WATER DENSITY AND ITS APPLICATIONS 



Recovering the bathythermograph. 



wind-driven, they are swiftest at the surface. In the main thermo- 

 cline and the upper part of the deep water layer, horizontal eddy- 

 type motion gradually decreases with depth, giving way to slower 

 deep water transport. These eddies, together with the sinking centers 

 in higher latitudes, set the dominant pattern of the current systems 

 of each ocean basin. Figure 2 is a diagrammatic representation of 

 the scheme of circulation, showing the density structure and the 

 direction of the currents in a combined wind-driven eddy and sinking 

 center. In the eddy currents centering in mid-latitudes, the density 

 surfaces slope downward toward the right in the Northern Hemi- 

 sphere and to the left in the Southern. Thus, in the center of 

 the eddies, layer depths are greater than around the outer edges 

 of the currents. Hence, where these currents border on the equa- 

 torial doldrum belt the thermocline is shallow and echo ranging 

 conditions are relatively poor. Sinking ordinarily occurs in a band 

 a few yards wide and perhaps as much as several miles long. This 

 phenomenon has been observed by surface vessels. It has the choppy 

 appearance of a tide rip and may be full of seaweed and other debris 

 brought into it by the converging surface currents. The downdrift 

 is strong enough to carry down debris of slight buoyancy, and the 

 surface currents may be strong enough to hold a drifting vessel in 

 the convergence in spite of a crosswind. 



Such convergences are probably important from the standpoint 

 of diving operations. Submariners use the term fresh pocket for a 

 place where the vessel suddenly and for no obvious reason begins 

 to sink and requires reballasting. This can happen any time when a 

 submarine goes too near the lower limit of a supporting density 

 gradient. Varying thickness and amount of temperature change 

 in the thermocline are no doubt responsible for some instances re- 

 ported. On the other hand, sometimes the submarine sinks very 

 rapidly and checks its descent with great difficulty and at a dan- 

 gerous depth. In such a case it may have encountered a convergence 

 in which there is no increase in density with depth, but also a down- 

 ward current of water comparable to downdrafts in the air that 



cause airplanes to lose altitude. From what is known about the 

 shape of these convergences, it seems likely that a submarine held 

 in one for any length of time has come into it on a nearly parallel 

 course and should therefore change course about 90 degrees to escape. 



DENSITY AND COASTAL CURRENTS 

 Very marked currents are produced by the melting of ice in salt 

 water. In the North Atlantic the arctic water of low salinity derived 

 from melting ice flows southward as the Labrador current, hugging 

 the right-hand shore. The deflection to the right owing to the earth's 

 rotation causes it to set into the bays on the Canadian coast. On the 

 south-east coast, so many wrecks have occurred near Cape Pine and 

 St. Shots Cove that mariners have believed there was a local mag- 

 netic disturbance. Such is not the case however, because these 

 disasters were mainly due to the effect of the density currents men- 

 tioned in the Sailing Directions for Newfoundland (Pub. No. 14). 

 Because of land drainage the density of coastal water is usually 

 less than at corresponding depths offshore. Along any coast where 

 there is a significant amount of freshening due to land drainage, 

 there is a general tendency for the fresh water to move offshore 

 on the surface and the denser (more saline) water to move in under- 

 neath. Thus, the salinity of the surface water increases from the 

 shore seaward, and there is an increase in salinity from the surface 

 downward. This is an unstable condition in that the density sur- 

 faces slope downward in an inshore direction; and the unequal pres- 

 sure gives rise to currents which, in general, behave according to 

 the principles of the circulation theory. 



Since the density surfaces slope downward toward the shore, the 

 directions of the currents are roughly parallel to the coast, with 

 the coast on the right-hand side of the direction of flow in the Nor- 

 thern Hemisphere and on the left-hand side in the Southern Hemi- 

 sphere. As a result of this prevailing condition, the Northern 

 Hemisphere coastal currents on the west side of the oceans are 



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