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 hydrodynamical 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-

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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.

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THE NORTHERN HEMISPHERE WESTERLIES In the middle latitudes of the northern hemisphere the normal zonal circulation is westerly from the ground up. This band of westerlies broadens in latitudinal extent with height, extending 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 longitude, and at some places where the sea-level geostrophic 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 hemisphere 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). latitude there is a continuous increase of the west- The axes of maximum west wind are shown by erlies with height up to the highest levels for which the heavy lines. The middle latitude west-wind we have data. maximum in all months lies north of the latitude Within the westerly belt an axis may be drawn at which the surface zonal circulation is zero. But showing the location of the strongest wind at each the sub-tropical maximum, which begins in Oc- level. In summer this axis has a simple form, tober and ends in May, in all these months lies at sloping equatorward from its position at sea level or near the latitude of zero surface zonal circula- at latitude 52°N to the center of maximum wind tion. just below the 200-mb level at 42°N. This center In some individual winters the double structure of maximum west wind (sometimes called the of the west-wind maxima is especially well de- westerly "jet stream") in summer lies about 4° of veloped. A good example of extreme development latitude poleward of where the sea-level zonal was the month of January, 1949 (Mintz, 1951, wind is zero, but 12° of latitude poleward of the FIG. 3). In that particular month the middle-lati- average latitude at which the surface zonal wind tude maximum reached from sea level to above changes from westerly to tropical easterlies. the 200-mb level, and the sub-tropical maximum In winter the axis of maximum west wind is not reached down to below the 700-mb level. Also, in a single continuous one. From the sea-level maxi- that month the latitudinal spread between the two mum at latitude 52 °N the axis slopes equatorward maxima was 20° or more. with height up to only about the 400-mb level. With respect to the distribution of westerlies at And this axis of maximum west wind, like the upper levels over the pole, it should be added that summer one, at all elevations lies north of the just as the normal low-level cap of polar easterlies average latitude at which the surface zonal wind sometimes gives way to low-level westerlies, so changes from westerly to tropical easterlies. But, also the westerlies in the middle and upper tropo- in winter, there is a second axis of maximum west sphere and lower stratosphere are not a perma-

