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208 the Observed Zonal Circulation of the Atmosphere 1 208 BULLETIN AMERICAN METEOROLOGICAL SOCIETY The Observed Zonal Circulation of the Atmosphere 1 YALE MINTZ Dept. of Meteorology, University of California, Los Angeles 24 ABSTRACT Observational data from many sources are brought together to form a composite picture of the distribution, from pole to pole and from sea level to the 5-centibar level, of the normal summer and winter zonal winds averaged around the earth. HE zonal circulation of the atmosphere distances in the FIGURE represent equal masses of is the zonal component of the wind, the air. The approximate elevation of each pressure Twest-east component, averaged completely surface is given by the scale for the International around the earth. In the meteorological litera- Standard Atmosphere at the upper left. The ture this circulation is also called the "zonal index" isotachs of the zonal wind are in speed units of or "general circulation" of the atmosphere. The meters per second, with westerly winds having zonal circulation varies with latitude and elevation. positive values and easterly winds negative values. It also varies with time, but this paper will de- The regions of easterly winds are shaded. Each scribe only the normal summer and winter condi- axis of maximum westerly wind or maximum tions. easterly wind is traced by a thin line. From this The great expansion of aerological observations FIGURE profiles of the zonal wind from pole to in recent years has led to the publication of nu- pole were taken for selected elevations. These are merous circumpolar maps of time-averaged pres- shown in FIGURE 2. sure and winds at several levels in the troposphere The analysis of the zonal wind field up to the and lower stratosphere, and of numerous vertical 5-cb level, which lies at about the height of 20 cross-sections of the time-averaged zonal com- kilometers, shows us the circulation through 95 ponent of the wind in different longitudes. This percent of the mass of the atmosphere. The cir- paper is an attempt to bring together the results culation in the remaining 5 percent of the mass of a number of these studies into a single com- of the atmosphere is not treated here. For a posite picture of the normal (time-averaged) survey and summary of what is observationally zonal circulation covering the whole earth. The known about the winds through this small remain- final picture thus obtained is objective and em- ing part of the atmosphere the reader is referred pirical and, except for the use of the geostrophic to a paper by Jenkins (1952). wind law, rests upon no deductions from hydro- dynamical theory. It is an attempt to represent SOURCES OF DATA the present state of our knowledge of the real at- North of 20° north latitude, the wind shown in mosphere against which different theories of the FIGURE 1 is the geostrophic zonal wind computed zonal circulation may be tested. from mean circumpolar maps (U.S.W.B., 1952; FIGURE 1 shows the normal zonal circulation, and U.S.W.B., 1944). Between latitudes 20°N from pole to pole and from sea level to the 5-centi- and 20°S the wind shown is essentially the zonal bar (50-mb) level, in summer and in winter. component of the mean pilot-balloon wind. The Northern hemisphere summer and southern hemi- data used in the latitude belt 20°N to 20°S were sphere winter (June-July-August) are on the taken from published circumpolar mean-wind left hand side of the FIGURE. Northern winter charts derived from pilot-balloon ascents, sup- and southern summer (December-January-Febru- plemented at the 30-cb level by observations of ary) are on the right. The radial ordinate in the the movement of cirrus clouds (Brooks et al., FIGURE (the earth's vertical ordinate) is pressure 1950) ; and from meridional cross-sections of the on a linear scale. The advantage of using pres- mean pilot-balloon wind at longitudes 20 °E and sure as the radial ordinate is that equal vertical 25°W (Ekhart, 1941), longitudes 78°E (Ven- 1 U.C.L.A. Department of Meteorology, Papers in kiteshwaran, 1950) and 100°E (Mintz and Dean, Meteorology, No. 18. The research reported in this 1952), and rawincross-sections at 165°E (Palmer, paper has been sponsored in part by the Geophysics Re- 1951). South of latitude 20°S, the data were taken search Division, Air Force Cambridge Research Center, under Contract AF 19(122)-48. from meridional cross-sections of the mean geo- Unauthenticated | Downloaded 10/10/21 06:50 PM UTC VOL. 35, No. 5, MAY, 1954 209 FIG. 1. The normal zonal circulation, or mean zonal wind averaged over all longitudes, in summer and in winter. Isotachs