ARCHIV Arch. Met. Geoph. Biokl., Ser. B, 27,265-279 (1979) Ft]R METEOROLOGIE

GEOPHYSIK UND BIOKLIMATOLOGIE �9 by Springer-Verlag 1979

551.515.13(*)

Department of Geography, The University of Adelaide, South Australia

A Synoptic Climatology of Satellite-Observed Extratropical Cyclone Activity for the Southern Hemisphere Winter

A. M. Carleton

With 9 Figures

Received April 11, 1979

Summary

Twice-daily infra-red satellite imagery for the winters (June to September) of 1973-77 are used to investigate cyclonic activity over the Southern Hemisphere. The analysis uses a previously developed satellite-observed cloud vortex classification scheme and "mean" patterns are derived for thb different signature types. It is found that cyclogenesis is pronounced in both lower-middle and high latitudes in winter and conforms generally to the locations of the major hemispheric long wave troughs. Cyclolysis is observed to be most significant in certain sectors of the Antarctic Ocean and to exhibit little inter- annual variation in spatial patterns for the period studied, i High frequencies of lower- latitude "cut-off" cyclones occur in the Australian-New Zealand region during winter. The results of this analysis are compared with previous non-satellite and satellite-derived studies for the Southern Hemisphere,

Zusammenfassung

Eine synopfische Klimatologie extratropischer Zyklonent~ifigkeit w[ihrend des siidhemi- sph~irischen Winters auf Grund von Satellitendaten

Zweimal t~glich aufgenommene Satellitenbilder im Infrarotbereich wurden fiir die Winter- monate Juni bis September 1973-77 ausgewertet, um die Zyklonenaktivit~t in der Siid- hemisph~ire zu untersuchen. Die Analyse bentitzte ein schon frtiher entwickeltes Klassi- fikationsschema yon Wolken-Vortices, die yon Satellitenh~hen aus beobachtet werden k6nnen. Mittlere Verteilungen der verschiedenen Klassifikationstypen wurden entwickelt. Es wurde festgestellt, dat~ Zyklogenese sowohl in niederen bis mitfleren Breiten, als auch in hohen Breiten wLhrend des Winters wirksam ist und im allgemeinen der Lage hemi- sph~irischer Langwellentr~ge entspricht. Zyklolyse ist besonders bedeutend in gewissen Sektoren des antarktischen Ozeans und zeigt w~hrend der untersuchten Zeitperiode lediglich geringe interannuale Ver~ndeflichkeit in ihrer r~iumlichen Verteilung. ,,Cut-off"- Zyldonen niederer Breiten treten mit grot~er H~ufigkeit in der Region yon Australien

18 Arch. Met. Geoph. Biokl. B. Bd. 27, H. 4

0066-6424/79 /0027/0265/$ 03.00

266 A.M. Carleton

und Neuseeland w~rend des Winters auf. Die Ergebnisse dieser Analyse werden mit denen friiherer Studien in der Stidhernisph~ire verglichen, die mit und ohne Hilfe yon $atellitendaten gcmacht worden waren.

1. Introduction

Extensive data-void ocean areas in the Southern Hemisphere have, until recently, hampered climatological research into the development and move- ment of the major synoptic-scale features on a hemispheric basis (see for example [2, 6, 18, 19, 20, 21, 22, 27, 29]). This situation has improved in recent years with the use of satellite data (for example [7, 8, 11, 13]). Troup and S treten [23] and Streten and Kellas [16] developed a cloud vortex classification system for use with once-daily ESSA visible channel hemi- spheric imagery, from which a synoptic climatology of Southern Hemi- sphere cloud vortices was derived for the summer and transition seasons [ 17]. The present paper seeks to complement the work of Streten and Troup by establishing a climatology of satellite-observed cloud vortices for the winter months (June-September) from an analysis of infra-red imagery for the five winters 1973-77.