Unauthenticated | Downloaded 10/10/21 06:50 PM UTC 212 BULLETIN AMERICAN METEOROLOGICAL SOCIETY nent feature in the polar region. Sometimes a deep this limitation the assumption is now made that the anticyclone develops close to or over the pole, with average of the zonal winds in the longitudes of a zonal circulation that is easterly at all levels over Australia and New Zealand are representative of the pole, persisting on occasion for weeks at a time. the mean zonal wind all around the southern hemisphere. With this assumption, it appears that the THE TROPICAL AND EQUATORIAL EASTERLIES zonal circulation in the southern hemisphere is, to a rough approximation, a mirror image of the one In the tropical and equatorial latitudes the zonal in the northern hemisphere. In summer, the prin- circulation is easterly. At sea level the easterlies cipal difference is one of magnitude, the southern extend from 38°N to 28°S latitude at the time of hemisphere westerlies showing greater speeds at northern hemisphere summer, and from 34°N to all heights than the northern hemisphere wester- 34°S at the time of northern hemisphere winter. lies. A second difference, in summer, is that the Near the ground the easterlies show a double southern hemisphere westerlies in the middle and maximum straddling the equator, with the stronger upper troposphere and lower stratosphere do not of the two maxima on the winter side of the reach all the way to the pole. In winter, the prin- equator. In each hemisphere the maximum east- cipal difference is that what in the northern hemi- erly wind at sea level is not only stronger but also sphere is a middle latitude west-wind maximum, in closer to the equator in winter than in summer. the southern hemisphere is a sub-polar one and The seasonal change in intensity of the northern very strong. These differences may be traced to hemisphere easterly maximum is especially large. the different distributions of land and sea in the two Between the two maxima the sea-level easterlies hemispheres. In the northern hemisphere there show a pronounced minimum centered on the is a proportionately larger fraction of land in mid- summer side of the equator. In a kinematic sense, dle latitudes, resulting in warmer middle-latitude this minimum is due to the recurving of some of summer temperatures and hence a smaller pole- the streamlines of the winter hemisphere trade ward temperature gradient and weaker upper-level winds as they cross the equator into the summer winds in the sub-tropical region. The strong west- hemisphere (cf. Mintz and Dean, 1952, FIGS. 1 wind maximum that is found in winter at the and 2). southern boundary of the Eurasian continent is The equatorial and tropical easterly belt nar- rows with height up to about the 300-mb level. completely absent in summer (cf. Mintz and Dean, Above this level it broadens again as the easterlies 1952, FIGS. 29 and 30). In the southern hemi- extend poleward into the summer hemisphere. sphere, on the other hand, the influence of con- Above the level of 70 to 40 mb easterlies prevail tinentality on the poleward temperature gradient, over all of the summer hemisphere. and hence on the strength of the upper-level zonal The larger of the two easterly maxima, which winds, is confined to the Antarctic landmass and in the lower levels is on the winter side of the results in the summer disappearance of the sub- equator, at higher levels crosses over into the polar west-wind maximum. summer hemisphere. The scant data that were One of the effects of the difference in the sea- used for the high tropical atmosphere have been sonal change of zonal circulation in the two hemi- interpreted as indicating a mean "easterly jet" spheres, is that integrated from pole to pole, and above 200 mb, located on the summer side of the neglecting the uppermost five percent of the mass equator and with a speed maximum of about 10 of the atmosphere, there is an increase in the total meters per second. Recent high-level rawin ob- angular momentum of the whole atmosphere from servations from Nairobi (Davies and Sansom, the time of northern hemisphere summer to the 1952) and from Singapore and Hong Kong (Hay, time of northern hemisphere winter. Computa- 1953) support this interpretation. tions show that this annual change in the total angular momentum of the atmosphere accounts almost exactly for the July to January decrease THE SOUTHERN HEMISPHERE WESTERLIES in the speed of rotation of the solid earth (the Up to this point, the analysis of the normal increase in the length of the day) that is observed zonal circulation has been based on data fairly by the astronomers (Mintz and Munk, 1951, well distributed in all longitudes around the globe. 1954). This provides an independent check on But south of 20°S latitude, as indicated earlier, the reliability of the zonal circulation described in data is available with comparable detail only for this paper, at least to the extent that it satisfies a the Australian-New Zealand region. Because of necessary condition.

