VOL. 34, No. 8, , 1953 357

Seasonal Changes in the Mean Pressure Distribution Over the World and Some Inferences About the General Circulation f


Meteorological Office, Harrow, Ms., England *

1. INTRODUCTION Atlantic 1921-1938 Atlantic 1855-1899, 1921-1938 TMOSPHERIC surface pressure is a Indian 1855-1943 north of 30°S measure of the weight of a vertical column 1921-1943 south of 30°S of air of unit area cross-section extending A Eastern Pacific to 160°E 1921-1938 to the outer limits of the 's atmosphere. A Western Pacific to J60°E 1921-1938 change in atmospheric surface pressure at a given place therefore represents a net mass transfer of All these observations were made by voluntary air per unit area to or from that place. The con- British ships. vergences and divergences which may occur at To obtain mean pressure values for 2° squares various levels within a column of air usually cancel over these , 2° grids were placed over the out to a large extent and it is only the relatively mean pressure charts in the atlases, and pressure small residual which appears as a change in surface values interpolated for each such square; pressure pressure. It is difficult to assess the particular values obtained in this way were meaned over level or levels at which convergence or divergence each 2° belt in each ocean. These means were used may occur; but it follows that if there is a change to construct isopleths in which the ordinates and in surface pressure, then there must be a net gain abscissae were and time (months) re- or loss of air within the column as a whole. A spectively, and the distribution of the mean belt study of the seasonal changes in the mean pressure of pressure was represented by contours. distribution over the earth's surface should, there- Subsequently the pressure values for each two- fore, give useful evidence of the seasonal net mass degree square were subtracted from the corre- transfer of air from one of the globe to an- sponding values for the following month and the other. The seasonal net mass transfer may be mean monthly tendencies so obtained plotted on partly an exchange between and ocean charts of the oceans. The marine data were sup- and partly an exchange between the two hemi- spheres. From the net mass transfer pattern over plemented over land by mean tendencies obtained the globe inferences may be drawn about the im- in the same way from pressure data in World portance of non-geostrophic flow as an inherent Weather Records (1920-1940). Mean isallobaric part of the mean general circulation. charts were then drawn up for the whole world, excluding the region south of 50° South latitude where data were scarce, for each two consecutive 2. DATA AND TECHNIQUES OF REPRESENTATION months of the . Finally the mean tendencies OF SEASONAL CHANGES OF MEAN PRESSURE DISTRIBUTION OVER THE OCEANS were converted into units of relative change of mass and integrated round the globe for, each ten The climatic charts of the oceans prepared in degrees of latitude, excluding the region south of the Meteorological Office include monthly charts 50° South latitude. The net changes in relative of the mean pressure distribution over the oceans. mass were plotted as a function of latitude for The isobars, which have been smoothed by eye, each month of the year and isopleths drawn. are based on mean pressure values computed for As a whole the periods covered by the different two-degree squares in the North Atlantic and five- sets of observations are fairly consistent, except degree squares elsewhere. The pressure observa- in the South and tions from which the charts have been compiled north of 30° S where observations for an earlier cover the following periods: period are also included. However, the inclusion t Published with permission of the Director, Meteoro- of such earlier observations is not expected to af- logical Office. fect seriously the broad pattern of the changes in * Present address: c/o Secretariat, World Meteorologi- cal Organization, Geneva, Switzerland. mean pressure from month to month.

Unauthenticated | Downloaded 10/04/21 01:25 AM UTC 358 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

FIG. 1. Mean latitudinal pressure pattern (Atlantic Ocean) in mb. with first two figs, omitted

FIG. 2. Mean latitudinal pressure pattern (Indian Ocean) in mb. with first two figs, omitted.

Unauthenticated | Downloaded 10/04/21 01:25 AM UTC VOL. 34, No. 8, OCTOBER, 1953 359

FIG. 3. Mean latitudinal pressure pattern (East ) in mb. with first two figs, omitted.

