are consistent with simple wave dynamics. The dominant velocity component and temperature for the lowest frequency band are nearly in quadrature. The band that includes periods from 64 to 32 days has a barotropic velocity structure consistent with the topographic Rossby wave description deduced from SOFAR floats, but with a significant velocity-temperature coherence in the thermocline.

Time-averaged horizontal correlation functions of velocity are isotropic when normalized by the covariances at zero separation. At the same time the covariances have significant anisotropy, suggesting that the dominant scales are the same for each direction while the energy content and energy fluxes vary significantly with direction. Within the lowest two frequency bands, the horizontal structure again has strong signatures of wave dynamics.

Full access
Keith Alverson and W. Brechner Owens

Abstract

Evidence for oceanic convection over Maud Rise in the Weddell Sea suggests that bottom topography may select the location and scale of deep convecting oceanic chimneys forced by seasonal large-scale atmospheric cooling. In this paper, the role of bottom topography in open-mean deep convection is studied using an idealized three-dimensional primitive equation model. A barotropic mean flow impinges on a Gaussian-shaped seamount in a stratified domain generating a Taylor cap (a region of topographically trapped fluid). Uniform surface cooling is applied throughout the domain. When the Taylor cap is tall enough to interact with the surface mixed layer, the local isolation from the advection of heat by the mean flow forms a conduit into the deep water. Convection within this region is significantly enhanced relative to ambient levels away from the seamount and to similar numerical simulations performed without bottom topography.

Given uniform background stratification, domed isopycnals are not important in the preconditioning process. However, when a surface intensification in the stratification exists, domed isopycnals associated with the Taylor cap circulation can also play a preconditioning role. In this case, the pycnocline is first ventilated over the seamount leading to rapid convective deepening into the weakly stratified deep water. An analytic formula for one-dimensional nonpenetrative convection into an exponential stratification profile is derived and compares well with results from the numerical model. Parameter dependencies for these topographic preconditioning mechanisms are discussed. These numerical results suggest that bottom topography can play an important role in selecting the location and horizontal scale of deep convection in the ocean.

Full access
W. Brechner Owens and Robert C. Millard Jr.

Abstract

An algorithm for converting the Beckman dissolved-oxygen probe variables with other data from a WHOI/Brown CTD/O2 system to oxygen concentration is presented. Improvements over earlier oxygen algorithms are inclusion of an oxygen current bias and time-lag correction using a nonlinear, least-squares calibration to the titrated oxygen bottle samples. The calibration technique uses a quasi-Newtonian minimization scheme available in scientific subroutine libraries such as the “IMSL” library.

Oxygen probe parameters are found to be stable over several (as many as 25) stations and give typical rms errors in the deep water of 0.005 ml l−1 which is roughly the expected error for the bottle samples. Oxygen current biases can be equivalent to concentrations as large as 0.8 ml l−1. Typical oxygen lag correction values, associated with diffusion times through the probe membrane are in the range of 4–10 seconds. Examples from the North Pacific where the shallow oxygen minimum severely tests the algorithm and from the North Atlantic are shown, demonstrating the effectiveness of the algorithm for a wide range of oxygen profile shapes.

Full access
William J. Schmitz Jr. and W. Brechner Owens

Abstract

It is demonstrated that the outcome of an intercomparison between data and the vertical distribution of eddy kinetic energy predicted by a previously developed numerical model of the MODE area is frequency dependent. In the range of periods from 50 to 150 or even to 400 days (one definition of the temporal mesoscale, the scale that the model was designed to simulate), the comparison is quite good. For periods in the range of 5 to 50 days, the agreement is poor. For periods longer than 400 days, the comparison is indeterminate. Earlier conclusions concerning the relation of model results to the MODE data should be qualified by stipulating frequency range, and future intercomparisons for any model in all regions should be conscious of the desirability of doing so across common frequencies.

Full access
Ellen D. Brown and W. Brechner Owens

Abstract

Momentum and energy transfers from the mesoscale horizontal velocity shear to the internal wave field have been deduced from an analysis of a closely spaced, 25 km, moored current-meter array. The correlation between the low-frequency horizontal shear and internal-wave-field continuum effective stress implies a significant horizontal eddy viscosity of O (106 cm2 s−1), somewhat larger than predicted by Müller (1976). A simple steady-state energy balance for the internal wave field using the observed correlation between the internal wave kinetic energy and the square of the low-frequency shear implies a 10-day relaxation time for the internal-wave Acid and a combined vertical viscosity and horizontal diffusivity not significantly different from zero. These estimates are within the experimental uncertainty of previous observational analyses.

