Monthly Weather Review June1960

DEPARTMENT OF COMMERCE WEATHER BUREAU FREDERICK H. MUELLER, Secretary F. W.REICHELDERFER, Chief MONTHLYWEATHER REVIEW

JAMESE. CASKEY, JR., Editor

Volume 88 JUNE 1960 Closed August 15, 1960 Number 6 Issued September 15, 1960

THE SEASONAL VARIATION OF THE STRENGTH OF THE SOUTHERN CIRCUMPOLAR VORTEX W. SCHWERDTFEGER

Department of Meteorology, Naval Hydrographic Service, Argentina [Manuscript received May 24, 1960; revised June 20, 19601

ABSTRACT

The annual marchof the strengthof the southerncircumpolar. vortexis shown to be composed of a simple annual variation(with the maximumoccurring in late winter)which dominates in thestratosphere, and a semiannual variation with the maximum at the equinoxes, which is the dominating part in the troposphere. This behavior of the circumpolarvortex is considered to be the consequence of the seasonal variation of radiation conditions and of the different efficiency of meridional turbulent exchange in the troposphere and stratosphere. It is suggested that the semiannual variation of the tropospheric vortex is an essential feature of a planetary circulation. The annual march of pressure with opposite phase values at polar and middle latitudes, can be understood as a consequence of the formation and decay of the great circumpolar vortex.

1. INTRODUCTION part of the period which can now be considered. The Some aerological data of the Southern Hemisphere and South Pole station has been taken as representative of the particularly those from the South Pole (Amundsen-Scott inner part of the SCPV, in spiteof the fact that inseveral Station) obtained during theIGY and its extension through months the center of the vortex cannot be found exactly 1959, make it possible for the first time to arrive at an es- over the Pole. This does not essentially affect the con- timate of the seasonalvariation in the strength of the clusions to be reached, asthe mean pressure gradients southerncircumpolar vortex (in the following abbrevi- (or gradients of height of isobaric surfaces) between 90’s. ated: SCPV) and suggest explanations of the variatiorx. and the center of the vortex remain always small in com- As a measure of the “strength” of the SCPV consider parison to the gradients between the inner polar zone and the geopotential height differences, along isobaricsurfaces, the middle latitudes. between the mean values of three longitudinally almost 2. RESULTS evenly spaced stations atabout 50’ S. (Invercargill, Marion Island, and Port Stanley), and the values at the Figure 1 shows the annual march of the height differ- SouthPole station. This measure seems tobe the best ences for 5 levels from 700 to 100 mb.; the monthlyvalues at hand as long as reliable and complete upper-air charts of a period of 3 years, April 1957 to March 1960, have for theSouthern Hemisphere arenot available. The been used. Table 1 and figure 2 give corresponding values mean monthly topographies of the isobaric surfaces over of theamplitude and phase of their first and second Antarcticapublished in two preliminary IGY Reports harmonic components, and the sum of the amplitudes of (Alvarez and Rastorguev [2] ; Alt, Astapenko, and Ropar the higher harmonics. As the data for TDO mb. were not [l])do not extend far enough toward the middle latitudes available for Marion Island, the analysis for the 200-mb. to give adequate measures; moreover, they cover only a level has been carried out with and without that station, 203

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TABLE1.-Harmonic analysis of theannual march of thestrength I600 of theSouthern Polar Vortex: Amplitude (r, in gpm.) andphase (4) of the first and second harmonic,and sum of theresiduals. April- 19,57 to March 1960. (a) H (Invercargill, Marion, Port Stanley) - H (Pole) (b) 3 (Invercargill, Port Stanley) - H (Pole) (c)AH (Invercargill, Marion, Port Stanley; 300-700 mb.) -AH (Pole; 300-700 mb.)

