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  • Author or Editor: Worth D. Nowlin Jr. x
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Arnold L. Gordon and Worth D. Nowlin Jr.

Abstract

Hydrographic data were obtained within the Bransfield Strait and adjacent waters during February and March 1975 by R/V Conrad and R/V Melville as part of FDRAKE 75. Within the Strait the circumpolar Deep Water is either missing or its influence is weak. The salinity maximum, oxygen minimum and silicate maximum present in the upper layers of the Strait attenuate toward the east, demonstrating the eastward decrease of Bellingshausen Sea influence. The Strait contains three basins separated from one another by sills less than 1500 m deep and from adjacent ocean areas by depth near or less than 500 m, except for a channel to the northeast of slightly over 1100 m depth. The deep and bottom waters of these basins, with depths to nearly 2600 m, are significantly colder, less saline, higher in oxygen and lower in nutrient concentrations than the deep exterior water adjacent to the Strait. These characteristics confirm Clowes' (1934) contention that the waters of these basins are renewed by local convection. Supportive evidence for post-bomb renewal is provided by tritium measurements from the easternmost basin of the Strait. Bottom (2566 m) tritum values are essentially the same as surface values, which are greater than expected for subsurface water which has not recently been in contact with the surface waters. Comparison of T-S relations suggests that one mixing component of near-surface water in the convective renewal of Bransfield bottom water is the same as that involved in Weddell Sea bottom water formation. The FDRAKE data set shows that the character of the deep and bottom waters is different within each of the three major basins, suggesting significant spatial (or temporal) variability of the convective events occurring in the Strait. Water mass distributions of the southern Drake Passage and the Weddell Sea are apparently not influenced by outflow of Bransfield basin water. Likewise, there seems to be no direct outflow of deep or bottom waters from the Bransfield basins into the Weddell-Scotia confluence Zone.

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Joseph L. Reid, Worth D. Nowlin Jr., and William C. Patzert

Abstract

The waters found within the southwestern Atlantic Ocean extend into it as separate lavers with markedly different characteristics. Along the western boundary the deeper waters, derived from the North Atlantic, are warm, highly saline, oxygen-rich and nutrient-poor. This North Atlantic Deep Water (NADW) lies within the density range of the Circumpolar Water (CPW) from the south, which is cooler, lower in salinity, very low in oxygen and very high in nutrients. Where the NADW and CPW meet in the southwestern Atlantic, the NADW separates the CPW into two layers above and below the NADW—each less saline, richer in nutrients and lower in oxygen than the NADW.

Above the upper branch of the CPW lies the Subantarctic Intermediate Water, which is lowest in salinity of all the layers. Beneath the lower branch of the CPW lies an abyssal layer derived from the mid-depths of the Weddell Sea. It is colder, less saline, lower in nutrients and higher in oxygen than the Circumpolar Water.

These layers appear to be separated vertically by density gradients which tend to be sharper at the interface than in the layers themselves. These maxima in stability, which result from the interleaving of water masses from different sources, extend over hundreds of kilometers: apparently vertical exchange processes are not strong enough to dissipate them.

Within the Argentine Basin the circulation of all except the abyssal layer appears to be anticyclonic and so tightly compressed against the western boundary that equatorward flow is observed just offshore of the poleward flow at the boundary. Waters from the north (within the Brazil current near the surface and from the North Atlantic at greater depths) flow southward along the western boundary and turn eastward near 40°S, part returning around the anticyclonic gyre and part joining the Antarctic Circumpolar Current. Likewise the Circumpolar Waters, which have entered from the Pacific, flow northward along the western boundary to about 40°S and then turn eastward, both above and below the NADW. The abyssal waters are derived from the Weddell Sea. Within the Argentine Basin they flow northward along the western boundary and turn eastward south of the Rio Grande Rise, and then southward on the western flank of the Mid-Atlantic Ridge; the abyssal flow is cyclonic beneath the anticyclonic upper circulation.

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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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