12 



UNDERWATER GUIDE TO MARINE LIFE 



their pulls are complementary and the highest tides, called "spring tides," 

 occur. This happens during the new and full moons. When the sun and moon 

 form a right angle with the earth as the center, their gravitational pulls partly 

 cancel each other and the lowest or "neap tides" occur. This happens during 

 the first and third quarters of the moon. 



WIND AND WAVC DIRECTION 



Fig. 2. The breaking of a wave on a sloping shore. 



Tides are erratic in occurrence and in height. In general they are lowest 

 at the equator and highest toward the poles. In most places high tides occur 

 twice a day, but in some places they occur only once. The highest tides in 

 the world occur in Nova Scotia's Bay of Fundy, followed closely by several 

 spots such as the Turnagain Arm near Anchorage, Alaska. In both of these 

 places, tides must travel up narrow channels in a short period of time. This 

 causes a tidal-wavelike rush of water called a "bore" to form. It is not known 

 exactly why extremely high tides form in some places and not in others. Pre- 

 sumably it has to do with the configuration of the ocean bottom. If the bottom 

 forms a trough, so that water can oscillate back and forth in it like water in a 

 bathtub, and is the right size, so that the period of this rocking vacillation 

 matches the period of the movements of the sun and the moon, water will 

 rock back and forth in the trough to give very high tides of up to 50 feet. 

 Bores will occur when the trough has a constricted end, up which tidal water 

 may rush. 



The currents in the sea have a more complex origin and are of greater 

 importance than waves and tides, even though they are usually not as 

 spectacular to look at. Horizontal currents traveling over the ocean's surface are 

 motivated primarily by prevailing winds, and their direction is influenced by 

 the rotation of the earth. The prevailing winds in both hemispheres blow from 

 the east along the equator, the trade- winds, and from the west along the horse 

 latitudes (30° north and south), the westerlies. The rotation of the earth 

 causes a deflection of these winds so that their paths become circular— clockwise 

 above the equator and counterclockwise below. In general, ocean currents 

 follow this wind pattern (/ig. 3), but are compficated by the conformation 

 of coasts and the ocean bottom. In the center of these great, rotating bodies 

 of water are eddies where the water just turns around and around and never 

 goes anywhere. One such eddy is the Sargasso Sea. 



Vertical currents are caused bv density diff^erences of water, that is, by 

 varying temperature and salinity. As water is heated near the equator, it becomes 