1200 Mean rl 91 Day max. rl

”- ~__ (a) 700 297 10 2600 25.VI 58 500 30 523263 n.v1 83 30 47 760267 18.VI 105 200 841 , 139 223 2.VIlI 119 815 128 219128 815 6.VIII 22 256 (b) % I 937 I 518 I 216 I 9.VIII I 244 I 28 I 1;; (e) 300-700 I 463 I 39 I 267 I 18.VI 300 I 48

in order to show that there is no significant difference if Marion Island is included or not. The values in section (c) of table 1 refer tothe meridional differences of the 100 thickness of the 700- to 300-mb. layer which are equivalent to the mean meridional tropospheric (virtual) temperature gradient between 50° and 90’s.; for the layer considered, a variation in thicknessof 25 gpm. corresponds to a variation of mean virtual temperature of 1’ C. The essential result is that the semiannual component is dominant in the troposphere, with the maximaappearing Dr. Jam Fob. Mush April biq J- Id, Aw. Sept ost No”. I: in the equinoctial months, (that is when the mean meridional FIGURE1.-The annualmarch of theheight differences (Inver- temperature gradient is greatest), and the simple annual cargill plus Marion plus Port Stanley) - H(S. Pole) for 700, 500, component is dominant in the stratosphere. (For a signifi- 300 and 200 mb., and R (Invercargill plus Port Stanley) - H(S. cance test of the semiannualperiodicity, it is better to Pole) for 100 mb. The location of the three subpolar stations is realize aharmonic analysis of the original series of36 marked by solid circles in figure 4. terms and to apply the method described by Brooks and Carruthers [3], (par. 18, 2), comparing the largest of the 18 possible harmonic components (that is, here the semiannual) with the “expectation”-value on the zero- hypothesis. This was done for the 300-mb. height differences (a=115 gpm.) and for the 300-700-mb. thickness differences (a=57 gpm.). It results that the ampli- mb mb tude of the semiannualharmonic component of both 10C series exceeds the expectation calculated at the 1 percent level.) As a matter of fact, much more evidence as to the first part of the above italicized statement has been published in recent years (Schwerdtfeger and Prohaska 118, 2oc 191, Loewe [Ill) and is implicit in other publications (Mintz and Munk [13], Hoffmeyr [7], Schumacher [171),

30( but for the present discussion it may be sufficient to refer to figures 1 and 2 and table 1. There may be no doubt at all about the second part of the above statement.

50 3. POSSIBLEEXPLANATION

7c The possible cause of this behavior of the SCPV can be tentatively assessed from the following considcrat’ions: In the troposphere, the meridional exchange (“Gro-%&US- FIGURE2.-Amplitudes and phases(days max.) of the2rst and tausch”) between middle and polar latitudes isvery secondharmonic component of theannual march of H (Inver- cargill plus Marion plus Port Stanley) - H (Pole), April 1957 to strong; the small annual range of tempzrature, about the March 1960. same at middle latitudes and polar zone of the Sout,hern

