Chapter X 



OFFSHORE TIDAL CURRENTS, REDUCTION OF CURRENT 

 OBSERVATIONS, AND CURRENT PREDICTION 



Paragraphs 



Rotary tidal currents 485-486 



Polar currents diaigrams 487-497 



Prediction of currents 492-496 



Harmonic analysis of tidal currents 496-491 



Nonharmonic reduction of current observations 500-505 



Wind currents 509 



OFFSHORE TIDAL CURRENTS 



485. Rotary tidal currents. — The tidal flow heretofore considered has 

 been that in a confined channel, in which the currents periodically 

 reverse their direction and pass through zero at each reversal. A 

 consideration of the tide producing forces, developed in chapter I, 

 shows that their direction is rotary rather than reciprocating. As is 

 perhaps to be expected, the action of these forces on the whole mass of 

 water in the oceans tends to produce rotary movements of the current 

 at offshore tidal stations. At such stations, the currents usually veer 

 around the compass during the tidal cycle, and have no periods of 

 slack water. These are called rotary currents. At most offshore 

 stations in the Northern Hemisphere the direction of the current 

 turns clockwise, and in the Southern Hemisphere, counterclockwise. 

 The velocity usually varies during the semidiurnal tidal cycle between 

 two maxima, in approximately opposite directions, and two minima 

 whose directions are nearly at right angles to the directions of the 

 maximum velocities. 



486. Nontidal currents. — The periodic tidal currents at offshore 

 stations are generally weak and may be m ach modified by permanent 

 currents of fairly constant strength and direction produced by the 

 circulation of ocean waters, and by temporary currents due to winds 

 and other meteorological causes. The Gulf Stream and the Japan 

 Current are well known permanent currents. 



487. Polar current diagrams. — Offshore currents are conveniently 

 represented by laying off the current strengths at say hourly intervals 

 on radiating lines (radii vectores) drawn from a common center (pole) 

 in the direction of the current. The curve through the ends of these 

 vectors is the polar curve of the current. The time is marked on the 



(249) 