are in meters per second. strophic zonal wind at longitudes 130°E and 150°E sizeable discrepancies between the geostrophic and (Gibbs, 1952) and at longitude 170°E (Hutch- pilot-balloon zonal winds. In general, the geo- ings, 1950), the mean pilot-balloon winds at Little strophic winds were larger, in the positive westerly America (Grimminger, 1941), and a single sum- sense, than the pilot-balloon winds. But the mer season mean of 426 pilot-balloon and rawin writer assumed that in these low latitudes the ascents taken at sea all around the border of pilot-balloon winds were the more reliable and Antarctica (Aerology, . ., Chief of Naval Op- in analyzing the transition zones gave them greater erations, 1948). In addition, from 60°N to 50°S, weight. the zonal components of the mean surface winds over all the oceans, as summarized from the wind THE NORTHERN HEMISPHERE POLAR observations in ships' logs (McDonald, 1938), EASTERLIES were averaged with longitude and used as a guide. As shown in FIGURES 1 and 2, over the North At latitudes 20°N and 20°S the data showed Polar region the normal zonal circulation near the Unauthenticated | Downloaded 10/10/21 06:50 PM UTC 210 BULLETIN AMERICAN METEOROLOGICAL SOCIETY ground has the form of a thin cap of weak easterly to the pole. This seems to have been the case in the winds. The cap extends over a somewhat wider years 1937-1938, which were the years that pro- region, but reaches to smaller heights, in summer vided Dzerdzeevski and Karelin with surface ob- than in winter. The speed maximum of this servations from the "North Pole" and "Sedov" easterly circulation is somewhat in excess of one expeditions and led them to the erroneous con- meter per second in summer and two meters per clusion that in the polar region the normal zonal second in winter. circulation near the ground was a westerly one The polar easterlies show up when computed (cf. Dzerdzeevski, 1946; Mintz and Dean, 1952). from the pressure field averaged from many years of observations, but are not a permanent feature In other years the cap of polar easterlies is found of the circulation. In some years the low-level cap when the average is taken over several months, of easterly circulation is replaced by low-level west- but contains periods of several days to several erlies that extend all the way from middle latitudes weeks in which the winds are westerly. FIG. 2. Profiles of the normal zonal circulation at selected elevations. Unauthenticated | Downloaded 10/10/21 06:50 PM UTC VOL. 35, No. 5, MAY, 1954 211 THE NORTHERN HEMISPHERE WESTERLIES In the middle latitudes of the northern hemi- sphere the normal zonal circulation is westerly from the ground up. This band of westerlies broadens in latitudinal extent with height, extend- ing on the one hand over the cap of polar easterlies and on the other hand extending equatorward with height over the tropical easterlies. Averaged around the earth, the southern boundary of the westerlies at sea level lies at about latitude 38°N in summer and 34°N in winter. But the local winds and the surface elevation vary with longi- tude, and at some places where the sea-level geo- strophic winds are easterly plateaus and mountains rise up as high as the 700- and 500-mb levels into FIG. 3. Time-latitude diagram of the normal zonal the upper westerlies. Because of this, the average circulation at the 500 mb level. Isotachs are in meters latitude at which the surface zonal wind changes per second. from westerly to easterly is only about 30°N in both summer and winter (cf. Mintz and Dean, wind that starts at about the 600-mb level at about 1952, FIGS. 5 and 6). 33°N and slopes equatorward with height through The westerlies increase in magnitude with the center of maximum wind at the 180-mb level height. In summer, in all latitudes the westerlies at latitude 28°N. As a result, in the layer be- reach their maximum strength between the 300- tween 600 and 400 millibars there are two west- and 200-mb levels and then decrease in intensity wind maxima. with height. Above the 50-mb level the winds, in The seasonal change from a single mid-tropo- summer, are easterly over all of the northern hemi- sphere west-wind maximum in summer to the sphere. In winter the pattern is more complicated. double maxima in winter is further illustrated in South of latitude 65°N, the west winds increase FIGURE 3. This FIGURE shows the 500-mb normal in intensity to about the 200-mb level and then de- monthly geostrophic zonal circulation in a time- crease in intensity with height. But north of this latitude diagram (source of data, U.S.W.B., 1952).
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