3. MEAN PRESSURE DISTRIBUTION OVER THE pressure is generally lowest from to OCEANS inclusive. The range of pressure is 5.2 mb. The westerly belt in the North Atlantic is charac- FIGURES 1-4 represent the mean pressure dis- terized by a minimum in and tribution throughout the year as a function of lati- and a maximum in May. The westerly belt at tude for the Atlantic, Indian, East and West Pa- 50° S is characterized by relatively low pressure cific Oceans respectively. The main features of from November to inclusive and by relatively the patterns are changes in mean pressure in the high pressure from to October inclusive but doldrum, subtropical and polar low these values are based on a much smaller number belts and northward and southward movements of of observations. The doldrum belt is characterized these belts with the . by a minimum in December and maximum in June, (a) Atlantic Ocean (52°S-60°N). In the July and . The range along the axis of North Atlantic there are two main seasonal maxima lowest pressure is 3.3 mb. and minima of pressure in the subtropical anticy- (b) Indian Ocean (48°S-20°N). North of the clonic belt; the maxima are in January and July the mean pressure distribution is domi- and the minima in and October. The range nated almost entirely by the effect of the continent of pressure along the axis of highest pressure is of . The maximum is in mid- and the 4.5 mb. In the corresponding belt of the South minimum in mid-. The range is 14.5 mb. Atlantic there is only one main maximum, in July South of the equator there is a maximum in July and August, and no well defined minimum although and August and a minimum in December in the

Unauthenticated | Downloaded 10/04/21 01:25 AM UTC 360 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

FIG. 4. Mean latitudinal pressure pattern (West Pacific Ocean) in mb. with first two figs, omitted. subtropical anticyclonic belt. The range here July and minimum in January. In the southern along the axis of highest pressure is 7.1 mb. In hemisphere in the subtropical belt there are maxima the belt of there is little evidence of a in April and ; the chief minimum oc- significant seasonal variation of mean pressure. curs in January and a secondary minimum in June; (c) East Pacific Ocean (57°S-53°N). In the the range is 4.6 mb. In the doldrums the maxi- northern hemisphere in the subtropical anticyclonic mum occurs in October and the minimum in Janu- belt there is a maximum in July and a minimum in ary; the range is 2.1 mb. October; these extremes are reflected in the polar The patterns in FIGURES 1-4 are common to all belt of westerlies where they occur in the same oceans in respect of certain broad features. Ignor- months. The range along the axis of highest pres- ing the partial effect of the Asian monsoon on the sure is 6.3 mb. In the the distribution in the West Pacific ocean, pressure is maximum in the subtropical anticyclonic belt oc- highest in the northern hemisphere anticyclonic curs in September and an ill defined minimum from belt in July and lowest in and March April to July. The range is 4.4 mb. There is no and again in October. In the corresponding belt significant variation of pressure in the belt of in the southern hemisphere pressure is highest westerlies. Pressure in the doldrum belt is fairly from July to September and lowest from Novem- uniform but a weak maximum occurs in August ber to April. In the northern hemisphere "west- and weak minima in April and May and in De- erly belt,'' pressure is highest from May to July cember. The range is 2.0 mb. and lowest from October to January. In the dol- (d) West Pacific Ocean (40°S-S0°N). In the drum belt it is highest from June to October and northern hemisphere in the subtropical anticyclonic lowest from December to May. The mean range belt there are maxima in July and April and min- of pressure along the axis of highest pressure is ima in February and June. The range is 3.1 mb. 4.6 mb. in the northern hemisphere and 5.3 mb. in In the belt of westerlies there is a maximum in the southern hemisphere. The mean range along

Unauthenticated | Downloaded 10/04/21 01:25 AM UTC VOL. 34, No. 8, OCTOBER, 1953 361

FIG. 7. Seasonal change of mass departure from normal (East Pacific Ocean).

