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Chris Fandry and R. Dale Pillsbury

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.

Using six long-term (35 weeks) current meter measurements at 2700 m in the Drake Passage, it is found that the baroclinic geostrophic transport of 100 Sv relative to 2700 m (Nowlin et al., 1977) should be corrected by an average of 27 Sv which is the contribution from the geostrophic barotropic transport. The total range of the 35 weekly estimates is 220 Sv and the rms error of the estimates due to insufficient spatial coverage is ±14 Sv. Two weekly estimates of the barotropic transport were made from 12 current meters all al approximately 2700 m across the passage, and varied considerably from the corresponding estimates made with only six current meters. The rms error for 12 current meters is only ±4.5 Sv assuming the rms through passage speed to be 3 cm s−1.

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James J. O'Brien and R. Dale Pillsbury

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Harry L. Bryden and R. Dale Pillsbury

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 be understood in terms of this observed variation in mean flow. The time-averaged mean flow at 2700 m depth is estimated to be 1.56±1.44 cm s−1 which implies that a transport of 39±36×106 m3 s−1 should be added to the geostrophic transport of about 100×106 m3 s−1 relative to 2700 m to obtain an estimate of the time-averaged total transport through the Drake Passage.

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Worth D. Nowlin Jr., Thomas Whitworth III, and R. Dale Pillsbury

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 compared with this latest estimate.

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R. Dale Pillsbury, Thomas Whitworth III, Worth D. Nowlin Jr., and Frank Sciremammano Jr.

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 suggest that there are distinct thermal and kinematic regimes in Drake Passage.

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