Tides and Winds
Since the early years of European settlement, Cook Strait has been notorious for its treacherous currents and high winds, often of gale force. The tidal currents now generated in the strait reach a velocity of about 8 knots westwards and 4 knots eastwards, but are very erratic depending on conditions of wind and weather. The strait lies between 41 degrees and 41 degrees 40 minutes South Latitude in the westerly wind belt known as the Roaring Forties and wind velocities of up to 150 m.p.h. have been measured in the vicinity of Wellington. As Cook Strait is the only large gap in the chain of mountains extending north-eastwards for 900 miles from Puysegur Point to East Cape, it is thus a natural channel through which air streams approaching central New Zealand are diverted and accelerated to pass between the North and South Islands. This local acceleration of the surface winds is most pronounced when the air approaches from directions between west and north-west or between south and south-east. The map below illustrates the pattern of flow through the strait on an occasion when the general air-flow aloft was from the west.
When the general air-flow aloft is roughly parallel to the mountain chain (and south-westerly flows are relatively frequent), a slight shift in direction causes a complete reversal of the surface winds in Cook Strait. The southerly change which is usually very sudden and on occasion violent, often arrives as a “line squall”. Winds from the north-westerly quarter predominate, however, being about twice as frequent as those from the southerly quarter. Strong gales (60 m.p.h. or more) occur about Wellington and in the strait about 30 times per annum, on the average, with maximum frequency in the late spring.