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in the ocean, zones where the surface waters of the ocean come together. They develop owing to unevenness in the wind field above the ocean and in the distribution of water density. They usually form at the junction of warm and cold waters and consequently are characterized by sharp horizontal gradients of temperature, salinity, density, and chemical and biological indicators (in certain cases the horizontal temperature gradient may be as much as 6°-7°C for several dozen meters). Owing to the uneven distribution of density, anticyclonic and cyclonic circulations of surface waters develop in convergence zones. In anticyclonic circulation there is intensive mixing and submergence of waters; in cyclonic circulation ascending streams of water develop and bring nutritional salts from the depths to the surface of the ocean. This creates conditions for high biological productivity in these zones.
There are four main permanent convergence zones in the ocean: the northern and southern subpolar zones and the northern and southern subtropical zones. The southern subpolar, or antarctic, zone, which encircles the earth between 50° and 60° S, is the longest. Convergence zones are very important for living organisms within the ocean because the submerging waters warm the deep layers of the ocean and enrich them with oxygen. All the chief hydrological characteristics of water layers located at depths of more than 100–150 meters are formed in convergence zones. The higher the latitude in which the convergence zones is located, the lower the temperature and the greater the density of the submerging waters and the deeper the layers they occupy in the ocean.
A. M. MUROMTSEV