Southern Drake Passage Research Articles

The stratification and water masses at Drake Passage

The waters at Drake Passage are known to be separated horizontally into four distinct zones of relatively small geostrophic shear by three fronts in which the shear and eastward flow are large. The two northern fronts, Subantarctic Front and Polar Front, that bound the Polar Frontal Zone comprise most of the transport of the Antarctic Circumpolar Current. Vertical sections of closely spaced (∼50 km) oceanographic stations made across Drake Passage in the summers of 1975 and 1976 produced quality measurements of temperature, salinity, oxygen, silicate, nitrate, and phosphate. Based on these data, a traditional description of characteristic distributions is presented. Good sampling resolution in the vertical allows maxima in hydrostatic stability to be traced laterally (approximately along isopycnals) between individual stations. Eleven strata of maximum stability are identified. Most vertically separate distinct water masses. The most pronounced stability stratum is associated with the seasonal pycnocline; north of the Polar Front, it separates the Subantarctic Surface Waters from Subantarctic Mode Water. To the south of the Polar Front the seasonal pycnocline is found slightly above the temperature minimum characteristic of the Antarctic Surface Waters in austral summer. This stability stratum extends northward near the base of the Subantarctic Mode Water. Another stability stratum extending across the entire passage forms the upper boundary for the Circumpolar Deep Water. South of the Polar Front, this stratum is at the base of the Antarctic Surface Water; to the north it is the lower boundary of the Antarctic Intermediate Water. Although Subantarctic Mode Water and Antarctic Intermediate Water are separated by a stability stratum north of the Polar Front, there is no such separation between Antarctic Intermediate Water and the temperature minimum of the Antarctic Surface Water south of the front. This suggests that Subantarctic Mode Water and Antarctic Intermediate Water are distinct and that the latter derives from the surface waters south of the Polar Front. At greater depths a horizontally discontinuous stratum is found between the Upper and Lower Circumpolar Deep Water. Two horizontally continuous stability strata are found near the base of the Circumpolar Deep Water, marking the transition to a high‐silicate variety of denser deep water from the Southeast Pacific Basin. The densest water mass found is Weddell Sea Deep Water, which flows through a gap in the South Scotia Ridge and along the southern Scotia Sea to fill the trough in the southern Drake Passage. It is overlain by a laterally continuous stratum of relative maximum stability.

Potential temperature, oxygen and circulation of bottom water in the Southern Ocean

The circulation of bottom and near-bottom water can be determined in a qualitative manner by inspecting parameter distributions. Bottom potential temperature and oxygen (which show a definite relation in the tp-O2 diagram) are very useful for this purpose in the portion of the Southern Ocean dealt with in this paper (20°W westward to 170°W and south of 50°S). Observations used are deeper than 3000 m and within 300 m of the bottom.There are two areas of cold water (<0·0°C): the more extreme emanating from the Weddell Sea and the other from the Ross Sea. The Weddell Sea Bottom Water flows to the east of the South Sandwich Trench into the Argentine Basin, though some water does enter the trench and flows northward The bottom waters of the Scotia Sea come from between the 3000–3200 m level of the Weddell Sea through a passage at 39°W in the South Orkney Ridge. A tongue of this water flows into the southern Drake Passage, confining to the northern Drake Passage the warmer bottom water from the Pacific Ocean. This warmer bottom water forms a rapid easterly current which enters the South Atlantic west of Shag Rocks. The cold Scotia Sea bottom water does not penetrate the Pacific Ocean but is topographically or dynamically restrained. Circulation in the eastern Scotia Sea is weak to moderate.A cold-water tongue in the Bellingshausen Basin is bounded by a band of warm water on its northwest and northeast sides. It is probable that this loop of warm water represents a strong bottom current from the southwest Pacific Basin which overflows the East-Pacific Ridge in the vicinity of a fracture zone. The flow within the cold-water tongue is moderate in its southwestern sections and weak in the southeast.Bottom photographic current evidence is in agreement with the circulation patterns as found from the potential temperature and oxygen distributions. The circulation found by relating the parameter distribution to velocity field is only in relative terms. Direct current measurements are needed to determine volume transport.