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Hemisphere, and especially the peculiar flat temperature curve of Antarctic stations during winter, is mainly due to the vigorous transport of subpolar air masses southward and the correspondingmovement of polarair masses northward. The importance of this macro-turbulent proc- ess, which ismainly a consequence of the meridional temperaturegradient, was recently stressed by Wexler [22, 231. This meridional temperaturegradient showsa characteristicannual march which, as willbe discussed later, appears to be related to the seasonal variation of the radiative parts of the heat budget of different latitudinal be,lts, tending to produce larger mean temperature (or thickness) contrasts between middleand polar latitudes during the equinoctial months than around the solstic,es. I I I 1 I I 0. There is a pronounced semiannual variation of the differ- W I~lII.bIur.llp.Iy.rIhr~hh~~~I*pr~ga'*o..~~ ences of daily totals of incoming radiation between middle FIGURE3.-Annual variation of 10" latitude differences of daily (or subpolar) and polar latitudes, as is shown by means totals of incoming radiation betm-een 30" and 90" S. The values of the theoretical values (Milankovitch [12]) in figure 3. are derived from table 1 of the work of Milankovitch [12], with Of course, a semiannual variation in the heat budget of Z=2 ly./min. different latitudinal belts can be attributed to the effects of incomingsolar radiation only if theother radiative (rz) is severaltimes greater thanthat which wouldbe termsdetermining the heat budget of thetropo-pheric needed, in thehypothetical case of absence of macro- air masses, particularly albedo and water vapor content, turbulent meridionalexchange, toaccount forthe ob- do not themselves show a pronounced semi-annual varia- served value of r2 in the annual march of the meridional tion. Thishappens to be so, as faras canbe deduced thickness differences (table 1, section c). Therefore, it from the scarce observations upto now available; besides, isstrongly suggested thatthe semiannualvariation of it may be important to mention that in the annual march the meridionaldifferences of absorbedradiation can of the meridional differences of surface temperatures, the bring about the seasonal change of the difference between amplitude of the first harmonic exceeds by far that of the the meantemperatures (or thicknesses) of middle and second harmonic. polar latitudes, the maxima occurring in the equinoctial In order to examine whether the meridional differences months. of those fractions of extraterrestrial radiation which are In the stratosphere, the conditions are different. The absorbedin the troposphere are of theright order of density of the air and the lapse ratebeing small (for more magnitude to be taken as a possible cause of the observed detailedreasoning see Rubin [15], Wexler (22, 231, and meridional temperature differences, the following assump- Palmer [14]) the compensating macro-turbulent meridional tions are made: that the layer between 300 and 700 mb. exchange of air, and therewith of heat, can not be suffi- absorbs about (a) 15 percent of the extraterrestrial radia- ciently efficient. Even if the difference between incoming tion at 5O0S., and 10 percent at 80° S., and (b) 20 percent solar radiation at middle and polar latitudes is greater in and 15 percent,respectively (these are onlyestimates, March and September than in May through August, the but are in reasonable agreement with Fritz 141, Liljequist dominating feature is that the cooling of the polar strato- [lo] andHanson [SI). Table 2 gives the differences of spherecontinues through midwinter. Therefore, the the amounts of absorbed radiation in ly./month between SCPV in the stratosphere forms again when the incoming latitudes 50' and 80° S. solar radiation at the highest latitudes diminishes notice- The essential result is that the amplitude of the second ablyand the differentialheating begins to favorthe harmonic of the annual march of the "differential heat- middle latitudes,and remainsuntil the springtime ing" exceeds that of the first harmonic, and that itsvalue heating of the polar stratosphere becomes stronger than

TABLE2.-Meridional diflerences (5Oo-8O0 S.) of estimated values of radiation absorbed in the 300-700-mb.layer, ly./month. (a) and (b): DiJerenf absorption conditions assumed, as specijied in the text.

Jan. Feb. Mar. MayApr. June July Aug. Sept. Oct. Nov. Dec.

1510 1960 2190 1790 1110 1790 2190 1960 1510 780 Bpo2010 148) 2090 1560 1430 ly./mo. if?) 1490 2250 28201980 12202360 1060 1480 26801760 2570 1290 Harmonic analysis: (a) r1=320 ly./mo.; rz=530 ly./mo.; +1=79'. +t=2€Q0=max. at Mar. and Sept. 20 (b) rl=160 ly./mO.; re=810 ly./mo.; +1=79"1 +e=258°=max. at Mar. and Sept. 21

