SOUTH INDIAN OCEAN 97 



spreads (locally up to 73 percent frequency) northward from the 

 stormy Westerlies to the more placid Variables, where high seas, lo- 

 cally generated, are much less frequent. 



The slackening of the Southeast Trades that takes place in northern 

 autumn, during their migration southward, is accompanied by a cor- 

 responding alteration in the prevailing state of the sea, so general 

 that the regions, within the Trades, where a high sea is reported in 

 frequency as great as 10 percent, are not only much less extensive in 

 January and February than in July and August, but appear to be 

 confined then to discontinuous pools. A high sea has, however, been 

 reported considerably more often in January and February (22 to 33 

 percent) than in July and August (10 to 24 percent) off the north- 

 western bulge of West Australia, where the winds of northern winter 

 average somewhat stronger (14 to 16 knots) than those of summer 

 (less than 12 knots). 



The period from January to March is also the chief season of tropical 

 cyclones in the South Indian Ocean. Storms of this character then 

 develop most commonly southward and eastward from the Seychelles 

 in about latitude 10° S., and follow a southerly course, most of them 

 passing east of Madagascar, but a few crossing the island, or following 

 the Mozambique Channel. This is illustrated by the storm tracks laid 

 down on the Pilot Charts for January and February. Tropical 

 cyclones may be responsible, at least in part, for the rather large 

 frequencies in which high seas are reported in winter southward and 

 eastward from Madagascar (10 to 16 percent). But the chance of 

 encountering the heavy seas they generate on any given voyage is 

 small, judging from the fact that only 139 of a dangerous character, 

 or about 3 per year, were reported for these months during the period 

 from 1848 to 1891. 



Unfortunately, the winter reports from the northern edge of the 

 West Wind Belt have not been numerous enough, nor distributed 

 evenly enough, to be of much significance. But gales are so much less 

 frequent there in northern winter than in summer as to suggest that 

 20-foot seas are not to be expected more than half as often from Decem- 

 ber through February, as in June, July, or August, along the steamer 

 route between South Africa and South Australia, where their reported 

 frequency is 17 percent for the year as a whole (table 8, p. 21). 



It seems further that a heavy swell is not as common along the 

 northern edge of the Westerlies in northern winter as it is in sum- 

 mer for the available percentages, derived from scattered data along 

 the route between South Africa and southern Australia, locate the 

 northern boundary of the area where a high swell is to be expected as 

 often as 40 percent or more of the time, in January and February, as 

 lying to the southward of latitude 40° S., except off South Africa and 