2. Data and Analysis

The hemispheric imagery utilised in the study was recorded by the ITOS (Improved TIROS Operational Satellite) Scanning Radiometer (SR), mounted on the NOAA series of sun-synchronous near-polar orbiting satel- lites [3 ]. The infra-red mosaics are composites of 12 I/2 north/south image swathes recorded twice-daily at 09 local time (L) and 21L. Particularly in winter, a better spatial and temporal coverage can be obtained with these data than is possible from visible imagery. Anderson et al. [ 1] note that vortices and frontal cloud bands appear geometrically similar in both visible and infra-red image types. Use can, therefore, be made of the Troup and Streten [23] and Streten and Kellas [ 16] vortex cloud classification in ana- lysing the infra-red data. Where possible the 09L visible mosaic was used as a check in interpreting the infra-red mosaics. The Streten-type system, presented in Fig. 1, is based upon the identification of characteristic vortex cloud signatures which are related to the evolution of extratropical depressions. Cyclogenesis may occur either as a development on a pre-existing frontal cloud band or in the absence of such a band (Fig. 1). Other workers had recognised the existence of these two modes of satellite- observed extratropical cyclogenesis (for example, [1, 12, 31]) while Petterssen and Smebye [ 10] discussed the dynamics of each type.

Extratropical Cyclone Activity for the Southern Hemisphere Winter 267

SOUTHERN HEMISPHERE STRETEN-TYPE PETTERSSEN & SMEBYE

SATELLITE OBSERVED CLASSIFICATION (1971) FORMATION TYPES VORTICES

(after Streten & Troup and Streten

& Kellas)

DESCRIPTIVE

COMX~ENTS

Wave on a frontal band, either marked by change in orientation and/or bulging of the band.

A

c

e~,.~ DY1 t Lt ,J

Dy 2

'Inverted corona' cyclone in isolation, without a clear slot.

Developing vortex with slo~ of clear air; either in isolation or on frontal band.

Vortex at maximum dev- elopment with spiral of clear air around centre.

Dissipating vortex with heavy vortical cloud; no spiralling. Cloud band not asymmetric~

Dissipating vortex with little central clo~; cloud band not asymmetric

Dissipating vortex with heavy cloud and marked asymmetry of frontal band; normally hook- shaped.

Dissipating vortex with little central cloud and marked asymmetry of frontal band.

Fig. 1. The Streten-type classification scheme for frontal cloud vortices appearing on Southern Hemisphere satellite imagery

18"

268 A.M. Carleton

Analysis of the hemispheric mosaics occurred in several stages. First, the interpreted twice-daily cyclonic vortex data were plotted onto 1 : 150,000,000 polar stereographic base maps. Second, monthly analyses were then derived by replotting all vortices appearing on the daily charts using a system of coloured crosses to indicate both vortex type and location. Points of initial cyclogenesis were circled. For each of the five winters, a series of four vortex maps was then constructed using the categories of cyclonic development defined by Streten and Troup [17]. These are: Early development stages (W, A, B types), maximum development stage (C-type), dissipating stages (D-types), frontless, "cut-off" cyclones (F/G-types), A "normalised frequency distribution" [9] of vortices (X) on polar stereo- graphic projections, can be obtained following Taljaard's [ 19] method which was based on that of van Loon [28]:

X = N (4 cos 45/r/cos ~),

where N is the number of vortices in each 5 ~ latitude by 10 ~ longitude "tessera" [28], ~7 is the number of months of data (four per winter) and

is the mid-latitude between each 5 ~ parallel. This procedure normalises the area of a 5 ~ latitude by 10 ~ longitude tessera to that at 45~ which is 438,000 km 2 . Isopleths of vortex frequency are then drawn for each of the four development categories listed above. Isopleth maps of the "mean" (five-winter) vortex distributions have been prepared for each category (Figs. 2, 6, 7, 9). In addition, Fig. 3 shows the two types of extratropical cyclogenesis (Streten's "W" and "A" modes) as the percentage departure from the five-winter hemispheric average for each type at 10 ~ longitude intervals. The mean winter frequencies of all vortex stages are expressed in 5 ~ latitude bands as the percentage of the hemi- spheric total for that type and are given in Fig. 4. The major features shown by these climatological analyses are discussed below and compared with the earlier results.

