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MONTHLY WEATHER REVIEW Editor, ALFRED J MONTHLY WEATHER REVIEW Editor, ALFRED J. HENRY ___- __ -_______ -- i VOI. 55. No. 5 CLOSEDJuly 2,1927 W. B. No. 927 MAT, 1927 ISSUED July 30, 1927 _______I_ THE NORTHEAST TRADE WINDS OF THE NORTH PACIFIC By E. A. BEALS CHAPTERI average is the result of many diverse pressure distri- butions. It is important that this truth be brought home QENERAL RELATIONS OF THE TRADE WINDS TO FRESSURE to writers on pressure distribution and its influence upon DISTRIBUTION the weather. Figures 1 and 2 have been prepared to show the average Since the relative pressure of the atmosphere determines pressure distribution and the prevailing winds in the the force and direction of the prevailing winds, it is Northern Hemisphere for January, a winter month, and obvious that the first step toward a proper understanding July, a summer month. The data are taken from the of the wind systems of the globe is an esaniination of United States Hydrographic Office Pilot Charts. the general pressure distribution. The pressure distribution in the Southern Hemisphere Broadly speaking, the chief features of world-pressure is similar to that shown in Figures 1 and 2 except that distribution are: (1) An equatorial belt or zone of dimhi- the large oceanic anticyclones are not dis osed symmet- ished pressure, (2) a belt or zone of high pressure around rically with those of the Northern HemispB ere; thus, for the world about 30’ both nort,h and south of the Equator, example, the anticyclone in the South Pacific is situated and (3) a second belt of low pressure about north latitude much nearer to the Continent of South America than the 60’ and a similar belt in the Southern Hemisphere in corresponding North Pacific anticyclone is to the Con- approximately the same latitude. tinent of North America. The belts of high pressure about 30’ from the Equator Recalling the statement previously made as to the are not continuous around the world but form a series existence of a zone of low pressure in the equatorid of the well-known semipermanent anticyclones, each with regions that is bounded on both sides by belts of higher its appropriate wind circulation. Similarly the belts of pressure, it will be readily understood that acting under low pressure about 60’ from the Equator are not con- the influence of gravity there will be a flow of air from tinuous but rather a disconnected series of cyclonic wind the higher to the lower pressure; that is, from about Bystems which reach their greatest development in winter lntitude 30’ N. and S. toward the Equator. The &ect and over the oceans rather than the continents. The best of the earth’s rotation on its axis is to cause these winds. known of these are found in the Northern Hemisphere to blow obliquely toward the Equator from the northeast along the Aleutian Islands in the Northeast Pacific and in the Northern Hemisphere and from the southeast in in the vicinity of Iceland in the Northeastern Atlantic. the Southern Hemisphere. These winds from their rele- These two well-known senlipenxianent cyclcones are tive constancy in direction are known as trade winds. known as the Aleutian cyclone or LOW and the Iceland The origin of the name, according to the Oxford Eng- cyclone or LOW, respectively. lish Dictionary, had nothing to do with “trade” in the The most prominent semipermanent anticyclones in the sense of “‘conmerce’ or passage for the purpose of trad-i Northern Hemisphere are the great Siberian ant>icyclone ing, though the importance of those winds to navigation: of winter, and winter only, and the two oceanic anti- led eighteenth century etyinologists and even navigators cyclones, the Azores in the Atlantic and the North so to understand them.” Pacific in the ocean of that name. The oceanic anti- The trades in their course Equatorward begin in the cyclones are best developed in summer. belts of high pressure, sonietinies designated the horae The wind circulation in these anticyclones is spirally latitudes, and end by giving way to the equatorial outward from the center in a clockwise direction; that of calnis or doldrums. Further reference to the doldrums the cyclone is exactly the opposite. will be made later in this discussion. Both the cyclones and anticyclones above mentioned The North Pacijic semipermanent anticyclone.