Unauthenticated | Downloaded 09/24/21 10:49 PM UTC 206 MONTHLY WEATHER REVIEW JUNE 1960 that of the surrounding latitudinal belt. It can be esti- the mass of theatmosphere), is nicely reflected inthe mated thatthat occurs duringthe month of October, mean annual variation of pressure at the surface. In 1955 morelikely in the second half of it, because thedaily and 1956, Schwerdtfeger and Prohaska[18,19] published an totals of incoming radiation are slightly less at the Pole analysis of the annual march of pressure for the world, untilthe second half of November but, on theother tryingto include for the first time the whole Southern hand, the ozone concentration is higher in the polar zone Hemisphere. It was shown that there exists a very pro- than in middle latitudes.From October to December, nounced semiannual oscillation over theextratropical when the annual and thesemiannual variation of the SCPV southernlatitudes, with oppositephase values in polar are running in phase, the vortex breaksdown in the upper (maxima at the solstices) and middle (maxima at the layers and goes towards itsminimum strength in the equinoxes) latitudes. This result, based on relatively short troposphere. series of data south of 50’ S., was supported by the data Thus, combining the considerations for troposphere and from an entirely independent study of Gordon [5], who stratosphere, it appears that the simple annual variation considered the monthly change of mass of different lati- in the strength of the SCPV should increase with height tudinal belts of the atmospherebetween the North pole and muchmore thanthe semiannual variation, as the et& 50° S. The correspondingvalues south of 50’ S. have ciency of meridional exchange diminishes with increasing been computed by Schwerdtfeger ([21] table 2). The height andthe summertime heating, because of the concept of a well marked semiannual pressure oscillation effects of ozone, is restricted to the upper levels. That in polar regions is now also confirmed by the somewhat is essentially what the data in table 1 confirm. The rela- longer series of observations available at the end of the tively small imrease with height of the second harmonic IGY. Schwerdtfeger and Prohaska [18, 191 suggested that component in the stratosphere can be attributed to the the semiannual component in the yearly marchof pressure lesser effects of the meridional differences in available over the extratropical part of the Southern Hemisphere is energy onthe difference between theheat budgets of directlyrelated tothe correspondingvariation in the middle andpolar latitudes, and partially, also, tothe strength of the circumpolar westerlies, and some evidence asymmetry in time and increasing annual range of the for this was brought forward by the authors and byLoewe yearly temperature curve of the polar stratosphere with [11]. It can now be said more explicitly that the annual height. march of surface pressure over polar and middle southern For completeness, theabove qualitative statements latitudes should be considered the consequence (the should besupported by numericalestimates of the de- “demonstration at the surface”) of the annual march of cisive terms.This is, of course, beyond theaim of the the strength of the SCPV. It does not seem necessary to present note. reproduce here the already published results of the harmonic analysis of the Southern Hemisphere pressurefield. 4. CONSEQUENCES However, it is interesting to point to the mean change of If the behavior of theSCPV is understood asthe pressure at the surface, related to the “breakdown” of the combined resultmainly of radiation processes and SCPV (from October to December, fig. 4). macro-turbulentmeridional exchange, some important (3) The semiannual variation of the mass of the polar consequences can be tentatively outlined: atmosphere is of such an order of magnitude (the ampli- (1) For the formation of the semi-annual variation of tude Gf the second harmonic amounts to about3 per mille the SCPV, which is dominant in the troposphere and still of the mass south of 60’ S., the maxima occurring at the quite noticeable in the lower stratosphere, the existence of solstices) that itseffect on the angularvelocity of the earth, the Antarctic continent is not an essential point (and, of i.e.,the length of day,must be noticeable. It appears course, neither is that of theother land masses of the that the calculations of Mintz and Munk [13] can thus be Southern Hemisphere). Generally speaking, and referring refined. * only to thesecond harmonic of the annual march, thesame (4) Finally, it should be remarked that the old notion of shouldhappen if theAntarctic did not exist. Therefore, classic climatology, not objected to until 1955 (Schwerdt- the semiannual wriationin the strength of the SCPV may be feger and Prohaska [18]), that the semiannual oscillation considered as anessential feature of a planetary circulation. of surface pressure may have its main origin in the equa- As far as the authorknows, this point was never mentioned torial zone, cannot be maintainedin the lightof the results in any work on the general atmospheric circulation, but from the Southern Hemisphere. it could have some importance for theoretical and perhaps Additional remark, referring to the occurrence of a semi- .also for experimental studies.* annualvariation in the strength of the tropospheric (2) The slow buildup and fast breakdown of the SCPV, circumpolar westerlies at high northern latitudes: In the with the semiannual variation the dominant characteristic case of the Northern Hemisphere, it can not be expected in thetroposphere (that is, in the layerof the main partof that the zones of equal heat budget be latitudinal belts and that the meridional temperature gradients at higher *Scherhag’s [16], contention, that “all essential features of a planetary circulation can be seen from the mean annual topography of the loo0 mb. surface [sic!] of the soutliern hemisphere,” can hardly be accepted. *This problem will be considered in a separate paper.

