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Donald V. Hansen

Abstract

It is shown that the instability waves regularly observed in the tropical Pacific and Atlantic oceans do not necessarily contain oscillations of half the local inertial frequency. Rather, it is hypothesized that the flow approaches the half-inertial conditions only at times when the instability waves develop sufficient baroclinic intensity to saturate the geostrophic vorticity limit of the gradient wind equation for anticyclonic flow.

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Mark S. Swenson
and
Donald V. Hansen

Abstract

Data from satellite-tracked drifting buoys and VOS/XBT profiles for the years 1979–95 were used to evaluate the seasonal cycle of how major oceanic processes redistribute heat in the cold tongue region of the tropical Pacific. The most active processes for the annual cycle are local heat storage and heat export by entrainment of upwelling and by mean meridional advection. Heat export by zonal advection, however, is not negligible, and meridional eddy heat fluxes associated with tropical instability waves effect a negative feedback that offsets a considerable fraction of that produced by the mean meridional advection. All of these processes mimic the essentially one cycle per year of the surface wind stress, as do those of the depths of both the bottom of the surface mixed layer and the thermocline. Because it is associated with poleward Ekman transports, upwelling, and baroclinic adjustment near the equator, the zonal wind stress component appears to be the more important. The meridional wind stress, while weaker in the annual mean, has a larger annual variation and, therefore, has equal influence on the annual variation of the scalar stress and perhaps the mixed layer thickness. The Monin–Obukov length is found to underestimate the mixed layer thickness considerably. Finally, the authors produce the first estimates of the seasonal cycle of eddy heat flux convergence, which plays a significant role in the evolution of the cold tongue, and show that the eddy heat flux convergence can be quantitatively modeled as eddy diffusion with a diffusivity derived from single-particle buoy statistics.

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David S. Bitterman
and
Donald V. Hansen

Abstract

Measurements of ocean surface currents derived from drift buoy trajectories are subject to errors caused by slippage of the buoy relative to the surrounding water. This slippage error is caused by a number of forces acting on the buoy and drogue element, one of which is the current shear in the water. Idealized model calculations are used to exemplify some effects of vertical current shear on drogues, and on the performance of drogued buoy systems in current shear. It is shown that shear enhances the performance of drogues, and that long drogues should perform better than short drogues in shear, but shear also can induce slippage by adding drag force to the buoy hull.

To establish environmental design parameters, average and rms current shear values between 9.7 and 22.5 meters depth were computed from Doppler acoustic current profiler measurements from the tropical Pacific Ocean. Largest values of shear (∼0.025 s−1 rms) were found near the equator in the eastern Pacific as expected. Elsewhere the shear was generally less than 0.02 s−1, mostly less than 0.01 s−1. Average values of shear were generally less than 0.007 s−1.

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Donald V. Hansen
and
Alan Herman

Abstract

Drifting buoy data from the eastern tropical Pacific Ocean are used to evaluate the degradation of sea surface temperature and current information incurred by reducing the number of transmissions from drifting buoys using the ARGOS system for position finding. Buoy locations are interpolated at uniform time intervals using an optimum interpolation method known as Kriging, which provides also an estimate of the rms position error. It is found that the published standard for surface current measurement for the TOGA Program (5 cm s−1) can be met with transmissions on one day of three in the Southern Hemisphere. Due to stronger mesoscale variability in the Northern Hemisphere the standard would be jeopardized by reducing transmissions even to one day of two. The standard for observation of sea surface temperature (0.1°C) can be met in either hemisphere with transmissions on one day of four. The Lagrangian decorrelation times for the Northern Hemisphere region of the eastern tropical Pacific are estimated as 4 days in the meridional direction, and 14 days in the zonal direction. It is recommended that transmissions be made on one day of three, and the time scale for the TOGA standard be revised accordingly.

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Donald V. Hansen
and
Pierre-Marie Poulain

Abstract

Satellite-tracked drifting buoy data are being collected by numerous investigators and agencies in several countries for the World Ocean Circulation Experiment-Tropical Oceans Global Atmosphere (WOCE-TOGA) Surface Velocity Program. By the end of the century, and thereafter, this global dataset will provide the best available climatology and chronology of the surface currents of the World Ocean. To expedite completion of research quality datasets for archival and dissemination, a data acquisition activity is being conducted at NOAA Atlantic Oceanographic and Meteorological Laboratory (AOML), Miami, Florida. At AOML, data from drifting buoys of cooperating operators are quality controlled and optimally interpolated to uniform 6-h interval trajectories for archival at the Marine Environmental Data Service (Canada). This report describes in detail the procedures used in preparing these data for the benefit of second- or third-party users, or future buoy operators who may wish to process data in a consistent way. Particular attention is given to provide quantitative estimates for uncertainty of interpolation.

