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FEBRUARY1961 MONTHLY WEATHER REVIEW 45

DESCRIPTION OF THE 50-MB. PATTERNS OVER IN

N. J. ROPAR 1 AND T. I. GRAY, JR.2

U.S. Weather Bureau, Washlngton, D.C. [Manuscript recelved June 24, 1960;revlsed November 25, 19601

ABSTRACT Seasonal variations in the 50-nlb. circulation over the during 1058 are discussed and the features ob- served in 1957 are compared with those found in 1958. Both years exhibited similar patterns in the fall and winter but differed somewhat in spring and summer. In 1957, the polar vortex of winter began to weaken in early Oct,ober and by November 21 a closed anticyclone at 50 mb. encompassed the continent. In 1958, the breakdown of the polar vortexmas more gradual and bythe end of Xovember the 50-mb. circulation was charactcrizcd by an irregu- lar pattern of closed cyclones and anticyclones rather than a dominant anticyclone.

1. INTRODUCTION 100-nleterint'ervuls and thcisotherms for every 5' Celsius. The nlapsincluded in this paperare excerpts Although it is too soon to receive the full amount of fro111 the series for t,he whole year which will be checked chec,ked dat'a from the Antarctic stations participating in and completed as the fi11wl data bec,ome arailablr. the IGY program, it is felt t'hat a sufficient number of' re- ports are available from t'he radio broadcast's in 1958 to 2. SUMMER 1957-1958 permit a preliminary analysis of t'he 50-mb. level over the continent (fig. 1). The 50-mb. chart, for January 15, 1958 (fig. a), is The present work is more or less a continuation of the fairlyrepresentat'ive of thesummer pattern at. the be- series of daily 50-mb. charts prepared by W. B. Moreland ginning of this year althoughthe circulation hadbeen of the US. Weather Bureau during his tourof duty as chief predominant81y anticyclonicfrom the middle to the end of of theAntarctic Weather Central at Little Americain December 1957. The flat contour gradient and relatively 1957. A comparison of t8hecircu1at)ion pat'terns for the two uniformtemperatures over the entire continent per- years reveals some significantdifferences in the spring and sisted until early Mnrch 1958. early summer seasons. A more detailed discussion of these The Low centered ut about 80' S., 100' E. apparently differences will be attempted later in this paper. hadmoved into t'his region fromthe west,ern coast of The series of charts for 1958 was plotted and analyzed Queen MaudLand. It' remainedover EastAntarctica at' IO-day intervals with data forone day on either side of but' exhibited a variable intensity and occasional break- the 10th day added as an aid in analysis. These charts oft' of secondar~-cells unt'il the middle of March, when a were basedon 0000 GMT data,although 1200 CMT data single pronouncedvort'ex and st,rong cyclonic circula- were used as an aid inanalysis. Extrapolations, by thick- tion developed over the whole continent. ness techniques, from levels between 100 mb. and 50 mb. The ridges along the coastal arms fluctuated in shape were carriedout whenever practicable to obt'ain better tmd extent, sometimes formingseparut'e small high coverage of the 50-inb. heightsand temperat'ures. This cells overthe continent. There appeared to be some method of analysis is similar to t'hat employed by Austin general westward drift to these ridges during t'he summer. and Krawitz [l] in their Northern Hemisphere st'udy, and The chart for February 5, 1958 (fig. 31, depicts the vari- byTeweles andFinger [2] intheir higher strnt'ospheric ationsin the syste~ns discussed above.The Low over analyses. t8hc Ross Seaseems to have been a part of t8helarger The area covered by the analysis is, of necessity, con- syst'errl cent'eredover . Rather prorni- fined tothe Antarctic Continent8 and adjacentmaters, nentridges appeared over the coast near D'Urville and since 50-mb. dnta fromthe islands in the Southern over theEllswort~h Highland with the possibility of a Oceans arequite Ineager. The contours are drawn ut srnall closed Highin the lat't'er area at about 74' S., ____._ 85' W. It should be recognized, however, thattrying I Present affiliation: St. Louis University, St. Louis, Mo. to rnaint~aincontinuity of thesefact'ors onchart's 2 Present afElintioI1: International Antarctic Bnalysis Center, Central Burcau of hi(>- teorology, Melbourne, (?.I., Victorh, Australis. drawn for IO-day intervals may not be too satisfactory.

