The very dense water of the Red sea and the Mediterranean makes the column of water salter and heavier and the level lower than in the ocean beyond the straits. Hence a strong surface-cur rent sets inwards through the straits of Bab-el-Mandeb and Gibral tar, while an undercurrent flows outwards, raising the temperature and salinity of the ocean for a long distance beyond the straits. Through the Bosporus and Dardanelles at the entrance of the Black sea, and through the sound and belts at the entrance of the Baltic, streams of fresh surface water flow outwards to the salter Mediterranean and North sea, while salter water enters in each case as an undercurrent. Wind and tide greatly alter the strength of these currents.
Strongly marked differences in density are produced by the melting of sea ice. 0. Pettersson has made a careful study of ice melting as a motive power in oceanic circulation, and points out that it acts in two ways ; on the surface it produces dilution of the water, forming a fresh layer and causing an outflow sea ward of surface water with very low salinity; towards the deep water it produces a strong cooling effect, leading to increase of density and sinking of the chilled layers. Similarly, warmer deep water is attracted polewards to the region of the frozen seas as a result of the flowing away thence of cold surface water. This
helps to explain the peculiar distribution of temperature and salt content around the poles.
Modern oceanography has found means to calculate quanti tatively the circulatory movements produced by wind and the distribution of temperature and salinity, not only at the sur face but in deep water. The methods, first suggested by H. Mohn and subsequently elaborated by V. Bjerknes, Sandstrom, Helland Hansen and others, have been very usefully applied in many cases by means of so-called dynamical sections, but they should be supplemented by direct observations of currents and of the funda mental processes and conditions underlying them. The determi nation of the exact relationship of cause and effect in the origin of ocean currents is a matter of great practical importance. The researches of Pettersson, Meinardus, H. N. Dickson and others leave no doubt, for example, that the variations in the intensity of the Gulf Stream, whether these be measured by the change in the strength of the current or in the heat stored in the water, produce great variations in the character of the weather of northern Europe.
The connection between variations of current strength and the conditions of existence and distribution of plankton are no less important, especially as they act directly or indirectly on the life conditions of food fishes.