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-3

.-2

13 =13 mb. MEAN OF 3 YEARS (3) FIGURE4.-Mean pressurechange (mb.) from October to December, Southern Hemisphere south of40' S. Hatching shows zone of negative values; stippling, zone of values>lO mb. For all stations with a record less than 20 years, the number of years used is put in parenthesis.

TABLE3.-Latidudinal means: (a)u-component of the geostrophic wind at 300 mb., average of 65", 70°, and 75' N., 1950-57, computed from Lahey, Bryson, et al. [9];(b) 500-1000-mb. thickness dijerences, 6O0-8O0N., (20 gpm. =lo C.),1900-39, computed from Jacobs [8].

M ar. Apr. May June July Aug. Sept.Jan.Aug. July June Feb.May Apr. Mar. Oct. Nov. Dec.

(a) 6.86.2 6.67.6 8.0 6.47.6 7.0 8.8 9.0 7.9 7.1!m./SeC. 124 1.79 196 200 168 143 168 200 196 1.79(b) 124 146169 155 l.w 135 gpm.

Harmonic analysis (a) rl=0.7 m./sec. max. at October 15; rz=0.9 m./sec., max. at Mar. and Sept. 28. (b) rl=ll gpm., &ax. at May 18; rz=30 gpm.,max. at Apr.and Oct. 6.