Unauthenticated | Downloaded 09/25/21 08:01 AM UTC 3. AUTUMN 1958 the coast of theentire continent andtheband of highest winds (40 to 50 kt.)extended from h3irny to flit,tle Thetransition from sumnleris first'observed on the America. The center of t'hc circulation at about 85' S., chartfor hfarch 15, 1958 (fig. 4). The development of 40' W. llad :L tendency to drift about) the geographicpole this single prominent vort'ex appeared beto rather grdual. ina general west. to eastdirection but was mostoften The first area to indicate R strengthening of the cyclonic locatedbetween theSoutll Pole iLnd t'heWeddell Sew. circulation was the coastal region fromD'Urville to and betwwen the Yout'h Pole and the Pole of Relative Little America withwinds of 30 to 40 kt. reported at 1n;lccessibilit.y (82' S., 59' E.) during the year. stations in this part of t'he continent in t'he period March Thc large-scale features which developed by the middle 14-16. of April were retained without much change throughout By April 15 (fig. 5), the circulation was stronger over the fall .

Unauthenticated | Downloaded 09/25/21 08:01 AM UTC FEBRUARY1961 MONTHLY WEATHER REVIEW 47

FIGURE2.-50-mt) chart, January 11-16,1958. FIGURE4."50-mb. chart, AIarch 14-16, 1958.

FIGURE3.-50-mb. chart, February 4-6, 1958. FIGITRE5.-50-mb. chart, April 14-16, 1958.

Unauthenticated | Downloaded 09/25/21 08:01 AM UTC 48 MOKTHLY WEATHER R'EVIEW FEBRUARY1981

FIGURE6.-50-mb. chart, July 14-16, 1058. FIGURE8.-50 mb. chart, October 14-16, 1958.

FIGURE7.-50-mb. chart, August 14-16, 1958. FIGURE0.-50-mb. chart,October 24-26, 1958. fact that the boundary of the polar zone at 50 n!b. seasons indicate that four, and occcnsionally five, major was about 68' to 69' S. Unfortunately, no wind re- troughs were more common. ports for this level are available at comparable latit'udcs By t'hemiddle of July the temperatures over the in- on thePalmer Peninsula to check the validity of this terior were between -85O C. and - 90' C., while the tern- hypothesis. prratures over the coastal areas ranged from -81' C. at The three-wave pattern shown on this chart seems to the Belgian station, on the At'lantic side of the cont'inent, be arepresentative feature of themaps for July- and to -65' C. at,D'Urville, on the Pacific side.Again, it August 1958, whereas themaps for the spring and fall might be noted that while the isotherms did not exactly

Unauthenticated | Downloaded 09/25/21 08:01 AM UTC FIGURE10.-50-mb. chart, November 4-6, 1958. FIGURE12.-50-mb. chart, November 27, 1958.

parallel the contours t,here was only LL slight cross-contour 16th. on the 14th hIcllurdo reported a wind of 145 kt. orientation and, therefore,very little advective temper- at 50 mb. ature change in the winter months. Although the band of strong winds over t'he coasts of Thechart, for August 15, 1958 (fig. 7), shows rather the continent varied in intensityand lateral ext'ent during well-defined jet,s on opposite sides of the continent with thewinter months, the winds over t'he coast of Wilkes winds of 50 t,o 60 kt,. over t,he coast of Queen Land Imd and Vict'oria Land were rarely reported as less than andwinds over 100 kt. (at 67 mb.) at D'Urville on the 25 to 30 kt. at 50 mb.