3. Results and Discussion

3.1 The Early Development (W, A, B) Vortices

Fig. 2 shows the mean distribution of the early development (W, A, B) vortex types. The major hemispheric long wave troughs as definded by Lamb [6] are closely associated with the "climatic frontal zones" [20], which from satellite elevations are evident as semi-permanent bands of cloud along which most cyclogenesis takes place [ 14, 15 ]. Thus the basic trough pattern is evident in Fig. 2 and compares closely with that found by Streten and Troup [ 17] for summer and the intermediate seasons. Major troughs extend from the South Atlantic Ocean into the Gran Chaco region of South America,

Extratropical Cyclone Activity for the Southern Hemisphere Winter

~ o r

269

Fig. 2. Mean distribution of the "early development" (W, A, B) vortices for the five winters 1973-77. Isopleth values refer to normalised vortex frequencies in each 5 ~ latitude by 10 ~ longitude tessera

through the Indian Ocean into the Gran Chaco region of South America, through the Indian Ocean from south-west of Australia, and in the Eastern Pacific linking up with the high latitude cyclogenetic region over southern Chile. A subsidiary zone of cyclogenesis is evident through the Tasman Sea and appears to be stronger in winter than at other times of the year (cf. [ 17]). This feature was also recognised in the non-satellite study of Karelsky [4] and Taljaard's [ 18, 21 ] analysis of cyclonic activity during the IGY. A further zone of early development vortex activity occurs south of Africa (Fig. 2). There is also a region of strong high latitude cyclogenesis in the south-west Pacific to the south-east of New Zealand. Thus the basic hemispheric three-

270 A.M. Carleton

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Longitude * E Longitude "W

Fig. 3. Initial points of wave (W) and "inverted comma" (A) cyclogenesis for all latitudes south of 20~ Vortex frequencies are presented as the percentage departure in each 10 ~ longitude band from the mean (five-winter) hemispheric average for each cyclogenetic type

wave pattern described for the other seasons by Streten [14] and Streten and Troup [ 17 ] also seems to characterise winter. The longitudes of maximum frequency for initially-observed points of wave (W) and "inverted comma" (A) cyclogenesis given in Fig. 3 confirm the areal pattern just described in Fig. 2. Thus cyclogenesis is strong in the Indian Ocean, in the longitudes of eastern Australia and the Tasman Sea, and in the South Atlantic near eastern South America. The multiple bands of cyclogenesis associated with the Pacific Ocean "trough'" and described by Streten and Troup [17] are also evident in winter (Fig. 2), while that south of Africa appears to be more variable in its longitudinal shuation. Inverted comma (A) cyclogenesis generally coincides with wave (W) cyctogenesis in all longitudes except over South America. This disparity is probably due to the fact that most "A" vortex developments occur over the oceans [17]. Here strong cold air advection to the west of the major frontal zones pro- duces the extensive cumulus fields within which most " A " cyclogenesis is observed.

Extratropical Cyclone Activity for the Southern Hemisphere Winter 271

20-

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~ 50- , ~ -

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Fig. 4. Mean (five-winter) distribution of vortex types as a function of latitude. Frequencies in each 5 ~ latitude band are presented as the percentage of the total for that type over all latitudes. Figures in brackets give the total numbers of cyclones for the five winters studied

Fig. 4 gives the mean latitudinal frequency of the W, A and "late formative" (B) vortices for the whole hemisphere. The distribution of the "W" cyclones (Fig. 4) shows a maximum frequency between 35 ~176 This is comparable with Streten and Troup's [ 17] distribution for the transition seasons which shows a strong concentration of these vortices in lower-middle latitudes, but contrasts with their summer distribution which exhibits no strong lati- tudinally dependent maximum. No real evidence is found for the zone of strong wave cyclogenesis at higher-middle latitudes described by Streten and Troup [ 17] for spring and autumn. Inverted comma (A) cyclones (Fig. 4)

