CURRENTS OF AIR AND OF ^^^ATER 



151 



northward-blowing winds throw the lighter 

 water out to sea and cause deeper water to 

 rise inshore. In addition, the southeast 

 trades that blow out from over the conti- 

 nent reinforce this circulation. A similar 

 combination of winds produces the upwell- 

 ing of water off the coast of Peru in the 

 eastern Pacific. 



The climate of both sea and land is af- 

 fected as a result of cooler water being 

 brought to the surface near shore. The biota 

 of both is also influenced, often strongly, by 

 changes in the food chain. The upwelling 

 water brings mineral nutrients to the lighted 

 zone of the sea, \\here they become avail- 

 able for phytoplankton, and a rich growth 

 of sea life usually develops. Man is affected 

 by the more productive fisheries of such re- 

 gions and frequently even more so by the 

 guano deposits from the dense populations 

 of sea birds that congregate around the rich 

 supply of food. The islands off the coast of 

 Peru afford a notable example. 



The climatic changes produced by the 

 transport of large masses of warm water 

 into Arctic latitudes or the opposite trans- 

 port of Arctic water into midlatitudes are 

 too well known to need more than passing 

 mention. The effect of the warm Gulf 

 Stream on the climate of northern Europe 

 and the chilling produced by the cold Lab- 

 rador current at similar latitudes on the 

 American side of the Atlantic make the 

 point. Their action can be duplicated in 

 many parts of the world, and such currents 

 control the geographic limits of whole 

 biomes. 



Tides and tidal currents have been dis- 

 cussed earlier (p. 84). These water move- 

 ments in the relatively fertile waters of the 

 continental shelf make possible, among 

 other things, an extensive development of 

 the sessile habit of divergent groups of ani- 

 mals, including protozoans, sponges, hy- 

 droids, sea anemones and corals, bryozoans, 

 barnacles, urochordates, and manv mollusks. 

 The communities of which such animals are 

 prominent members, together with asso- 

 ciated worms, snails, crustaceans, and the 

 like, have their basic food supply carried 

 to them primarily by tidal and other local 

 currents. Manv burrowing organisms are 

 similarlv served. The notable absence of the 

 sessile habit among land animals bears wit- 

 ness to the relative paucity of air-borne life 

 as contrasted with aquatic plankton. 



Oceanic currents have been much studied 



both by physical and by biotic oceanogra- 

 phers. The well-documented information on 

 the subject is too extensive and too com- 

 plex to have even its basic principles fully 

 outhned here. The established principles, 

 especially of physical oceanography, have 

 been expressed mathematically in many 

 cases, but to consider them further now 

 would take us adrift from our main course. 

 The interested student is referred to Sver- 

 drup, Johnson, and Fleming's book (1942). 



WATER MASSES 



Density differences set up by thermosa- 

 line forces produce vertical convection cur- 

 rents. These are important in the vertical 

 migration of plankton organism.s that may 

 show diurnal depth movements of consider- 

 able extent (p. 139). Convection is even 

 more important in establishing a homoge- 

 neous layer of surface water, the depth of 

 which depends on the strength of the con- 

 vection currents. The existence of surface, 

 as distinct from deeper, waters serves to 

 introduce the present day concept that the 

 ocean, like the atmosphere, is composed of 

 a set of recognizable masses. The water 

 masses of the sea are identified by their 

 temperature-salinity characteristics, just as 

 air masses are known by temperature- 

 humidity differences. 



Figure 29 shows schematically the dis- 

 tribution of the oceanic upper water masses. 

 Typically, the oceans have a relatively 

 shallow surface layer extending down about 

 100 to 200 meters. The temperature-salinity 

 values vary greatly within this layer, and 

 great seasonal variations occur in areas with 

 variable climates. Other water masses have 

 relatively stable temperature- salinitv values. 

 In many ways the Antarctic Ocean furnishes 

 a helpful introduction. In the subantarctic 

 region five vertically arranged v/ater masses 

 can usually be distinguished as follows: 

 (1) subantarctic upper water, (2) subant- 

 arctic intermediate water, (3) upper deeper 

 water, (4) lower deeper \\'ater, and 

 (5) bottom water. 



The subantarctic mass of upper water 

 extends northward far into each bordering 

 ocean, taking its place, according to densitv 

 relations, between the given central water 

 and the deep water. In the Atlantic, but 

 not in other oceans, the intermediate water 

 mass originating in antiboreal regions ex- 

 tends beyond the equator and reaches as 

 far as 20 degrees north latitude. 