Unauthenticated | Downloaded 09/25/21 08:01 AM UTC 5. SPRING 1958 It is in the spring season that the most, radical changes take place inthe Antarctic stratospheric circulation. The rapid warming at very high levels in 1958, described by Hanson [3],was also not'iceable at 50 mb. and helped to produce the event'ual breakdown of the winter cyclone. The rate of warming at 50 mb. appcars to have been about the same at bot'h the coastal and interior stat,iorls inearly October and this act'ed tomaintain the band of highwinds encircling thecontinent from the coast of to and the shown on the chart for October 15, 1958 (fig. 8). Winds of 80 kt. were reported at the Xorwegianst'ation, over 100 kt. at hlirny and D'Urville, and 100 kt. at Hallett, in the period October 14-16, 1958. It mightbe noted in passing that t'hc double center configuration of t'he polar vort'ex (fig. 8) was not a common occurrencealthough a similar pattern was evident on October 5, and once in each of May, June, and August'. On October 25, 1958(hg. 9),the center of thepolar vortex had shifted t'o about 75' S., 50' E. and a ridge had FIGURE14.-50-mb. chart, July 15,1957. developed over the western Ross Sea and inland to near theSouth Pole. Theband of highwinds had moved inlandover Wilkes Land and the . The two features weakened gradually and appeared to move apparentwarm air advection and subsidence accom- panying this change in circulation caused a rise in tem, aroundthe cont'inent from west to east until a rather irregular pattern of isolatcd Lows and ridges was estab- peratureto -37' C. at Wilkesand to "35' C. at lished byDecember 5, 1958 (fig. 13).This t'ype ol McXIurdo ascontrasted with temperatures of -60' C. circulationwas characterist'ic ol theremainder of to -65' C. onthe ot'her side of thecont'inent. The Dc- temperature atthe South Pole rose from -68' C. to cember 1958. -53' C. in this 10-day period with a 6' increase taking 6. COMPARISON OF THE 50-MB. PATTERNSIN place from the 24th to the 25th asa result of the change in 1957 AND 1958 circulation. Therelatively colder air associated wit'h t'he polar Thcrc is quite a bit of similarity between thc 50-mb. vortex spread eastward and apparently acted to inhibit chart's of thefall and winter seasons 01 1957 and 1958. t,he warm air advection and help produce a breakdown Thepolar vortex ovcr thc contincnt developed rather of thedeveloping ridge. The modification of the t'em- gradually and mas well established by t'hc end of April peraturegradient allowed a circumpolar flow to be of each year. A comparison of the charts for July ol both re-established. The chart for November 5, 1958 (fig. lo), years (fig. 6 and fig. 14) shows very lit'tle difference in the shows the center of the Low near 84' S., 40' E. with a character of t'hewinter circulation. It should be noted circulationsimilar totllc winter pattern. The temper- that differences in detail, such as the positions of troughs atures had decreased somewhat over the coast of Wilires and ridges, occur on most of the charts for the two years. Landand remained about the same over the Polcand It' is in the spring and sunmer that a pronounced dif- coast of Queen Maud Land. The jet over Wilkcs liand ferencein the 50-1nb. patternsis observed. In thefirst and t,he Ross Sea persisted until the 25th(fig. 11) whcn the part of October 1957, a ridge developed over t'lle coast of pattern began t,o weaken and a ridge formed over East Wilkes lmld ant1 themajor low center was displaced Ant'arctica. towtrdthe Palmer Pcninsula. The nlove~nent ol these A series of daily chart's was analyzed from Xovember systems was accolnpanied by a shilt8of the jet :tcross the 27 to December 1 todetermine il a radicalchange in SouthPole with a11 orientationhorn about, 20' E. to circulat,ion occurred which might not have bccn detected 160' FIT.long. By Xovember 17, 1957 (fig. 15), the ridge int,he usual 10-day map interval. This series showed was nlorefirnlly established over EastAntarct'ica, and that the ridge over East Antarctica continued to develop on November 21, 1957 (fig. 16), :t closed anticyclone was and t,here was a possibilit'y of a closed High about 80' S., ccnt'eredabout 78' S., 110" E. witheasterly winds ob- 125' E., while the Low was displaced to a posit'ion just served at all coastal stations. This pattern was substan- south of the Palmer Peninsula wit'h a trough extending tidy retained until January 1958. over Queen lllaud Land toward Mawson (fig. 12). These In 1958 there was no major change in the winter type

Unauthenticated | Downloaded 09/25/21 08:01 AM UTC FIGURE16.-50-n1b. chart, November 21, 1957.

