GENERAL CIRCULATION OF WINDS OVER THE TROPICAL REGIONS OF THE PACIFIC 



11 



Upper Winds Over South American 

 High-Pressure Area 



The upper winds in the great area extending from 

 longitude 100° east to the coast of Chile wo>ild appear 

 from the few flights available to be from the southwest. 

 Along the Chilean coast at Iquique, Caldera, Valparaiso, 

 and other sea-level stations, the prevailing wind blows 

 from the southwest. In flights 30 to 34, made between 

 longitudes 100° and 120° west, the southwest stratum 

 was observed in all except flight 32. The thickness of 

 the southeast trade wind layer above which southwest 

 winds blew was 1.2, 0.7, and 3.9 km on three flights. 

 Northerly winds persisted to 5.7 km in flight 32, proba- 

 bly owing to a small low-pressure center which devel- 

 oped nearer the South American coast, the barometer 

 falling from 767.3 to 765.4 mm in a change of 250 km in 

 the ship's position. 



During January and February, while the Carnegie 

 was in the South Pacific, the center of the great whirl of 

 winds around the South Pacific high-pressure area was 

 just south of Easter Island. The observed wind veloci- 

 ties above 3 km were very light over this area, and the 

 directions generally from east or north in flights made 

 north of latitude 30° south, whereas south of this latitude 

 winds were generally westerly. The two high flights re- 

 vealed light southeast winds from 8 up to 11 and 12 km. 

 This South American high-pressure area with cloudless 

 sky is like the "Azores high," a region of strong down- 

 ward currents, but with very light horizontal movements 

 to the highest levels of the troposphere. 



Trades and Antitrades 



Between Longitudes 100° and 140° West 



in Latitudes 15° to 20° South 



Not only the balloon flights, but also the Greenwich 

 noon observations made from latitudes 15° to 20° south, 

 show that the surface winds constitute an easterly cur- 

 rent of extraordinary uniformity in direction from longi- 

 tudes 100° to 140° west. The eastern half of this section 

 is entirely free from oceanic islands, and in the western 

 half only the low coral islands of the Tuamotu Archipel- 

 ago raise themselves a few meters above the surface of 

 the ocean. Their effect, however, on the great atmos- 

 pheric circulation is probably out of all proportion to 

 their size. 



From longitudes 100° to 120° west the surface winds 

 have a southerly component and a force of 3 to 5 Beaufort 

 force (4 m to 12 m per second), whereas from longitudes 

 120° to 140° west the winds are more nearly due east or 

 with a northerly component and a slightly reduced veloc- 

 ity, varying usually between the limits of 2 and 4 Beau- 

 fort force (2 m to 7 m per second). The southerly com- 

 ponents generally disappeared from 1 to 2 km above the 

 surface, so that the winds developed into a great easter- 

 ly drift with its greatest velocity at the bottom. From 

 longitudes 120° to 135° west, above this easterly drift, 

 setting in at a mean level of about 3 km, is a west-north- 

 west wind. The shift from east to west-northwest winds 

 Is not abrupt, but above a layer of stagnant or ligHtly 

 moving air. The exact height at which the northwest cur- 

 rent makes its appearance seems to be variable from 2 

 to 6 km. The velocity of the northwest current in the 



stratum from a height of 6 to 7 km is of the order of 6 to 

 10 mper second. 



The flights showed that the northwest current prob- 

 ably reached 8 km, but an upper limit was not indicated. 

 The southeast trade winds exhibited the remarkably high 

 constancy of 0.95 or more to a height of 1.5 km. 



Trades and Antitrades in the 

 Central Area of the South Pacific Ocean 



From longitudes 133° to 140° west the data indicate 

 that above the southeast trades an inflow of air from 

 south-southwest sets in at levels from 2.6 to 3.3 km and 

 continues from a southerly direction to considerable 

 heights. One flight reached a height of 5.5 km and an- 

 other 10 km in the southerly current. 



Although further observations may not confirm the 

 existence of a southwest current above the trades at a 

 point so far to the east in the Pacific as the meridian of 

 133° west, it may be noted that at Apia Observatory, 

 Samoa, 30° farther west, a long series of observations 

 has shown that at a height of 12 km a very strong south- 

 west current exists throughout the year, but its lower 

 level varies with the season. From January to June, the 

 same season in which the Carnegie observations in this 

 area were made, the upper winds at Apia begin to show 

 a definite southerly component at heights between 4 and 

 6 km. After turning through southeast, the winds pass 

 into the southwest quadrant between 7 and 11 km. In the 

 distance to Samoa unfortimately few flights reached this 

 height, but in flight 71 south-southwest winds were foimd 

 from 3 km until the balloon was lost at 4.5 km. In flight 

 77, above a very thick calm stratum, south winds devel- 

 oped at 6.6 km and continued to 12.5 km, whereas flight 

 80, made near Samoa, had south- southwest winds above 

 6 km. 



In passing through the Society Islands in the belt be- 

 tween latitudes 20° and 12° south and from longitude 

 140° west to Apia, Samoa, the surface and lower -level 

 winds, though mostly between east and north, were fre- 

 quently from the northwest quadrant. The typical south- 

 east trades were rarely observed. This northerly com- 

 ponent of the atmosphere at the lower levels has been 

 confirmed both by the Apia upper -air observations and 

 by the Carnegie observations made on her second visit 

 to Samoa in November 1929. 



Winds in the Equatorial Region 

 of the Pacific Ocean 



Two series of flights were made in the equatorial 

 region of the Pacific, the first in April-May along the 

 meridian of 175° west and the second in October -Novem- 

 ber in longitudes 145° to 155° west. Along both these 

 cross sections easterly winds were found to prevail at 

 the surface, being for 5 ° on either side of the equator 

 more commonly east -northeast than from a southerly 

 direction. The east -northeast surface layer near the 

 equator varied from 400 to 800 m in thickness, and had 

 a mean velocity at 500 m of between 5 to 12 m per sec- 

 ond, thus setting up a vigorous circulation in a region 

 where doldrums might be expected. 



Above the easterly winds to the south of the equator 

 and more especially from latitudes 7° to 15° south. 



