Thomas Whitworth

On the meridional extent and fronts of the Antarctic Circumpolar Current

Large-scale features of the Antarctic Circumpolar Current (ACC) are described using all historical hydrographic data available from the Southern Ocean. The geopotential anomaly of the sea surface relative to 1000 db reveals the highly-sheared eastward flow of the ACC and the strong steering of the current by the ridge system around Antarctica. The near-surface property distributions differentiate the ACC waters from the warmer and saltier waters of the subtropical regimes. The Subtropical Front (STF), interrupted only by South America, marks the northern most extent of subantarctic waters. Distributions of properties on isopycnal surfaces show an abrupt end to the characteristic signal of the Upper Circumpolar Deep Water (UCDW), as this water mass shoals southward and is entrained into the surface mixed layer. This sharp water mass boundary nearly coincides with the southernmost circumpolar streamline passing through Drake Passage. To its south are the weakly-sheared circulations of the subpolar regime. Inspection of many hydrographic crossings of this transition reveals that the poleward edge of the UCD W signal is a reasonable definition of the southern boundary of the ACC. At Drake Passage, three deep-reaching fronts account for most of the ACC transport. Well-established indicators of the Subantarctic Front and Polar Front are traced unbroken around Antarctica. The third deep-reaching front observed to the south of the Polar Front at Drake Passage also continues with similar characteristics as a circumpolar feature. It is called here the southern ACC front. Stations from multiple synoptic transects of these circumpolar fronts are used to describe the average property structure within each ACC zone. Between the STF and the southern boundary of the ACC, the shear transport of the circumpolar current above 3000 m is at all longitudes about 100 Sv (1 Sv = 106 m3 s−) eastward.

On the circulation and stratification of the Weddell Gyre

The availability of new, high-quality, hydrographic data has prompted a re-examination of the circulation in the Atlantic sector of the Southern Ocean. Dynamic topography maps and tracer distributions on selected isopycnal surfaces show that the Weddell Gyre is a large, elongated cyclone located south of the Antarctic Circumpolar Current (ACC), extending northeastward from the Antarctic Peninsula. Patterns of geostrophic shear and a southward turn of the ACC mark its northeastern end near 30°E.

Weddell Sea shelf water in the Bransfield Strait and Weddell-Scotia Confluence

Abstract The unusual stratification of the waters in the Weddell-Scotia Confluence between the Scotia and Weddell Seas and in the Bransfield Strait is traced to the influence of shelf waters from the northwest Weddell Sea. The shelf waters span the density range encompassed by the warm, salty Circumpolar Deep Water (CDW) of the Antarctic Circumpolar Current, and the colder and slightly fresher CDW in the Weddell Sea. An isopycnal mixture of these three source waters flows eastward from the tip of the Antaractic Peninsula in to the Weddell-Scotia Confluence region, and westward north of the Peninsula, where it flows downslope to renew the deep waters of the Bransfield Strait. This mixing scheme can occur year-round, in contrast to some previous explanations of the stratification in the region, which relied on the (unobserved) winter convective overturn of the water column.

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...

