Bill Taylor, of Queenstown, asks :-
In carefully chosen raft and rowing expeditions across oceans the current helps a lot. What is the speed of some of the faster ocean currents, where are they and what causes them?
Robert Hall, a Hydrographic Surveyor in the Royal New Zealand Navy and then the commanding officer of the survey ship HMNZS Takapu, responded.
Currents flow at all depths in the oceans but in general the stronger currents occur in an upper layer which is shallow in comparison with the general depths of the oceans. The surface current is horizontal only and the direction of circulation is clockwise in the Northern hemisphere and counter-clockwise in the Southern hemisphere.
The main cause of surface currents in the open ocean is the direct action of the wind on the sea surface. A close correlation accordingly exists between current directions and those of the prevailing winds. Winds of high constancy blowing over extensive areas of ocean will naturally have a greater effect in producing a current than will variable or local winds. Thus, the North-east and South-east Trade Winds of the two hemispheres are the main spring of the mid-latitude surface current circulation.
In the Atlantic and Pacific Oceans the two Trade Winds drive an immense body of water West over a very wide width of some 50 degrees of latitude, broken only by a narrow belt of east going Equatorial counter-current, which is found a few degrees North of the equator in both these oceans. A similar transport of water to the West occurs in the South Indian Ocean driven by the action of the South-east Trade Wind.
There is probably no region in any part of the open oceans where the currents experienced do not at times attain a rate of at least one knot during periods of strong winds. Within the major currents of the World maximum rates derived from ship drift records are generally found to be in the range of 2-4 knots although rates of 5 knots are not uncommon. The duration and extent of these higher values are not generally known. The most well known of the higher current rates in the Atlantic Ocean is the Gulf Stream. In the Pacific Ocean we have, amongst others, the Japan Current. The Indian Ocean has the Somali and East African Coast Currents, especially during the SW Monsoon. In the area of Suqutra are the strongest known currents in the world and rates of 7-8 knots have been recorded.
For details of other currents I recommend The Mariner's Handbook and the RNZN's New Zealand Tidal Stream Atlas. Both can be purchased through chart sales agents throughout New Zealand.