a volume of water commensurable with the area and depth of a body of water and having relatively homogeneous physical-chemical characteristics forming under certain physical-geographical conditions. The basic factors forming water masses are the heat and water balance of a given region, and consequently the basic indicators of water masses are temperature and salinity. Analysts also consider the factors indicating the content of oxygen and other hydrochemical elements in water masses; these aid the investigation of the water mass dispersion from its place of origin and transformation. The characteristics of water masses do not remain constant but are subjected within certain limits to seasonal and long-term variations and also to change in space. To the degree that they are dispersed from their place of formation, water masses are transformed by the action of thermal and water balance, and they mix with the surrounding waters.
There are both primary and secondary water masses. The former are those whose distinctive characteristics are formed under direct atmospheric influence and are characterized by the largest boundaries of variation within some volume of water; the latter are water masses formed as a result of the mixing of primary water masses, and their traits are the most homogeneous. The following water masses are found in the vertical structure of the Pacific Ocean: surface (primary), down to 150-200 m deep; sub-surface (primary and secondary), from 150-200 m to 400-500 m; intermediate (primary and secondary), from 400-500 m to 1,000-1,500 m; deep (secondary), from 1,000-1,500 to 2,500-3,000 m; and bottom water masses (secondary), deeper than 3,000 m. The boundaries between the water masses are the Pacific front zones, which are division and transformation zones traced according to the increasing horizontal and vertical gradients of the basic water mass indicators.
Each ocean has its own characteristic water masses. For example, the Atlantic Ocean has the Gulf Stream, Northern Tropical, Southern Tropical, and other surface water masses, the Northern Subtropical, Southern Subtropical, and other subsurface water masses, the North Atlantic, South Atlantic, and other intermediate water masses, the Mediterranean deep water mass, and others; the Pacific Ocean has the Northern Tropical, Northern Central Subtropical, Southern Tropical, and other surface water masses, the Northern Subtropical, Southern Subtropical, and other subsurface water masses, Northern Pacific, Southern Pacific, and other intermediate water masses, and the Pacific deep water mass.
The T, S-curve and isopycnic methods are used to study water masses in order to establish the homogeneity of temperature, salinity, and other indicators on the curve of their vertical distribution.
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A. M. MUROMTSEV