272 A.M. Cafleton

Fig. 5. The "range" of "early development" ON, A, B) vortices. Isopleths represent the difference between the lowest and the highest normalised values in each 5 ~ latitude by 10 ~ longitude tessera over the five winters 1973-77

cover a broader range of latitudes, but tend to predominate at higher lati- tudes as was also found by Streten and Troup [17]. Thus these vortices occur most frequently between about 45~176 in winter. High frequencies of the B-stage (Fig. 4) also cover a broad latitudinal range as cyclones form- ing in lower latitudes move south-eastward. The greatest frequencies are from about 45~176 which compares with Streten and Troup's [17] summer distribution of this vortex type. There is considerable geographical and interannual variability from this mean pattern. Fig. 5 gives the range of frequency of the early development (W, A, B) vortices over the five winters. Strong variability is evident for cyclogenesis

Extratropical Cyclone Activity for the Southern Hemisphere Winter

qno~-

273

, , , ~ v o

Fig. 6. Mean distribution of the "mature" (C) vortices for the five winters 1973-77. Isopleth values refer to normalised vortex frequencies in each 5 ~ latitude by 10 ~ longitude tessera

in the Indian Ocean to the south-west of Australia, and for the regions of higher latitude cyclonic developments near eastern Antarctica and over southern South America. The least variation in these winters occurs in connection with the trough over South America and that through the Pacific.

3.2 The Mature (C) Vortices

The five-year mean distribution of the "maximum development" (C) stage vortices is given in Fig. 6. The general hemispheric wave pattern is still evident, with regions of maximum frequency located south of Australia,

274 A.M. Carleton

Fig. 7. Mean distribution of the "dissipating" (D) vortices for the five winters I973-77. Isopteth values refer to normalised vortex frequencies in each 5 ~ latitude by 10 ~ longitude tessera. Note the absence of data over most of Antarctica due to the lack of contrast between the ice-covered surface and the cloud signatures

south-west o f Africa, south-west of Sou th America , in the South At lan t ic Ocean and through the Tasman Sea to the south-west of New Zealand. The la t i tudinal frequencies o f these vort ices are given in Fig. 4 which shows m ax imum concent ra t ions be tween abou t la t i tudes 55 ~ 1 7 6 al though high values occur as far nor th as 35~ This d is t r ibut ion compares with that found by St re ten and Troup [ 171 for the o ther seasons. A region of high f requency is also loca ted nor th o f the Ross Sea (160~ - 170~ in winter and results mainly f rom higher la t i tude cyclogenesis (Fig. 6).

Extratropical Cyclone Activity for the Southern Hemisphere Winter

Q N o r

275

Fig. 8. The "range" of "dissipating" (D) vortices. Isopleths represent the difference between the lowest and the highest normalised values in each 5 ~ latitude by 10 ~ longitude tessera over the five winters 1973-77

3. 3 The Dissipating (D) Vortices

Figs. 7 and 4 give the mean distribution of the dissipating (D) cyclone stages and show that the highest frequencies surround Antarctica at about 55 ~176 The major region for cyclolysis is centred near the coastline of eastern Antarctica from about 100~ 70~ and apparently results from the con- fluence of cyclones originating in both the Atlantic and Indian Oceans, as shown by Astapenko [2] for the IGY. Other centres of cyclolysis are found in the Ross Sea (160~176 the Bellinghausen Sea (100~176 and the Weddell Sea (60~176 With the exception of the Weddell Sea maximum,

276 A.M. Carleton

Fig. 9. Mean distribution of the frontless "cut-offf' (F/G) vortices for the five winters. Isopleth values refer to normalised vortex frequencies in each 5 ~ latitude by 10 ~ longitude tessera

these regions of cyclonic decay were also found by Streten and Troup [ 17]. It appears that the Weddell Sea is more important for vortex dissipation in winter than in the other seasons (cf. [25]). Fig. 7 also shows some more minor centres o f D-stage activity in lower latitudes, principally that west of Australia, resulting from cyclones developing near Madagascar, and from south of Tasmania to the south-west of New Zealand. The range of fre- quency of the D vortices over the five winters is shown in Fig. 8. The high latitude cyclolyt ic regions just described show a tendency to persistence and with little shift in location from one winter to another. There is, however, strong variability in intensity o f the Ross, Bellingshausen, Wedde!l,

Extratropical Cyclone Activity for the Southern Hemisphere Winter 277

and western Antarctic cyclolytic centres, but that near eastern Ant- arctica exhibits least interannual variation. The middle latitude region of vortex dissipation in the Tasman Sea clearly fluctuates quite markedly in intensity (Fig. 8).