Full access
Brian K. Arbic and W. Brechner Owens

Abstract

Interdecadal temperature variability of the Atlantic Ocean is investigated by differencing hydrographic sections taken from the 1920s through the 1990s. A comprehensive reanalysis of North Atlantic sections and the inclusion of South Atlantic sections show that warming seen previously in the North Atlantic extends to the South Atlantic. The largest statistically significant changes occur on pressure surfaces between 1000 and 2000 decibars (db). Over this pressure range and for latitudes between 32°S and 36°N, temperatures have warmed by ∼0.5°C century−1. At 48°N a cooling of ∼3°C century−1 occurred between the 1950s and 1980s.

These isobaric temperature trends are decomposed into ones along surfaces of constant neutral density, and ones due to the vertical movement of neutral surfaces. The two components are associated with different processes. In the southern North Atlantic (8°–36°N) the subthermocline warming between the 1950s and 1980s appears to be due primarily to downward displacements of neutral surfaces, while the South Atlantic changes occur primarily along density surfaces. The downward displacements in the North Atlantic occur throughout the 1000–2000-db layer, suggesting a volumetric increase (decrease) in the water masses above (below) the intermediate layer. Since calculated wind-driven displacements of the thermocline do not agree with this analysis, a change in deep water formation rates is the most likely explanation. The South Atlantic warming trend can be extended further back in time and is due to isopycnal advection, which has a much slower signal propagation speed than does the displacement mechanism for the North Atlantic changes.

This suggests that warming in Atlantic intermediate waters is due not only to climatic forcing changes over the last four decades, but also to changes on centennial timescales. These oceanic climate changes have origins in both the northern and southern polar seas.

Full access
Bruce A. Warren and W. Brechner Owens

Abstract

Sections of closely spaced CTD stations along Longs. 165°W, 175°W and 175°E, in combination with 14-month current records from the central longitude, define two deep, nearly zonal currants, with speed increasing upward, in the subarctic Pacific. One flows eastward above the Aleutian Rise and Aleutian Trench, and appears to be a concentration of geostrophic flow forced by the bottom topography. The other flows westward along the Aleutian Island Arc, and is the northern-boundary current predicted by deep-circulation theory. Both currents reach to the sea surface, the boundary current being simply the deep part of the Alaskan Stream. The current records were too few to permit better than rough estimates of volume transports but to the extent that they could be combined with thermal-wind calculations they suggest, at 175°W, (1) a transport of 28 × 106 m3 s−1 for the Alaskan Stream, of whch 5 × 106 m3 s−1 was found below 1500 m, and (2) a transport of around 20 × 1O6 m3 s−1 for the eastward jet, of which some 5 × 106–10 × 106 m3 s−1 was estimated below 1500 m.

The deep water in the area surveyed was so nearly homogeneous that salinity, oxygen, and nutrients could generally be calculated from potential temperature within measurement error, these additional properties were therefore of only limited use in tracing the deep flow. However, temperature maps at depths of 2 and 4 km demonstrate continuity of the two deep currents across the 60° of longitude between Japan and the Gulf of Alaska. The eastward jet can be tracked back through the Emperor Seamount chain to the Zenkevich Rise off Japan, while the deep Alaskan Stream can be followed downstream to Long. 180°, where it separates from the boundary and flows due westward to the Emperor Seamount chain, which it rounds to the north, prior to its becoming the southward flowing deep western boundary current of the subarctic Pacific. Other details of the water-property fields are described in the text, and comparisons are made with the deep subpolar boundary flow of the North Atlantic.

Full access
James C. Mcwilliams, W. Brechner Owens, and B. Lien Hua

Abstract

A formalism is presented for making estimates of a variety of mesoscale quantities—streamfunction, potential vorticity, and both linear and nonlinear terms in the dynamical balance equations for heat and potential vorticity—from measurements made during the POLYMODE Local Dynamics Experiment (LDE). The formalism is based upon the dynamical assumptions of geostrophic and hydrostatic balance and the methodology of multivariate optimal estimation theory. A particular novel result is the derivation of optimal estimators for quadratically nonlinear quantities, such as the advection terms in the dynamical balance equations.

Two statistical representations are formulated that are appropriate to different subsets of LDE data: a vertical modal representation for spatially extensive estimates of the most energetic mesoscale motion (for use during the two-month intensive phase of LDE), and a vertically local representation for the somewhat smaller scale motions that can be estimated from the thermocline moored array over a fifteen-month period. The empirical parameters for both statistical representations are estimated, primarily from the moored array over the full ocean depth. A hierarchy of statistical assumptions is considered in order to select estimators that are an appropriate compromise between an accurate depiction of observed mesoscale statistical structure and simplicity of the estimator formulae. This formalism is being used to make the estimates from LDE data, which are reported on in companion papers.

Full access