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David S. Bitterman
and
Donald V. Hansen

Abstract

Three drift-buoy designs have been deployed since 1988 in substantial numbers in the tropical Pacific Ocean by United States participants as part of the Tropical Ocean Global Atmosphere (TOGA) Pan Pacific Surface Current Study. These include the Low Cost Tropical Drifter designed and built at the Atlantic Oceanography and Meteorological Laboratory, the Low Cost Drifter (LCD) designed and built by the Massachusetts Institute of Technology Draper Laboratories, and the Minister Drifter designed and built at the Scripps Institution of Oceanography and built by Tecnocean Inc., San Diego, California, which has subsequently become known as the World Ocean Climate Experiment standard drifter. This report contains an evaluation of the performance of the sea surface temperature measurement system carried by these buoy designs. Based on comparisons of the monthly mean SST derived from the available XBT and CTD casts and on intercomparisons of among each of the buoy types, all three designs appear to include a warm bias in the surface temperatures they report. The LCD showed a larger mean bias and diurnal variation from solar heating than the other two buoy types. This difference is probably due to the location chosen for its sensor, resulting in poor thermal contact with the surrounding water.

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Gregory Han
,
Donald V. Hansen
, and
Jerry A. Galt

Abstract

A qualitative evaluation is made of the output from a finite-element, steady-state diagnostic model to observed time-averaged currents. The model uses a vorticity balance equation with linear bottom friction and inputs observations of near-bottom currents on the model boundary, density field and bottom topography. The output is the near-bottom (barotropic) velocity field over the entire modeled region. Velocity profiles are constructed using the thermal wind equation with the observed density field from May 1976 and a turbulent closure scheme model of Mellor and Durbin to reproduce the top and bottom Ekman layers. Transport is computed in layers above and below the pycnocline by integrating the geostrophic velocity profile and adding the Ekman layer transport.

Comparisons of the modeled bottom velocities at three moorings interior to the region and modeled vertical profiles of velocity at the interior moorings and the four boundary moorings to the observations at those points, show favorable agreement. Along-isobath flow is modeled more accurately than cross-isobath flow. Along-isobath flows, both in the shelf valley and outside the valley, are well represented, but near-bottom flows in the valley in two of the patterns are strongly ageostrophic and thus are not in agreement with model results. Errors >100% are found for weak flow events and the accuracy of the vertical shear in the velocity deteriorates away from the time of the density observations.

The model results are useful for calculating the advective transport of dissolved and suspended constituents in the water. Though the accuracy of the point velocities have a median relative error of ∼50%, the transport calculation is probably more accurate.

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Thomas H. Kinder
,
James D. Schumacher
, and
Donald V. Hansen

Abstract

Drift buoys with shallow (17 m) drogues, released during May 1977 and tracked by satellite, delineated an eddy in the southeastern Bering Sea. Located above complex topography having a depth range of 200 to 3000 m, the eddy had a diameter of about 150 km. Mean rotational speeds ∼50 km from the eddy's center were 20 cm s−1, but speeds up to 50 cm s−1 were measured. A CTD survey during July defined the eddy from 200 to 1500 m depth in temperature and salinity distributions, but no hydrographic evidence for the eddy existed at the surface. A geostrophic calculation relative to 1500 m agreed qualitatively with drifter data, but was ∼5 cm s−1 less than mean drifter speeds. Examination of the T-S correlation showed that water masses at the eddy's core were the same as those at its periphery, in contrast with a cyclonic ring observed nearby in July 1974. The last drifter left the eddy in October, and a second CTD survey in February 1978 showed that the eddy had either dissipated or moved.

An earlier STD survey of the region in summer 1971 had shown neither an eddy like that seen in 1977 nor a ring like that seen in 1974. In spite of the ubiquitous inclusion of permanent eddies and steady currents in Bering Sea circulation schemes, recent evidence from synoptic data suggests that the hydrographic and velocity fields are highly variable on scales of 50 to 200 km and a few weeks to a few years. While we do not understand the generating mechanism for this eddy, current instability, wind forcing and topographic interaction all remain plausible candidates. Because of the eddy's size and location, we believe that it formed within the southeastern Bering Sea.

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Thomas C. Royer
,
Donald V. Hansen
, and
David J. Pashinski

Abstract

Drifting buoys and dynamic topography over the continental shelf in the northern Gulf of Alaska are used to describe the coastal circulation and flow in the offshelf Alaska Current. One permanent anti-cyclonic eddy is detailed along with several shorter period features. In general, the buoys progressed shoreward and to the west from a release point near the continental shelf break. There was a tendency for the drifters to terminate their trajectories near or inside Prince William Sound.

Precipitation, runoff and wind stress suggest that the buoys' drogues are affected by entrainment. It is hypothesized that drifters move shoreward until their drogues encounter the offshore-moving, ageostrophic upper layer. Their position stabilizes between the onshore and offshore flow and their subsequent movement is parallel to this interface. Though drifters might not be monitoring surface flow, it can be inferred that the surface flow is offshore here.

The behavior of drogued, drifting buoys enables them to be especially valuable in flow along frontal regions.

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