degree squares along the belt, c/> is the mid-latitude FIG. 5. Seasonal change of mass departure from normal of the two-degree belt, and I is the length of a side (Atlantic Ocean). of a two-degree square at the equator. FIGURES 5-8 represent the seasonal changes of the axis of lowest pressure in the doldrum belt is mass as departures from the mean for the Atlantic, 2.5 mb. Indian, East Pacific and West Pacific Oceans re- Pressure distribution charts along the lines of spectively within the areas covered by FIGURES FIGURES 1-4 have been drawn by Lettau [10] for 1-4. three chosen meridians, , mid-At- FIGURES 5-8 show certain features common to lantic and East respectively, for the polar all oceans. In the northern hemisphere, excluding year 1932-33. This technique yields information the Indian Ocean, there is excess of air in or of the general trend of the changing pressure field summer and a general deficit in or winter. throughout the course of the year. In the southern hemisphere there is an excess reaching a maximum generally in August and Sep- 4. RELATIVE MASS OF AIR OVER THE OCEANS tember and a deficit in the summer months. The trend in the northern hemisphere appears to be We may convert pressure into units of mass. followed in the mean by a similar trend one month Thus M — p n cos /2, where M is the mass across a two-degree belt of latitude of the ocean, p is the later in the southern hemisphere. The seasonal mean pressure in millibars, n is the number of two-

FIG. 6. Seasonal change of mass departure from normal FIG. 8. Seasonal change of mass departure from normal (Indian Ocean). (West Pacific Ocean).

Unauthenticated | Downloaded 10/04/21 01:25 AM UTC 24 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

bo c rt JC

O £

Unauthenticated | Downloaded 10/04/21 01:25 AM UTC VOL. 34, No. 8, OCTOBER, 1953 363 changes in the relative mass of air shown in FIG- May-June.—The Asian low extends into the URES 5-8 suggest that the dominant feature is a whole of the and also into the West Pa- seasonal exchange of air between the northern cific Ocean while an extension from the Arctic hemisphere land masses and the oceans of both covers the North American continent. Relatively hemispheres. small highs cover the Atlantic Ocean west of France and the and also the Gulf of

5. DISTRIBUTION OF MEAN MONTHLY ISALLOBARS . In the southern hemisphere a high covers the South Atlantic Ocean, South and the Figures 9-10 show the distribution of the change Indian Ocean. in mean pressure from April-May and from Oc- June-July.—The Asian isallobaric low has be- tober-November by means of isallobars. These come less extensive. A second low now ex- two pairs of months are chosen for illustration as tends from the Arctic north of the Bering representing the highest rates of change. The across Northern , and outstanding features for each month throughout to the British Isles. Highs cover the central the year will, however, be discussed briefly in turn. North Atlantic and the whole of the North Pa- Dec ember-January.—An extensive isallobaric cific Oceans. In the southern hemisphere there low is centered over the Western Pacific in tem- is a belt of isallobaric high across the Atlantic and perate ; another low extends from Ice- Indian Oceans and a low to the west of . land to the . The summer isallobaric July-August.—An isallobaric high has replaced low is beginning to appear over Southern Asia. the low over the Asian continent. A low covers the Isallobaric highs cover extreme , Arctic and the Central North Atlantic and North Alaska and . In the southern Pacific Oceans. Highs cover the South Pacific hemisphere there is an isallobaric low in the In- and Indian Oceans. dian Ocean. August-September.—An intense isallobaric high January-February.—An isallobaric low covers covers the Eurasian land mass while lows cover the and Alaska and also extends east- the North Atlantic and North Pacific Oceans and wards from mid-Atlantic to the Pacific coast of the Barents Sea. Asia, chiefly between 20° and 50° latitude. Isal- September-October.—The intense isallobaric lobaric highs are centered over Northern Scandi- high is maintained over Asia. An extensive low navia and the Northwest Pacific Ocean. covers the North Atlantic Ocean from 30° to the February-March.—A deep isallobaric low cov- Arctic and an intense low is centered in the Gulf ers and extends southwards to the Indian of Alaska. In the southern hemisphere a belt of Ocean and westwards across Europe, North Af- low extends round the globe, mainly north of 40°. rica and the North Atlantic Ocean south of 50° to October-November (see FIG. 10).—The Asian the Pacific Coast of the North American continent. high extends north-eastwards across the Arctic to An intense isallobaric high covers the whole of the Alaska. A low extends from Greenland to the Arctic while a second high covers the whole of the British Isles and another covers the Western Pa- Pacific Ocean north of the . In the south- cific in temperate latitudes. A belt of low extends ern hemisphere a high covering southern around the globe in the southern hemisphere. appears to extend southwards. November-December.—A high remains over March-April.—The extensive isallobaric low Asia and an extension from it stretches across over the Eurasian continent is maintained while a and the North Atlantic Ocean south low of less extent covers that part of the East Pa- of 30°. An intense low is centered in the North cific Ocean bordering the United States and Cen- Atlantic Ocean south of Greenland and another tral America. An extensive high centered near low covers the North Pacific Ocean in temperate Iceland extends southwards over the North At- latitudes. A belt of low almost encircles the globe lantic while another high is centered over the West in the southern hemisphere. Pacific Ocean in temperate latitudes. Although no attempt will be made to explain in April-May (see FIG. 9).—An extension of the detail the patterns described interesting inferences Asian isallobaric low covers the Arctic and also may be drawn from them. The main feature that that part of the West Pacific Ocean bordering the becomes evident is the vast seasonal exchange of Asian coast. An extensive high covers Europe air from continent to ocean and from hemisphere and the North Atlantic Ocean. In the southern to hemisphere. In the northern hemisphere the hemisphere small highs cover the major land flow from the to the oceans begins in masses. February and continues through July while the