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THE ANTARCTIC EASTERLIES can properly ask the question." But upon the cor- rect formulation and solution of this dynamical Over the Antarctic continent aerological data is problem depends our ability to make rational physi- scanty. But there is enough information to tenta- cal predictions of the day to day and week to week tively delineate the pattern of the zonal wind dis- deviations of the zonal circulation from its normal tribution in a rough fashion (cf. Court, 1951). state and hence to obtain a physical basis for ex- In both winter and summer the sea-level pres- tended weather forecasts. The reader will find an sure, averaged with longitude, is at a minimum at excellent discussion of the complexity of this prob- about latitude 65 °S and poleward of this latitude lem, and profound philosophical insight into the the observed sea-level winds are in fact easterly. pathways that may lead to its solution, in an ar- But in winter the poleward temperature gradient ticle by Professor V. Starr published in the Com- of middle latitudes is believed to continue all the pendium of Meteorology (Starr, 1951). way to the pole, so that the extrapolated subter- ranean sea-level high is a "cold high" and changes REFERENCES with height, over the pole, into a "cold low" aloft. As a reasonable guess, the complete reversal of Aerology, Flight Section, Chief of Naval Operations, 1948: "Aerological Observations and Summaries the poleward pressure gradient has been placed at for the Antarctic from 15 December 1946 to 15 about the two-kilometer elevation, with the higher March 1947." NAVAER 50-1R-214. Washington, central regions of the Antarctic landmass pene- D. C. Brooks, C. E. P., Durst, C. S., Curruthers, N., Dewar, D., trating into the upper polar low. The easterly and Sawyer, J. S., 1950: "Upper winds over the winds are therefore confined, in winter, to a world." Met. Office, Geophysical Memoirs, No. 85. peripheral ring on the flanks of the continent. In London. Court, A., 1951: "Antarctic atmospheric circulation." the interior of the continent the surface winds and Compendium of Meteorology, pp. 917-41. Boston. upper air winds are westerly. Davies, D. A., and Sansom, H. W., 1952: "Easterly jets In summer, on the other hand, the normal pole- over East Africa." Weather, Vol. 7, pp. 343-4. Dzerdzeevski, B. L., 1946: "On the distribution of at- ward temperature gradient is reversed over the mospheric pressure over the central regions of the Antarctic continent. The polar high, in this sea- Arctic." Meteorologia i Gidrologia, No. 1, pp. 33- son, is a "warm high." Consequently, in summer 38. (0 raspredelenii atmosfernogo davlenia nad tsentralnoi Arktikoi.) there must normally be a deep anticyclonic circula- Ekhart, E., 1941: "Die Stromung der Luft iiber Afrika tion and easterly zonal circulation at all levels over und den angrenzenden Gebieten; Ein Beitrag zur the central polar region. Kenntnis der allgemeinen Zirkulation." Wiss. Ber. Reichsamt fur Wetterdienst, Reihe A., Nr. 10. Berlin. THE PHYSICAL BASIS OF THE ZONAL Gibbs, W. J., 1952: "Notes on the mean jet-stream over Australia." Journal of Meteorology, Vol. 9, pp. CIRCULATION 279-284. Grimminger, G., 1941: "Meteorological results of the The further accumulation of aerological data in Byrd Antarctic Expeditions 1928-30, 1933-35: the years to come will undoubtedly necessitate ad- Summaries of data." M. W. Rev. Supplement No. 42. Washington, D. C. justments or changes in some of the details of Hay, R. F. M., 1953 "High-level strong easterlies over the zonal circulation described in this paper. It is Singapore and Hong Kong." Weather, Vol. VIII, unlikely, however, that revisions will be necessary No. 7, pp. 206-08. Hutchings, J. W., 1950: "A meridional cross section for in any of the major features. Especially does it an oceanic region." Journal of Meteorology, Vol. seem established that in the equatorial and tropi- 7, pp. 94-100. cal latitudes the normal zonal circulation is east- Jenkins, C. F., 1952: "A survey of available information erly and in the middle latitudes it is westerly. on winds above 30,000 feet." Air Force Surveys in Geophysics, No. 24. Air Force Cambridge Re- This is the mean state of motion that the atmos- search Center. Cambridge, Mass. pheric fluid, gravitationally held to a rotating earth McDonald, W. F., editor, 1938: Atlas of Climatic Charts that is differentially heated by the sun, has taken of the Oceans. U. S. Weather Bureau. Washing- ton, D. C. for itself. It is Nature's own solution to a compli- Mintz, Y., 1951: "The geostrophic poleward flux of cated thermodynamical-hydrodynamical problem. angular momentum in the month of January 1949." On the other hand, the dynamics or physical Tellus, Vol. 3, pp. 195-200. basis of the normal zonal circulation is so complex, Mintz, Y., and Dean, G., 1952: "The obseved mean field of motion of the atmosphere." Geophysical Re- so many processes act and interact, that meteorolo- search Papers, No. 17. Air Force Cambridge Re- gists have not yet agreed as to how the problem search Center. Cambridge, Mass. should be formulated, let alone solved. It is an in- Mintz, Y., and Munk, W., 1951: "The effect of winds and tides on the length of the day." Tellus, Vol. 3, stance in which "we know the answer before we pp. 117-121.