-The must not be confused with the traveling cyclones and anti- annual meandering of this formation is shown in F’ c clones of the daily weather map. The latter preserve Here again attention is directed to the injunction?Zk t ceir form and travel hi a definite direction over the expressed that the North Pacific anticyclone should not be earth’s surface for a time. The former, on the other thought of as a definite entity, but rather as the montbl hand, must not be thought of as having a dishict entity average pressure over the ocean in certain geographic3 which is continually preserved. Rather the should be positions. In Januaiy the center of highest aver* thought of as representing the average niont Kly pressure pressure is found as indicated in the figure; its su 88- distribution in their respective areas, and this inon thly quent monthly mean positions are indicated, and a Phe 53672-27-1 211 Unauthenticated | Downloaded 09/29/21 03:02 AM UTC 212 MONTHLY WEATHER REVIEW MAY,1927. connectin them shows that the center makes a certain may persist for a day or so, after which they slacken mount of westing and north5g up to August when it is until another impulse is received. found in the position indlcated; a movement back The semipermanent cyclone over the Aleutians attains toward its winter position is then initiated, and the its maximum geographic estent and intensity in January, cycle.. of change begins anew in the succeeding year. as may be seen from Figure 1. It disappears in June and FIG. l.-IsobaIs and prevailing winds, Northern Hemir%here,January The movement of anticyclones over the Northeast is not a disturbing factor between May and September. Pacific is mostly in an easterly direction; it must happen Trade-wind boundaries.-The regions where the north- then that when fresh bodies of relatively cool air are east trades prevail in the North Pacific niay be traced in brought into the western or northern front of an existing Figures 1 and 2, which show the prevailing wind direction anticyclone its wind circulation is increased thereby and for each 5' square for the months of January and July, there is a distinct response in the northeast trades which respectively. Unauthenticated | Downloaded 09/29/21 03:02 AM UTC MAY, 1927 MONTHLY WEATHER REVIEW 33% In winter the Siberian anticyclone is responsible for the compass. In other words, winds from tihe n&, the northeast winds of the southeast coast of Asia. east, as recorded on the Pilot Charts, may have bee4 The trades are weakest in September, when the north- recorded on the ship's log anywhere between NNE: east monsoon winds of Asia do not exist and the North and ENE., and the same discrepancy will apply to other! Pacific anticyclone is rapidly diminishing in strength. directions. FIG.2.--lsobars and prevelhg winds, Northern Hemisphere, July Recording trade-wind observations at sea.-Although On account of the wide range between the direction a$ weather observers on board ship record wind direc- recorded on the Pilot Charts and those which might have j tions to 16 points of the 'compass, they are published actually occurred, it will be difficult to determine thb; on the North Pacific Pilot Charts on an 8-point scale; precise drift of the northeast trades. However, where' therefore a northeast wind on the Pilot Chart may an east wind prevails in one 5O square and a northeast' be a wind coming anywhere from 22% to 67% on in the next adjacent 5O square, it is reasonable to; Unauthenticated | Downloaded 09/29/21 03:02 AM UTC 2x4 MONTHLY WEATHER REVIEW May, ks2Q presume that the northeast wind had an easterly cam- ponent and the east wind a northerly component and that the true direction of both winds was not far from eaat- northeast . These facts should be considered when examining the wind directions both on the Pilot Charts and those of Figures 1 and 2, the latter having been taken directly from the former. With these c.onditions in mind, and remembering that winds in the. Northern Henlisphere blow spirally outward from antic,yclones in a clockwise direction, it is probable that the directions of the trades at the estreme eastern side of the ocean have a marked northerly component. As one goes west within the trade-wind be.lt the winds veer more and more unbil t,hey have a decidedly easterly c.0ni ponen t . The southeast trades which blow spirally outward, but in a counterclockwise, direction, froin the semipermanent ant,ic.yclone in the eastern portion of the South Pacific follow the same system as the northeast trades. In the eastern part OF that ocean t>heyhave a decided southerly' component, but in going west they become more and more easterly, until at the northern e,dge of the southeast trades they merge wit,h the corresponding winds at the FIG.I.-Isobars and prevailing winds of the Pflciflc Ocean (after Werenskiold) Unauthenticated | Downloaded 09/29/21 03:02 AM UTC MAY,1927 MONTHLY WEATHER REVIEW FIG.5.-hlean isobars and prevailing winds over the Paciflc Ocean, July (after Werenskiold) these longitudes, for 90’ or more in winter and for 20° Meteorological textbooks of North American origin in summer, there is no belt of calms, but after passing to describe a narrow belt of calms, called the doldrums, aa the west of about 140th nieridian of east longitude in esisting between the northeast and the southeast trades winter and the 180th nieridian in simmer another soralled and extending across both the Atlantic and the PRCSC.
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