Unauthenticated | Downloaded 10/04/21 01:25 AM UTC BULLETIN AMERICAN METEOROLOGICAL SOCIETY

Unauthenticated | Downloaded 10/04/21 01:25 AM UTC VOL. 34, No. 8, OCTOBER, 1953 365

& aG >

bjo C3 a

Unauthenticated | Downloaded 10/04/21 01:25 AM UTC 366 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

FIG. 12. Convergence pattern showing zonal changes in mass from one month to the next X (1.25 X 1010) metric tons. reverse process begins in August and continues Becker [3] confirms that whilst the distribution through January. The exchange is most pro- of pressure over the continents of both hemispheres nounced at the when the rate of change shows a strong annual variation (high pressure in of mean temperature is probably a maximum. winter, low pressure in summer of the hemisphere The North American continent appears to play in question) the marine subtropical areas of high an insignificant part in the seasonal exchange of pressure have their strongest development in both air as compared with the Eurasian land mass. hemispheres in the northern summer. This fact FIGURE 11 shows the annual range of mean pres- is explained thus that the run off .of air masses sure. Maxima occur in the region of Iceland, the from the large continents of the northern hemi- Aleutians and over the central Asian land mass. sphere caused by the summerly decrease of pres-

Unauthenticated | Downloaded 10/04/21 01:25 AM UTC VOL. 34, No. 8, OCTOBER, 1953 367 sure does not only lead to an enlargement of the REFERENCES marine high pressure areas of the northern hemi- [1] Hohn, H.: Eine equatoriale Westwind Zone als sphere, but encroaches also on the southern hemi- Glied der allgemeinen Zirkulation. Zeit. Met., 3, 1949, pp. 240-246. sphere, causing there too an enlargement of the [2] Widger, W. K.: A study of the flow of angular marine subtropical areas of high pressure. momentum in the atmosphere. J. Met.. 6, 1949, pp. 291-299. [3] Becker, R.: Der planetarische Jahresgang der mari- 6. SEASONAL EXCHANGE OF AIR BETWEEN THE tim-subtropischen Hochdruckkerne. Ann. Met., 2, HEMISPHERES 1949, pp. 48-51 (English Abs. p. 48). [4] Stewart, H. J.: Periodic properties of the semi- The changes in mean pressure from one month permanent atmospheric pressure systems. Q. Appl. to the next for each ten-degree square were con- Math., 1, 1943, pp. 262-267. verted into units of relative mass and integrated [5] Glenn, A. H.: Circulation and convergence in the equatorial zone between 95 °E and 160°E, Decem- around the globe. If the total of mass so obtained ber-February. B. Amer. Met. S., 28, 1947, pp. 453- is plotted for each month of the year as a function 464. [6] Burkhart, K.: Eine statistische Untersiichung at- of latitude, isopleths may be drawn as in FIGURE mospharischer Zirkulations-schwankungen. Ann. 12. The pattern which appears shows net relative Hydr., 68, 1940, pp. 1-13 (Abs, B. Amer. Met. S., mass divergence and convergence and indicates 