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Mintz, Y., and Munk, W., 1954: "The effect of winds U. S. Weather Bureau, 1944: Normal Weather Maps, and bod;ly tides on the annual variation in the Northern Hemisphere Upper Level. Washington, length of day." Month. Not. Roy. Astr. Soc., D. C. (10, 13, 16, and 19 kilometers). Geophys. Suppl. (in press). U. S. Weather Bureau, 1952: "Normal weather charts for Palmer, C. E., 1951: "On high-level cyclones originating the Northern Hemisphere." Technical Paper No. in the tropics." Transactions, Amer. Geophys. 21. Washington, D. C. (Sea-level, 700, and 500 millibars.) Union, Vol. 32, pp. 683-96. Venkiteshwaran, S. P., 1950: "Winds at 10 kms. and Starr, V. P., 1951: "The physical basis for the general above over India and its neighborhood." Memoirs circulation." Compendium of Meteorology, pp. India Meteorological Dept., Vol. 28, part 2. po. 541-50. Boston. 55-120.

the gradient. To some extent this can be done by close CORRESPONDENCE attention to the pressure change but otherwise development must be anticipated.

Thermal Gradient Forecasts: Suggestions for a Coordinated Program to Study Differential Advection as a "Pre- 1. Geostrophic advection based on prognostic pressure charts. dictor" of Precipitation and other 2. Regions where the ageostrophic component is strong Meteorological Features such as between a cold front and the following high. 3. Modifications introduced by stability. C. S. GILMAN For the first approximation the thermal gradient does U. S. Weather Bureau, Washington, D. C. not seem to be as important as the wind, and studies of the wind and pressure fields should expediently precede For years many practicing forecasters have felt that close attention to the thermal gradient field. However, advection is closely related to precipitation and pressure as refinement becomes desirable, it will be necessary to development. Closely allied, of course, is the effect of extend the concepts of deformation and frontogenesis to differential heating. Works in the last few years by include the non-linear cases involved with the trajectories Bleeker and Andre, Austin, M. F. Harris and G. A. in actual situations. Lott has supplied support for that feeling. Certainly the importance of temperature contrast is emphasized by the Attempt to apply life-history concept to advection areas most fundamental and universally accepted laws of mete- and other synoptic features: orology. It seems to the writer that the time is ripe to make a coordinated attack on this field of ignorance. Confluence, low-level jets. The following suggestions are made as to what such a Some case studies might concentrate on a differential- program should consist of: advection area as an entity, its formation, development, weakening, and disappearance. Simultaneous Relations: Also the vertical structure and weather that accom- panies it. Case studies of rainfall situations. Studies of advection situations emphasizing life history Mathematical study of the dynamics involved in differ- of advection areas. ential advection: Experience in forecasting areas of vertical motion by the trajectory method has shown that their movements The study would, no doubt, have to be numerical rather are sometimes erratic but can frequently be anticipated than analytical at the present stage of development. The if care is taken in analysis and extrapolation of the wind writer sees little possibility for use of machine methods field. No doubt other important features related to wind until at least several models have been carried through field development have not even been touched upon. by hand.

Prognostic Relations: Methods for coordination: Pressure forecasts—(1) Based on present Analysis Panel discussions—especially involving interchange of Center charts. Attempt to improve this first approxima- ideas between practicing meteorologists and researchers. tion by concentration on maximum gradients. (2) At- Committee for coordination with a permanent secre- tempt to develop a much better 12-hour prognostic by tary. close attention to isallobaric areas in connection with im- The attention of meeting-program chairmen is invited minent changes in advection and instability fields. It is to the possibility of these panel discussions. The writer believed that some improvement would be almost auto- believes that either the Society or some other society matic if concentration was placed on predicting the maxi- could well establish such a committee and he would be mum pressure gradient as such rather than on forecasting willing to serve as a "clearing house" for suggestions the pressure at different places and then subtracting to get from all interested in the project.

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