circulationuntil 0ct)ober 25, 1958 (fig. I)), when a ridge developed over the Ross Sea area and t'he jetwas diverted across Wilkes T~nddirectly toward . How- 1957 ever, in this case, the low center was displaced to about 75" S.,50" E. as the nlajor ridge built over the Ross Sea. The colder air associated with the Low apparently sprend to the east inhibiting thc continucd development of the ridge. The consequent, nlodificntion of thetemperature gradient over Wilkes Land and the Ross Sea area helped to produce a more uniform circultrtion centered near 84' S., 40" E. (fig. 10). The permanent breakdown of the pronounced circum- polar flow apparently was well-established by Novcmbcr 27 (fig. 12) with the build-up of a high cell over Wilkes Land at about 80" S.,125 "E., and the movement' of the low center to just sout'h of t,he PalmerPeninsula. By FIGURE17.-50-mb. temperatures (" C.) at 10-dayintervals, 1957 and 1958, at hfirny (solid line), Sout,h Pole (dashed-dotted line), December 5, 1958 (fig. 13), as wtrs noted before, the cir- and IIallett (dot,ted line). culation pattern became rather unorganized and remained this way throughout the month. The significant difference between the spring and sum- Hcrc again the t'wo years were rather similar in the rela- mer patterns in the two years was that: (1) at no time tivcly gradual decrease of t,emperature beginning in the in thespring and sunlmer of 1958 did an anticyclone fall and cont,inuing to a minimum reached in late July to dominatethe circulation over the entire continent as mid-August. As was observed in the circulat.ion pat,terns, happenedin 1957, and (2) t,l:e find breakdown of the a significant differencc appeared in t'he t'enlpcrntures for strong polar vortex took place almost a month lat,cr in the spring and early summcr seasons. In 1957, all three 19.58 and at a more gradual rate than was observedin stationsexhibited a sharprise of t'emperature from the 1957. first of Octobcr wit'h the maximum reached at' about As a point of interestJin the comparison of the two October 25 andat the South Poleand Hallet't about years, a graph of the 50-1111). temperatures at thrce stations November 5. Fromthis point) t'he temperature began (South Pole, Mirny, and Hallet t) was prepared (fig. 17). idling slowly. Inspring and early summer 1958 the These t'emperatures, for both years, were plotted at 10- t,emperaturc rose rapidlyin Oct'ober at' theSouth Pole day intervals onthe, same basis as t,he maps for 1958. ant1 Hallctt but by about Oct'ober 25 the increase had

Unauthenticated | Downloaded 09/25/21 08:01 AM UTC 52 MONTHLY WEATHER REVIEW FERRUARY1961 stabilized and the warming was gradual until the maximuminpreparing the final version of thepaper. The 1958 wasreached in mid-November atHallett and early series of charts was plotted by Mr. C. F. Fopay. December at the South Pole. The temperature at Mirny increasedrelatively gradually during this period to a REFERENCES maximum in early December. 1. J. M. Austin and L. Krawitz, ‘L50-Millibar Patterns and Their Relationship to TroposphericChanges,” Journal of Meteor- ACKNOWLEDGMENTS ology, vol. 13, No. 2, Apr. 1956, pp. 152-159. The authors are grateful to Mr.W. B. Moreland for his 2. S. Tewelesand F. G. Finger, “An AbruptChange in Strato- permission to use his analyses as a part of this paper and spheric Circulation Beginning in Mid-January 1958,” Monthly Weather Review, vol. 86, No. 1, Jan. 1958, pp. 23-28. for his helpful suggestions during discussion of this work. 3. K. J. Hanson,A Case Study of theExplosive Stratospheric Thanks are also due Mr. M.J. Rubin for his interest and WarmingOver the Antarctic, October 1958, (Submittedfor aid in this project, and to Mr. W. S. Weyant for his help Publication), 1958.

New Weather Bureau Publication

The CooperativeU7eather Observer, 1J.S. Weat’herBureau, Washington, D.C., 1960, 116 pp.For sale by Superintentlent of Docurncnts, U.S. Government Printing Office, price 65 cents. A booklet recognizing and honoring the voluntary services of the coopera- tive weather observers over the Unit’ed States. It contains an explanation of early weather observational progranls in the United States and the part played bythe volunteer observers. Biographical sketches of observerswith long service records are included and many examples of devotion to duty are cited. Institutions and organizations with long records of observations are also recog- nized.

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