3.4. The Frontless "Cut-Off" (F/G) Vortices

The areal distribution of the quasi-stationary, "cut-off" (F/G) cyclones is shown in Fig. 9 and latitudinal frequencies are given in Fig. 4. These vortices predominate between about 30~176 in the Australian-Tasman Sea- New Zealand-Pacific regions and are associated with frequent meridional blocking situations (see, for example, [5, 6, 19, 21, 22, 24]) which apparent- ly result from the marked thermal effect of the Australian continent on winter tropospheric temperatures in this sector (for example, [26, 27, 29, 30]). By comparison, relatively few F/G vortices are found for similar latitudes in the remainder of the hemisphere, and Streten and Troup,s [17] study did not show such high concentrations in the other seasons. Fewer high latitude systems of this type are observed in winter compared with the other seasons [17] and this is probably due to the generally expanded, if not actually stronger, zonal wind circulation at this time [27, 29]. Thus F/G vortices are concentrated near Enderby Land (50~176 apparently a semi- permanent preferential location for these types [17], south-west of Cape Horn, north of Dronning Maud Land (0 ~ ~ and near eastern Antarctica at 120~176 (Fig. 9).

4. Concluding Remarks

The winter climatology of satellite-observed cloud vortices for the years 1973-77 tends to confirm the major features of cyclonic activity described by Taljaard [19, 20] for the IGY. While there are also broad similarities between these patterns and those found by Streten and Troup [ 17] for the summer and intermediate seasons, some strong differences are also observed.

The analysis indicates: 1) High frequencies of cyclogenesis at both lower-middle and high latitudes; the former predominantly of the wave (W) type and the latter mainly of the inverted comma (A) variety. Both patterns of cyclogenesis tend to corre- spond with the locations of the major hemispheric long wave troughs de- scribed by Streten[ 14] and Streten and Troup [ 17]. Cyclogenesis through the Tasman Sea is evidently strongest in winter. 2) Increasing cyclonic development generally occurs with increasing latitude in the vicinity of the major frontal axes of the hemisphere. 3) Cyclolvsis in winter reaches a maximum in Antarctic coastal waters with

278 A.M. Carleton

highest values south o f Australia, in the Ross, Bellingshausen and Weddell Seas and to the south o f Africa. The Weddell Sea maximum is insignificant at other times o f the year. 4) Winter is a favoured season in the Australia-New Zealand-Pacific regions for low-latitude "cut-off"-cyclonic developments (F/G vortices), contrasting strongly with the observed patterns for summer and the transition seasons by Streten and Troup [17]. Quasi-stationary frontless cyclones at high l~ttitudes are rarer in winter which probably reflects a more vigorous zonal circulation in these latitudes at this time.

Acknowledgements

I would like to thank Mr. Neff Streten (ANMRC, Melbourne), Dr. Peter J. Lamb (University of Adelaide), Professor Roger G. Barry (INSTAAR, University of Colorado, Boulder) and Mr. Harry van Loon (NCAR, Boulder) for their encouragement and in- valuable assistance and comments.

References

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3. Environmental Data Service: Environmental Satellite Imagery. Key to Meteoro- logical Records Documentation No. 5.4. Publ. by NTIS, Virginia, U. S. A. (1973-77).

4. Karelsky, S.: Monthly and Seasonal Anticyclonicity and Cyclonicity in the Australian Region - 15 Years (1946-60) Averages. Meterological Study 13, Bureau of Meteorology, Australia (1961).

5. Kerr, I. S.: Some Features of Upper Level Depressions. Technical Note No. 106, New Zealand Meteorological Service (1953).

6. Lamb, H. H.: The Southern Westerlies: a Preliminary Survey; Main Characteristics and Apparent Associates. Quart. J. R. Met. Soc. 85, 1-23 (1959).

