96 FREQUENCY OF WAVE CONDITIONS 



SOUTH INDIAN OCEAN 



The seasonal alternation in the prevailing state of the sea is of the 

 same order in the South Indian Ocean as in the North, i. e., it is high 

 the oftenest in northern summer, when the northern boundaries of the 

 Southeast Trade Wind Belt and of the Westerlies have both reached 

 their most northerly limits for the year; when the average velocities 

 of both these wind systems are at their highest ; and when gales of 

 force 7, or stronger, are the most common in high latitudes. 



The sea pattern (pi. XVII) exhibits a rather definite north-south 

 alternation in midocean at this time of year. In the equatorial belt of 

 calms, the sea is high for generally less than 5 percent of the time; 

 in the axis of the Southeast Trades Belt, a high sea is reported in 

 frequency greater than 10 percent (greater than 20 percent of the time 

 where the Trades blow the strongest) and 20-foot waves have been 

 reported 3 percent of the time for the year as a whole (table 8, p. 21) ; 

 in the Variables, the frequency of high seas averages somewhat less, 

 though varying widely as reported from square to square; and finally, 

 along the northern edge of the Westerlies, the sea has been classed as 

 "high" in 20 to 50 percent of the reports for July and August, together. 

 It is also likely that the sea runs higher than 8 feet for more than half 

 the time along the main sweep of the Westerlies, right across the 

 southern Indian Ocean, from the offing of South Africa, past south- 

 ern Australia and Tasmania ; waves of 20 feet, and higher, have been 

 reported from another source (table 8, p. 21) in 17 percent frequency, 

 for the year as a whole on the route between the Cape of Good Hope 

 and southern Australia. 



The chief departures from this fundamentally latitudinal pattern 

 are : (a) the sea is much less often high (only occasionally so reported) 

 and much more often low (54 to 90 percent, pi. XVIII) in the waters 

 between northwestern Australia and the East Indian island chain to 

 the north than it is farther westward in this same latitudinal belt, 

 where the Trades are better developed; and (b) neither of the two 

 relatively smooth belts — the equatorial and that of the Variables — 

 extends westward as far as the African coast, though high seas are 

 hardly more common there than they are in midocean, at correspond- 

 ing latitudes. 



The swell runs high much more often in northern summer than the 

 sea does, throughout the South Indian Ocean as a whole, notably along 

 the Southeast Trades from the approximate longitude of northern 

 Sumatra (95° E.) to Madagascar (overage 37 percent for high swells, 

 and about 18 percent for high seas) ; this is no doubt due to the same 

 reason that the Trades swells are high more often than the Trades seas 

 in other parts of the oceans (pp. 79 and 85). And a high swell often 



