restricted access 3. Conveyor Belts of the Seas: The Prevailing Winds and Currents
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3 Conveyor Belts of the Seas The Prevailing Winds and Currents [The goddess Circe sent] us . . . a favorable breeze, which sprang up from astern and filled the sail of our blue-­ prowed ship. All we had to do . . . was to sit still, while the wind and the helmsman kept her straight. With a taut sail she forged ahead all day . . . [and] through the darkness. Thus she brought us to the deep-­ flowing River of Ocean and the frontiers of the world. —Homer, The Odyssey, circa 750–700 BC A fundamental matter to keep in mind is that measured distance is not the ultimate issue with respect to ocean crossings; rather, sailing time (along with ease) is, and in these terms effective ocean width is not the same in opposing directions, in every part, or at every season. Traditional sailors tended to talk in terms of so many days’ sail, and only sec­ ondarily (if at all) of measured distance. For example, a fifteenth-­ century Chinese chart of the Indian Ocean indicated degrees of separation between places by recording how many watches were involved. The Norse spoke of 12-­ hour sailing days rather than linear distances. To the extent that linear distances are given in early sources, they do not refer to situations involving the ocean’s major belts of winds and currents. The emphasis on sailing time was because “map” distance is only one of the important variables determining the time it takes to traverse an ocean. The other vital variables are the vectors, velocities, and consistencies of the winds and of the surface oceanic currents encountered, especially the winds. Among scholars concerned with questions of transoceanic interinfluences, the Norwegian experimental voyager Thor Heyerdahl was particularly prominent in stressing (even overstressing) the importance of such phenomena with respect to likely directions and routes of crossings. It is apt at this point to summarize the relevant characteristics of these large-­ scale air and water movements. The prevailing wind patterns of the world are produced by regional and seasonal atmospheric pressure differences, which, in turn, reflect differences in solar insolation according to differences in latitude and times of year. The most important of the pressure zones are the enormous subtropical high-­ pressure cells, one of which lies over each of the major ocean basins (that is, North Atlantic , South Atlantic, North Pacific, South Pacific, and south­ ern Indian), between roughly 20 and 40 degrees north and south latitude (fig­ ure 3.1). Winds spiral outward from these highs toward the equatorial low-­ pressure trough and Prevailing Winds and Currents / 33 toward the subpolar low-­ pressure belts while being deflected according to the Coriolis effect.1 The net result is that in the middle latitudes, on the poleward flank of each of these highs a zone of prevailing approximately westerly winds exists over each ocean basin, and that on the equatorward side of each high is a belt of approximately easterly winds—the trade winds (so called owing to their former importance to merchant shipping). On the east­ ern and west­ ern flanks of these high-­ pressure cells are zones of equatorward-­and poleward-­ blowing winds, respectively. Further, major high-­ pressure cells over the poles produce the so-­ called polar easterlies over the boreal oceans, which extend into the subpolar latitudes, winds that would have favored discovery of the more northerly Atlantic islands and north­ ern America from mainland northwest­ ern Europe and from the British Isles. These tropical trades, midlatitude westerlies, and subpolar easterlies annually shift a number of degrees northward and southward , following (up to a point) the 47-­ degree seasonal shifts in latitude of the overhead sun. Sea Carousels: The Principal Winds and Currents The prevailing wind systems, although not always completely consistent—especially the more poleward ones—do operate most of the time. The trade winds are particularly reliable in most mid-­ ocean equatorial areas, at least for much of the year. The drag of the prevailing winds across the surfaces of the seas generates directional drifting of the surface waters, although at a very much lower Figure 3.1. Generalized map (Mercator projection) of the principle surface ocean currents: (a) Japan/North Pacific currents, (b) California/North Equatorial currents, (c) Equatorial Countercurrent, (d) Peru/South Equatorial currents, (e) Antarctic Drift, (f) Irminger Current, (g) Gulf Stream/North Atlantic current, (h) Canary/North Equatorial currents, (i) Benguela/South Equatorial currents. Courtesy of Stephen C. Jett. 34 / Chapter 3 velocity than that of the winds themselves. The vectors of the...