with a lower-level low, and a low may have one or 

 more distinct troughs radiating from it. (24) 



2. A long depression of the sea floor, hav- 

 ing relatively gentle sides, normally wider and 

 shallower than a TRENCH. (26) 



TRUE NORTH . The direction from any point on the 

 earth's surface toward the geographic North Pole; 

 the northerly direction along any projection of the 

 earth's axis upon the earth's surface, e.g., along 

 a longitude line. Except for much of navigational 

 practice (which uses MAGNETIC NORTH) , true north is 

 the universal 0° (or 360°) mapping reference. True 

 north differs from magnetic north by the magnetic 

 DECLINATION at that geographic point. (24) 



TRUNK BUOY . A mooring buoy having a pendant ex- 

 tending through an opening in the buoy, the ship's 

 anchor chain or mooring line being secured to this 

 pendant. (17) 



TRUNK ROUTE DECCA . See DECTRA. 



T-S CURVE . See TEMPERATURE-SALINITY DIAGRAM. 



TSUSHIMA CURRENT . That part of the KUROSHIO flow- 

 ing northeastward through Korea Strait and the Sea 

 of Japan, following the northwest coast of Japan, 

 and then curving southeastward to rejoin the main 

 part of the Kuroshio. (17) 



TRUMPET BUOY . A buoy provided with a trumpet hav- 

 ing a distinctive tone and an adjustable character- 

 istic. (17) 



TRUNCATED LANDFORM . A landform cut off, especially 

 by erosion, and forming a steep side or cliff. (27) 



TSUNAMI (TIDAL WAVE) ■ Tsunamis are ocean waves 

 produced by sudden, large-scale motion of a portion 

 of the ocean floor, as by volcanic eruption, earth- 

 quake, or landslide. If they are caused by a sub- 

 marine earthquake, they are usually called seismic 

 sea waves. Either a tsunami or a storm wave that 

 overflows the land is popularly called a tidal 

 wave, although it bears no relation to the tide. 

 Tsunamis usually occur in series, gradually 

 increasing in height until a maximum is reached 

 between about the third and eighth wave. Follow- 

 ing the maximum, they again become smaller. Waves 

 may continue to form for several hours, or even 

 days. In deep water the wave height of a tsunami 

 is probably never greater than two or three feet. 

 Since the wave length is usually considerably more 

 than 100 miles, the wave is not conspicuous at sea. 

 In the Pacific, where most tsunamis occur, the wave 

 period varies between about 15 and 60 minutes , and 

 the speed in deep water is more than 400 knots. 

 When a tsunami enters shoal water, it undergoes the 

 same changes as other waves. Because of the great 

 speed of a tsunami in relatively deep water, the 

 slowing is relatively much greater than that of an 

 ordinary wave created by wind. Therefore, the in- 

 crease in height is also much greater. Tsunamis 50 

 feet in height or higher have reached the shore, 

 inflicting widespread damage. (12) 



TUBEWORMS . Segmented marine worms some of which 

 secrete calcareous tubes. (15) 



TUFA . A chemical sedimentary rock composed of 

 calcium carbonate or silica. Tufa is included 

 with rock on bottom sediment charts. On U. S. 



Navy Hydrographic charts for the western coast of 

 France the notation T may mean tangue, a calcareovis 

 deposit derived from a variety of marine organisms. 



TUNICATES . A subphylum of marine invertebrate ani- 

 mals which includes ascidians or sea squirts, and 

 salps , Ascidians are either compound or simple, 

 and many species attach to submerged objects. 

 Several species of salps are bioluminescent . (15) 



TURBIDJTES . Derived by TURBIDITY CURRENTS from 

 the lands or from higher submarine terrain. (27) 



TURBIDITY . The state or condition of having the 

 transparence or translucence disturbed, as when 

 sediment in water is stirred up, or when dust, 

 haze, clouds, etc., appear in the atmosphere because 

 of wind or vertical currents. (17) 



TURBIDITY CURRENT . A gravity current resulting 

 from a density increase by suspended material. 

 Also called SUSPENSION CURRENT, and DENSITY CURRENT. 

 (27) 



TURBULENCE . The state or condition of being vio- 

 lently agitated or disturbed, as a stream which 

 meets an obstacle, or air flowing over an uneven 

 surface. (17) 



TURBULENT FLOW . Fluid motion in which random 

 motions of parts of the fluid are superimposed 

 upon a simple pattern of flow. All or nearly all 

 fluid flow displays some degree of turbulence. 

 The opposite is STREAMLINE FLOW. (17) 



TWO RANGE DECCA . Two-range Decca is a variant of 

 the DECCA NAVIGATOR system. It was developed 

 specifically for surveying and unlike conventional 

 Decca, which utilizes permanently based transmit- 

 ters ashore, the master is carried in a ship and 

 the shore based equipment is designed for portabil- 

 ity, as well as for the essential accuracy. 



Since only two stations are used, the dial 

 meter display consists of only 2 movements instead 

 of the usual 4 in the standard Decca Navigator. 

 (29) 



TWZO. Trade Wind Zone Oceanography. 



TYPE OF TIDE . The characteristic form of the tide 

 with special reference to the relation of the diur- 

 nal and semidiurnal waves. Tides are sometimes 

 classified as diurnal, semidiurnal, and mixed, but 

 there are no sharply defined limits separating the 

 groups. The tide is said to be diurnal when the 

 diurnal wave predominates and only a single high 

 and single low water occur each day during the 

 greater part of the month. The tide is semidiurnal 

 when the semidiurnal wave predominates and two high 

 and two low waters occur each tidal day with a 

 relatively small inequality in the high and low 

 water heights. In the mixed type of tide the diur- 

 nal and semidiurnal waves are both important fac- 

 tors and the tide is characterized by a large in- 

 equality in the high or low water heights or in 

 both. There will usually be two high and two low 

 waters each day, but occasionally the tide will 

 become diurnal. Also applicable to tidal currents. 

 (14) 



TYPHOON ■ A tropical cyclone originating in the 

 western Pacific Ocean, particularly in the vicinity 

 of the South China Sea or to the eastward of the 

 Phillippine Islands. Such a disturbance is called 

 a BAGUIO in the Philippine Islands. (17) 



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