R. Dale Pillsbury

Structure and Transport of the Antarctic Circumpolar Current at Drake Passage from Short-Term Measurements

Abstract Three-week average speeds from an array of current meter moorings which spanned Drake Passage were used in conjunction with geostrophic calculations to estimate the short-term transport of the Antarctic Circumpolar Current. Closely spaced hydrographic stations show that the current consists of three vertically coherent bands of relatively high speed within the generally eastward flow. These bands separate four water mass regimes which have distinct T-S relationships at depths above the core of the Circumpolar Deep Water. The geostrophic transport relative to 3000 db averaged 95×106 m3 s−1 for five transects of the Passage and is consistent with previous measurements. Referencing the geostrophic transport to the current meter measurements gives an adjusted transport of 124×106 m3 s−1 to the east. This estimate is about midway between values obtained in the two previous attempts to adjust relative transport through Drake Passage to observed velocities. The previous estimates are reconsidered and co...

Variability of Deep Flow in the Drake Passage from Year-Long Current Measurements

Abstract To investigate the reasons for the wide variation in previous estimates of transport of the Antarctic Circumpolar Current through the Drake Passage, an analysis of the spatial and temporal variability of currents at 2700 m depth is made from year-long current measurements on six moorings in the Drake Passage. The currents are found to vary over time scales of about two weeks and over spatial scales shorter than 80 km. An average of the six down-channel velocity components is used to estimate the spatially averaged down-channel velocity, or mean flow, at 2700 m. This mean flow varies from 7.6 to–2.9 cm s−1 and has a root-mean-square (rms) amplitude of 2.0 cm s−1 about its time-averaged value. Provided the geostrophic transport relative to 2700 m depth remains constant in time, these variations may be interpreted as temporal variations of 2 60×106 m3 s−1 in total transport with an rms amplitude of 50×106 m3 s−1. The wide variation in previous estimates of transport from short-term measurements can ...

Currents and Temperatures as Observed in Drake Passage During 1975

Abstract Current and temperature records from 10 meters on six year-long moorings deployed during February 1975 in Drake Passage are examined and discussed in the context of hydrographic data from that area. The mean flow directions are consistent with those from geopotential anomaly charts, showing a northward flow in the central passage and eastward through-passage flow in the south and north. Directly measured vertical shear below 1000 m is remarkably uniform with depth in the central passage. Periods of high shear correspond to periods of high speed and are associated with lateral shifts in the velocity cores imbedded in the Antarctic Circumpolar Current at Drake Passage. Fluctuations in temperature and current are highly correlated in the vertical. Although meters near 2700 m separated by 80 km or more show only a few significant horizontal correlations for record-length statistics, there appear to be coherent fluctuations in the central passage during winter. Temperature and speed variability sugges...

On the Estimation of Absolute Geostrophic Volume Transport Applied to the Antarctic Circumpolar Current

Abstract An objective method of estimating the geostrophic barotropic volume transport across an oceanographic section is developed and applied to the transport of the Antarctic Circumpolar Current through the Drake Passage. The total geostrophic transport through a passage can be broken up into a baroclinic component, calculable from hydrographic data, and a barotropic component which is calculated objectively using direct current meter measurements at some fixed level. The influence of measurement noise is incorporated into the calculation of the rms error in the objective estimate of barotropic transport which depends on the number of current meters at the fixed level, the “correlation length” of the through passage velocity component, the noise variance and noise scale. For measurement noise variance not exceeding 10% it is shown that an accurate estimate (rms error ≤20%) of the barotropic transport can be made when the spacing of the current meters is less than or equal to the correlation length. Usi...

Observations of kinetic energy levels in the antarctic circumpolar current at Drake Passage

Based on 31 nearly year-long records of current in Drake Passage the kinetic energy levels in the deep water (below 2000 m) across the passage and through the water column in the central passage are examined. The energy spectra show no significant temporal variability; by contrast, the spatial differences are pronounced, with more fluctuation kinetic energy (KF) in the northern than the southern passage and more above 1000 m than below. Partitioning by frequency bands shows that approx. 28% of the KF results from fluctuations with periods between 2 h (the Nyquist period for the sampling rate) and 2 days and that the energy level for this band is rather uniform across the passage. Large values of KF at northern passage locations result primarily from more activity in the period band between 2 and 50 days. Although that band accounts for almost half of the total KF, a large fraction (23%) of observed kinetic energy is associated with longer periods. The long-term records allowed examination of the representativeness of results—cumulative plots of KF and kinetic energy of mean motion (KM) vs time indicate that the kinetic energy densities reach equilibrium values (for specific long-term records) only after intervals of the order of 4 months. As in the case for the Gulf Stream system, abyssal KF values in the Antarctic Circumpolar Current system at Drake Passage are one or two orders of magnitude greater than KF values in the interior of the North Atlantic subtropical gyre. Moreover, deep KF values south of Cape Horn equal deep water values beneath the Gulf Stream. Values of KM from records deeper than 2500 m increase southward from northern to central locations in Drake Passage. In the central passage KM increases upward from values near 10 cm2 s−2 at 2700 m to over 300 cm2 s−2 at 300 m. KF increases from deep water values near 25 cm2 s−2 to just over 200 cm2 s−2 near 300 m. Relative to mid depths, ratios of KF to KM increase near the surface and near bottom in the central passage.

Measurement of the flow through the Strait of Gibraltar

Abstract As part of the Gibraltar Experiment, eight subsurface moorings equipped with twenty-eight current meters were installed in the Strait of Gibraltar from October 1985 to April 1986. Another four moorings with fifteen current meters were installed from May 1986 to October 1986. Velocity, temperature, pressure and conductivity were measured at half-hourly intervals. Processing of these measurements and some initial results on the two-layer exchange between the Atlantic and Mediterranean through the Strait of Gibraltar are described in this paper.

Oceanographic studies supporting the assessment of deep-sea disposal of defueled decommissioned nuclear submarines

Based on criteria developed by the international Atomic Energy Agency (IAEA), potential disposal sites for defueled, decommissioned nuclear submarines appear to exist in deep water south of the Mendocino Fracture Zone within 200 nautical miles of the United States Oceanographic measurements in the water column and at the sea floor in a study area (W-N) at 39 5°N, 127 5°W will allow the operational and radiological consequences of deep-sea disposal to be compared with land burial of old submarines. The W-N studies also are yielding new data that will provide insights to the deposition and early diagenesis of distal hemipelagic sediments