Water Masses And Thermohaline Circulation

The water column in the deep ocean is stably stratified into several layers which are traditionally identified by extrema (cores) in vertical profiles of temperature, salinity, oxygen or nutrients. The distinguishing characteristics of each core layer reflect the unique properties input at the source region and the subsequent modifications of the water mass within that layer. The conservative properties, such as potential temperature and salinity, are modified only by diffusion and mixing as they sink and spread. Non-conservative properties, such as oxygen and nutrients, have their source values altered by biological or chemical processes. Nonetheless, extrema in the non-conservative properties persist over great distances and are very useful as water mass tracers. A subsurface oxygen maximum, for example, usually indicates that water within the layer occupied by that maximum was more recently exposed to the atmosphere than the waters immediately above and below. Maxima in oxygen are usually accompanied by minima in phosphate and nitrate. Some extrema are not true cores that can be identified with a particular source region, but instead are "induced" by underlying or overlying true cores (Gordon, 1967b). When highly oxygenated surface water sinks to intermediate depths (e.g., Subantarctic Mode Water), an induced oxygen minimum core layer is formed between the maximum at the surface and that at intermediate depth.

Since water parcels tend to flow along surfaces of constant potential density, the individual core layers tend to conform to these surfaces which ascend toward the sea surface in polar latitudes. Here, the waters are subjected to the modifying influence of interactions with cold Antarctic air and ice. The classical picture of the meridional circulation driven by thermohaline exchanges in the Antarctic was given by Deacon (1937, fig. 1). Relatively warm and salty deep waters flow with a southward component of motion toward Antarctica. South of the Polar Front, this water mass, which is here referred to as Circumpolar Deep Water, rises to shallow depths and is cooled, freshened, and oxygenated to form both Antarctic Surface Water and Antarctic Bottom Water. Subsequently, these water masses spread northward within the surface and bottom layers, respectively. Within the Polar Frontal Zone, the layer containing Antarctic Surface Water descends below the warmer and less dense Subantarctic Surface Water. North of the Polar Frontal Zone, this layer is identified by a relative minimum in salinity and is referred to as Antarctic Intermediate Water. The thermohaline circulation is believed to play an important role in the redistribution of heat from the equator to the pole and in the ventilation of the deep and abyssal waters.

Recent studies (e.g., Reid et al., 1977; Schlemmer, 1978; Sievers and Nowlin, 1984) have demonstrated that core layers coincide with local minima in vertical profiles of stability, while the boundaries between layers coincide with stability maxima. This technique for tracing water masses can also be implemented in terms of the distribution of potential vorticity (e.g., Keffer, 1985). These studies have revealed that the major water masses mentioned above consist of numerous sublayers that are distinct in more subtle ways.

The following discussion of the various water masses in the Pacific Sector will be arranged more or less in order of decreasing density. That is, it begins with a description of the very cold, dense waters observed over the continental shelf, continues with a discussion of Antarctic Bottom Water, and proceeds generally upward and northward through the various water mass layers. Table 3.1 enumerates the water masses of the Pacific Sector of the Southern Ocean and summarizes their characteristics. Where no geographical limits are noted, the water mass can be found throughout the Pacific Sector.

Waters over the Antarctic Continental Shelf

During summer, when the coastal region is accessible for sampling, all of the waters over the Antarctic continental shelf exhibit temperatures within a few degrees of the in situ freezing point, but the salinities vary considerably. The variations in salinity are probably due to geographic differences in exchanges across the air-sea interface. Highest salinities seem to occur in regions of persistent wintertime leads and polynyas which prolong the exposure of the sea to the cold Antarctic atmosphere. During summer, the cold shelf waters are overlain by a layer of relatively warm surface water that is diluted by melting ice. According to

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