23, 1942, p. 316). the magnitude of the net exchange of air between [7] Ramanathan, K. R, and K. P. Ramakrishman: The general circulation of the atmosphere over India the hemispheres. The greatest net divergence and and its neighbourhood. Ind. Met. Mem, 26, Pt. X, the greatest net convergence occur in subtropical 1939, pp. 189-245. latitudes. In the northern hemisphere maximum [8] Reuter, F.: Die halbjahrige Luftzirkulation im NO- Passatgebiet des Nordatlantischen Ozeans uber net divergence occurs from April to May and Teneriffa. Ann. Hydr., 67, 1939, pp. 67-70. maximum net convergence from October to No- [9] Dalldorf, H.: Tropospharischer Meridionalaustausch vember ; both occur at 30° latitude. In the south- in den gemassigtem Breiten. Arch. deut. See- warte u. Marineobs., 57, Nr. 10, 1937, p. 21. ern hemisphere maximum net divergence occurs [10] Lettau, H.: Luftmassen- und Energieaustausch from October to November and maximum net zwischen niederen und hohen Breiten der Nordhalb- convergence from April to May; both occur at 20° kugel wahrend des Polarjahres 1932/33. Beit. Phys. fr. Aim., 23, 1935, pp. 45-84. latitude. The latitude at which the net annual [11] Jaw, J. J., S. Y. Tao and Y. S. Kao: Statistical stud- transfer of air is greatest is also 30° in the northern ies on the general circulation in . hemisphere and 20° in the southern hemisphere. /. Chinese Geoph. S., 1, 1949, pp. 140-158. If we confine the net mass divergence to a layer [12] Rossby, C.-G.: Air displacements and intensity changes of atmosphere vortices. /. Mar. Res., 7, from the surface to 1 km the net transfer which 1948, pp. 175-187. occurs during the months and over the 10° belt [13] Namias, J., and P. F. Clapp: Normal fields -of con- where divergence is greatest is equivalent to an vergence and divergence at 10,000 ft. level. /. Met., 3, 1946, pp. 14-22. unbalanced net meridional circulation of about 1 [14] Shaw, N.: Manual of Meteorology, Cambridge, vols. meter/sec. II and III, pp. 213, 413 resp.

Maud, 1918-25. Scientific Results edited by H. U. NEWS AND NOTES Sverdrup: Volume IA: Special reports, 279 pp. Volume IB: Magnetic, atmospheric, electric and aurora Maud Expedition Reports Still Available results, 215 pp. Volume II: Meteorology, discussion, 331 pp. In view of the increased interest in the Arctic it appears Volume III: Meteorology, tables, 527 pp. appropriate to draw attention to the valuable results from Volume IV: Oceanography, 421 pp. the Maud Expedition, because the reports of this expedi- Volume V: Biology, 268 pp. tion were published in such a manner that they are not as generally available as they deserve to be. About 150 complete sets can still be obtained from the The reports comprise five volumes under the general University Library, Bergen, Norway, at a price of $35.00 title: The Norivegian North Polar Expedition with the (Norw. kr. 250).—K. C. S.

Unauthenticated | Downloaded 10/04/21 01:25 AM UTC