The so-called sigma-transformation is a change of coordinates widely used in three-dimensional marine modelling. A new vertical velocity, u similar to sub(3), is usually associated with this transformation. It is shown that the vertical velocity may be split into two contributions. The first one, called upsloping velocity, is merely induced by the geometry of the basin. The other one is due to intrinsic upwelling mechanisms and is proportional to u similar to sub(3). Examples are provided by the results of the three-dimensional model of Geohydrodynamics and Environment Research.
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