Unauthenticated | Downloaded 09/24/21 10:49 PM UTC MONTHLY WEATHER REVIEW JUNE1960 latitudes show the equinoctial maxima at all longitudes. 7. W. L. Hofmeyr, “Upper Air over the Antarctic,” Meteorology Nevertheless, even there the effect of stronger meridional of the Antarctic, Weather. Bureau, Pretoria, 1957, Chapt. 10, temperaturedifferences around the equinoxes is evident pp. 173-208. 8. I. Jacobs, “Monatsmittel der Absoluten und relativen uber der in the mean values (table 3). Considering the pronounced Nordhemisphare und ihre monatlichen hderungen, Folge 2,” regional contrasts in surface conditions of this latitudinal Meteorologische Abhandlungen des Institutfur Meteorologie belt, the fact that r2>rl may be interpreted as supporting undGeophysik der Freien Universitat Berlin, vol. IV, No. the notion that the semiannual variation of the meridional 2, 1958. differences of absorbed short-wave radiationis at least one 9. J. F. Lahey, et al., Atlas of SO0 mb. Wind Characteristics for the Northern Hemisphere, and SummaryData Tabulation of SO0 of the causes of the semiannual variation of the strength mb. Flow Characteristics for the NorthernHemisphere, De- of the circumpolar vortices. partment of Meteorology, University of Wisconsin, 1960. 10. G. H. Liljequist, “Energy Exchange of an Antarctic Snowfield, ACKNOWLEDGMENTS Short Wave Radiation,” Norwegian-British-Swedish Antarctic Expedition, 19@-1956, Scientific Results, vol. 2, Part I, sect. The author is grateful for the generous help and support A, Norsk Polarinstituut, Oslo, 1956. 11. F. Loewe, “Comments on, ‘The SemiannualPressure Oscilla- of the Carnegie Institution of Washington which has made tion, Its Cause and Effects’,” Journal of Meteorology, vol. 14, it possible to visit the Institute of Geophysics, University No. 2, Apr. 1957, pp. 187-188. of California at Los Angeles, andthe US.Weather Bureau, 12. M. Milankovitch,“Mathematische Klimalehre, in Koeppen- PolarMeteorological Research Unitat Washington,to Geiger,’’ Handbuch der Klimatologie, vol. 1, Part A, Gebr. work on IGY interdisciplinaryresearch. The present Borntrager, Berlin, 1930, p. 176. 13. Y. Mintz and W. H. Munk, “The Effect of Winds and Bodily study was carried out at U.C.L.A. and completed at the Tides onthe Annual Variation in the Lengthof Day,” Monthly U.S.W.B. The author is indebted to Dr. C. E. Palmer, Notices of the RoyalAstronomical Society, Geophysical Sup- R. Taylor, and Dr. S. Venkateswaran at Los Angeles, and plement, vol. 6, No. 9, Apr. 1954, pp. 566-578. to Dr. H. Wexler, M. J. Rubin, and K. J. Hanson, and the 14. C. E. Palmer,“The Stratospheric Polar Vortex in Winter,” anonymous reviewers of the Monthly Weather Rewiew at Journal of Geophysical Research, vol. 64, No. 7, July 1959, pp. 749-764. Washington,for the interest theyhave shownand the 15. M. J. Rubin, “SeasonalVariations of theAntarctic Tropo- helpful criticisms they have offered. pause,” Journal of Meteorology, vol. 10, No. 2, Apr. 1953, pp. 127-134. REFERENCES 16. R. Scherhag, “Probleme der allgemeinen Zirkulation,” Geophy- sics, vol. 6, No. 3/4, 1958 pp. 539-557. 1. J. Alt, P. Astapenko, and N. J. Ropar, “Some Aspects of the 17. N. J. Schumacher, “Aerology”, Norwegian-British-Swedish Antarctic Atmospheric Circulation in 1958,” IGY Report No. Antarctic Expedition 1949-1952, Scientific Results, vol. 1, Part 4, National Academy of Sciences, World DataCenter A, 1, 1956. Washington, D.C., 1959. 18. W. Schwerdtfeger and F. Prohaska, “AnBlisis de la marcha 2. J. A. Alvarez and V. I. Rastorguev, “Description of the Ant- anual de la presi6n y sus relaciones con la circulaci6n atmos- arcticCirculation from April to November 1957,” ZGY ferica enSudamerica austral y la Ant4rtida. METEOROS, Report No. 1, National Academy of Sciences, World Data vol. 5, No. 4, Oct./Dec. 1955, pp. 223-237. Center A, Washington, D.C., 1958. 19. W. Schwerdtfeger and F. Prohaska,“Der Jahresgang des 3. C. E. P. Brooks and N. Carruthers, Handbook of Statistical Luftdrucks auf der Erde und seine halbjahrige Komponente,” Methods in Meteorology; H. M. Stationary Office, London, Meteorologische Rundschau, vol. 9, 1956, NO. 314, pp. 33-43, 1953, 413 pp. (par. 18, 2). NO. 9/10, pp. 186-187. 4. S. Fritz, “Solar Radiant Energy and Its Modification By the 20. W. Schwerdtfeger and F. Prohaska, “The Semi-Annual Pressure Earthand Its Atmosphere,” Compendium of Meteorology, Oscillation, Its Cause and Effects,” Journal of Meteorology, American Meteoro:ogical Society,Boston, Mass., 1951, pp. vol. 13, No. 2, Apr. 1956, pp. 217-218. 13-33. 21. W. Schwerdtfeger, “Der siidliche Polarwirbel,” Meteorologische 5. A. H. Gordon, “SeasonalChanges in the Mean Pressure Dis- Rundschau, vol. 13, 1960 (in press). tribution Over the World and Some Inferencesabout the 22. H. Wexler, “The ‘Kernlose’ Winter in Antarctica,” Beophysica, General Circulation,” Bulletin of the American Meteorological ~01.’6, NO. 3/4, 1958, pp. 577-595. Society, vol. 34, No. 8, Oct. 1953, pp. 357-367. 23. H. Wexler, “Seasonal and Other Temperature Changes in the 6. K. J. Hanson, “Radiation Measurement on the Antarctic Snow- Antarctic Atmosphere,” QuarterlyJournal of the Royal field, a Preliminary Report,” Journal of Geophysical Research, Meteorological Society, vol. 85, No. 365, July 1959, pp. vol. 65, No. 3, Mar. 1960, pp. 935-946. 196-208.

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