On the deep western-boundary current in the Southwest Pacific Basin

Abstract The principal system of deep western-boundary currents in the subtropical South Pacific is that in the Southwest Pacific Basin, which transports Circumpolar Deep Water northward and Pacific Deep Water (at mid-depths) southward. The WOCE PCM9 current-meter array was placed across this system at Lat. 32° 30′S in order to measure the mean transports of those components and their variations. The array, consisting of 60 current meters on 20 moorings, extended 1000 km eastward from the Tonga–Kermadec Ridge, and remained in place for 22 months, between February 1991 and December 1992. The instruments were situated approximately at 2500 m, 4000 m, and close to the bottom. CTD sections (including dissolved oxygen and nutrients) were occupied along the array during its deployment and recovery, and, in between, by the WOCE transpacific section P6. Density sections were used to construct objectively-mapped fields of geostrophic velocity, which were adjusted using current-meter data as integration constants to provide snapshots of the full velocity field. The resulting adjusted transports sometimes differed substantially from relative geostrophic transports, but agreed quite well with transports calculated from current records alone. A time series of volume transport was derived from objectively-mapped three-day-averaged currents. The boundary-current system at PCM9 was essentially 700 km wide, with flow most intense on the flank of the Tonga–Kermadec Ridge, where the maximum mean velocity vector had a magnitude of 9.6 cm s −1 . The time-averaged transport, integrated horizontally across the array and from 2000 m to the bottom, was 16.0×10 6 ±11.9×10 6 m 3 s −1 northward. Of this roughly 15.8×10 6 ±9.2×10 6 m 3 s −1 was northward flow of Circumpolar Deep Water, and 0.2×10 6 ±5.1×10 6 m 3 s −1 was northward flow of Pacific Deep Water. Even for the 22-month mean, however, the velocity field was strikingly banded vertically, and there was little impression of a zero-velocity surface following the demarcation between Circumpolar Deep Water and Pacific Deep Water across the section; only in the horizontally integrated sense was there a correspondence between water masses and the variation of transport with depth. The very large variability in transport is associated with prominent oscillations of periods near 50 days, 20 days, and 10 days, as well as with strong events distributed irregularly across the array that lead to a concentration of spectral energy in a band between 40 and 200 days. The origins of these disturbances are not known. While unexpectedly large changes in the density field near the Tonga–Kermadec Ridge were observed from one cruise to another, the huge fluctuations in transport seemed to be connected more with velocity signals varying only slowly with depth. No measurable changes in water-mass properties were detected by the cruises during the 22 months of deployment, but the salinity was about 0.01 lower at the salinity maximum in the Circumpolar Deep Water than it had been 25 years earlier. The direct, long-term transport measurement suggests that the total upwelling at 2000 m north of 30° S is 13×10 6 m 3 s −1 , corresponding to an areally-averaged vertical velocity of 1.0×10 −5 cm s −1 . This is substantially smaller than earlier values, and it helps to reduce estimates of the global deep upwelling closer to those of the global deep downwelling. The small value of Pacific Deep Water transport in the boundary-current system, relative to that of Circumpolar Deep Water, implies, within the framework of the Stommel–Arons dynamics, that little of the deep water entering the Pacific from the Antarctic returns southward at mid-depths. If so, then some present-day circulation schemes and budgetary constructions need to be re-assessed.

Energetic plumes over the western Ross Sea continental slope

Rapid descent of dense Drygalski Trough (western Ross Sea, Antarctica) shelf water over the continental slope, within 100 to 250 m thick benthic plumes, is described. Speeds of up to 1.0 m/s are recorded flowing at an average angle of 35° to the isobaths, entraining ambient Lower Circumpolar Deep Water en route. This process is predominant in determining the concentration and placement of the shelf water injected into the deep sea as a precursor Antarctic Bottom Water. Nonetheless, a 4-hour duration pulse of undiluted shelf water was observed at depth (1407 m) directly north of the Drygalski Trough, moving at around 90 degrees to isobaths, and at a speed of 1.4 m/s. Thus the export of Ross Sea shelf water to the deep sea is accomplished within plumes descending at moderate angle to isobaths, punctuated by rapid downhill cascades.

Antarctic Bottom Water production and export by tides in the Ross Sea

[1] Current measurements in the western Ross Sea show that Antarctic Bottom Water is produced at the outer edge of the Continental Shelf by tidal stirring of Antarctic Surface Water, Circumpolar Deep Water and Shelf Water. Current amplitudes during spring tides are greater than 1 m/s, sufficient to carry Circumpolar Deep Water onto the shelf from offshore and stir at least the lower portion of the water column to a nearly homogeneous mixture. The Antarctic Bottom Water produced every day during spring tides does not appear to have a seasonal signal. Its export rate is estimated to be 1.95 ± 1.85 × 10 6 m 3 /s.

Forced resonant undulation in the deep Mascarene Basin

Abstract Current meters moored for 19.5 months at Lat. 20°S in the deep water of the western Mascarene Basin recorded a distinct, large-amplitude [O(10 cm s −1 )] undulation of bimonthly period, propagating westward at 7 cm s −1 . Its characteristics demonstrate that it was a barotropic Rossby wave of relatively large meridional scale. Simple theory accounts for it as having been forced by local wind-stress curl at one of the resonant frequencies of the Mascarene Basin. A sharp bimonthly peak is also prominent in spectra of TOPEX/POSEIDON sea-surface height in the Mascarene Basin, but is not seen to the eastward, as is consistent with the local generation. Fluctuations of 45-day period reported earlier in the upper ocean just northeast of Madagascar might have been generated through a similar process, but with frequency shifted by the South Equatorial Current.

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...

Slight northwestward inflow to the deep South Fiji Basin

Abstract Inflow to the South Fiji Basin from the deep boundary current east of New Zealand and the Kermadec Ridge occurs only in the potential-temperature interval 2.0–2.2°C, at a rate of about 0.2 × 106 m3 s−1.