Summary And Conclusions

In this chapter, we have presented a general qualitative description of the major water masses and the large-scale features of the mean circulation in the Pacific Sector of the Southern Ocean. The circulation is dominated by the wind-driven eastward flow of the Antarctic Circumpolar Current. In the Southwestern Pacific Basin, this current consists of at least two relatively narrow jets of intense flow associated with sharp horizontal gradients (fronts) in the property field.

These jets appear to be steered and perhaps maintained by the bathymetry. The Subantarctic Front, which meanders northward east of the Campbell Plateau, and the Polar Front, which follows the northern flank of the Pacific-Antarctic Ridge, move close together to cross the ridge system above the Udintsev and Eltanin Fracture Zones. In the Southeastern Pacific Basin, the jets appear to weaken and become more diffuse, and their continuity west of Drake Passage cannot be demonstrated with available data.

South of the Antarctic Circumpolar Current, the data are sparser in part due to the presence of ice. Here, a generally acknowledged feature of the large-scale mean flow, the Ross Sea Gyre, is only poorly resolved by the existing data. The strength, persistence and areal extent of this feature remains to be determined. The structure and persistence of circulation near the Antarctic continental slope throughout the Pacific Sector is also largely unresolved. Further north, the mechanism responsible for maintaining the tongue of low-salinity surface water extending westward from the southern coast of Chile remains a mystery.

The major influences on the water masses of the Southern Ocean are derived from the North Atlantic and the Weddell Sea. Far downstream in the South Pacific, the relatively minor changes experienced by these water masses can be attributed to diffusion and mixing. The most distinguishing contribution of the Pacific Sector to the properties of the Southern Ocean is the oxygen minimum and nutrient maximum of the Upper Circumpolar Deep water. Most of the dense water formed over the continental shelf in the Ross Sea appears to be dynamically constrained to circulate over the shelf. The small amount of high-salinity shelf water that flows off the shelf and down the continental slope contributes to the formation of high-salinity bottom water. However, the influence of this water appears to be confined to a limited region near the source and is therefore minor compared to the influence of Weddell Sea Bottom Water. Because of its similarity to Weddell Sea Bottom Water, the extent of influence of low salinity bottom water formed near the Adelie Coast and in the eastern Ross Sea has not yet been clearly established.

In conclusion, we have been only marginally successful at describing even the large-scale mean picture. Clearly, more strategically-placed modern hydro-graphic stations are needed to help bring this mean picture into sharper focus. The extrema in phosphate and nitrate profiles are particularly useful in identifying all of the major water mass layers within the deep and bottom waters of the Southern Ocean. The increasing availability of high-quality nutrient and transient tracer data provided by modern hydrographic stations should be very helpful in the investigation of water mass formation and spreading. However, the most critical data deficiency in the Pacific Sector of the Southern Ocean is the lack of direct current measurements throughout the water column to quantify better water mass spreading, heat flux and associated phenomena. In this remote region, such data are exceedingly sparse. Future efforts to fill this void are essential.

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