7. Lamond, M. H., et al.: GARP Basic Data Set Analysis Project: the Second Experi- ment - June 1970. Austral. Met. Mag. 20, 193-262 (1972).

8. Martin, D. W.: Satellite Studies of Cyclonic Development Over the Southern Ocean. Technical Report No. 9 International Antarctic Meteorological Research Centre. Bureau of Meteorology, Melbourne (1968).

9. Neal, A. B.: MSL Cyclones and Anticyclones in November 1969 and June 1970. In [6]. Austral. Met. Mag. 20, 217-230 (1972).

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12. Rutherford, G. T.: Occlusion Sequences South of Australia. Proceedings of the Inter-Regional Seminar on the Interpretation of Meterological Satellite Data. Publ. on behalf of WMO by Bureau of Meteorology, Melbourne (1969).

Extratropical Cyclone Activity for the Southern Hemisphere Winter 279

13. Streten, N. A.: Some Aspects of High Latitude Southern Hemisphere Summer Circulation as Viewed by ESSA 3. J. Appl. Met. 7, 324-332 (1968).

14. Streten, N. A.: Some Characteristics of Satellite-Observed Bands of Persistent Cloudiness Over the Southern Hemisphere. Mon. Weath. Rev. 101,486-495 (1973).

15. Streten, N. A.: Some Aspects of the Satellite Observation of Frontal Cloud Over the Southern Hemisphere; in Conference Handbook of the Two Day Workshop on Fronts, Royal Met. Soc. (Australian Branch), Melbourne 3.2-3.9. (1977).

16. Streten, N. A., Kellas, W. R.: Aspects of Cloud Pattern Signatures of Depressions in Maturity and Decay. J. Appl. Met. 12, 23-27 (1973).

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18. Taljaard, J. J.: Cyclogenesis, Cyclones and Anticyclones in the Southern Hemisphere During the Period June to December 1958. NOTOS 14, 73-84 (1965).

19. Taljaard, J. J.: Development, Distribution and Movement of Cyclones and Anti- cyclones in the Southern Hemisphere During the IGY. J. Appl. Met. 6,973-987 (1967).

20. Taljaard, J. J.: Climatic Frontal Zones of the Southern Hemisphere. NOTOS 17, 23-33 (1968).

21. Taljaard, J. J.: Synoptic Meteorology of the Southern Hemisphere; in Newton, C. W. (ed.) Meteorology of the Southern Hemisphere. Met. Mono. 13 (35). American Met. Soc. (1972).

22. Taljaard, J. J., van Loon, H.: Cyclogenesis, Cyclones and Anticyclones in the Southern Hemisphere During the Winter and Spring of 1957. NOTOS 11, 3-20 (1962).

23. Troup, A. J., Streten, N. A.: Satellite-Observed Southern Hemisphere Cloud Vortices in Relation to Conventional Observations. J. Appl. Met. 11,909-917 (1972).

24. van Loon, H.: Blocking Action in the Southern Hemisphere. Part 1. NOTOS 5, 171-175 (1956).

25. van Loon, H.: On the Movement of Lows in the Ross and Weddell Sea Sectors in Summer. NOTOS 11, 47-50 (1962).

26. van Loon, H.: Mid-Season Average Zonal Winds at Sea Level and at 500 mb South of 25 degrees South, and a Brief Comparison With the Northern Hemisphere. J. Appl. Met. 3, 554-563 (1964).

27. van Loon, H.: A Climatological Study of the Atmospheric Circulation in the Southern Hemisphere During the IGY, Part 1.1 July 1957-31 March 1958. J. Appl. Met. 4,479-491 (1965).

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29. van Loon, H.: A Climatological Study of the Atmosperic Circulation in the Southern Hemisphere During the IGY, Part II. J. Appl. Met. 6,803-815 (1967).

30. van Loon, H.: Wind in the Southern Hemisphere; in Newton, C. W. (ed.) Meteorology of the Southern Hemisphere. Met. Mono. 13 (35). American Met. Soc. (1972).

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Author's address: A. M. Cafleton, The Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, U. S. A.