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DUMONT DURVILLE, ADELIE LAND, ANTARCTICA 315

transmit the instructions, possibly leading to changes in observation times and methods. Other factors, such as urbanization or vegetation and modification of the environment, are more favourable.

Taking into account these difficulties, this study is limited to the basic meteorological parameters, such as pressure, temperature, wind, and sunshine hours, which can be assumed to have been carefully measured. Unfortunately, other parameters, such as humidity, cloudiness and snow cover, cannot be used because there are too many missing or incorrect data.

(i) Before 1970, the mean wind speed is 11 m s- and the interyear deviation is relatively weak; (ii) After 1972, with a value of 10 m s - l , the mean wind speed is weaker, but the deviation is stronger.

The present meteorological station was built in 1964 and the anemometer tower has not been modified since. However, before 1970, French meteorological stations were equipped with the 'Papillon' anemometer which overestimated the wind speed. This problem, well known by French climatologists, can explain the discontinuity observed around 1970, as Dumont d'Urville is a place where the wind is very strong, but it cannot explain the increase of the deviation between years after 1970.

Figure 2(a) shows the evolution of the annual mean temperature. The coefficient a=0.0261 of a regression function y = at + b is statistically significant and corresponds to a mean warming trend of 0.78"C over the 32 years.

Considering possible inhomogeneity factors, as mentioned above, the temperature series can be assumed to be relatively homogeneous. It is possible that this evolution results from a long-period oscillation of climate, such as the Briickner cycle (Hushcke, 1959), but may be a warming trend. Following Jones (1990) the best guess that can be made is that Antarctic air temperatures now appear to be warmer, by at least 1"C, than those

The graph of mean wind speed (see Figure 2(b)) shows the presence of two periods:

Years

Figure 2. Mean annual temperature (a) and mean annual wind speed (b) from 1957 to 1989. A significant warming trend of 0.78"C is apparent over the period.

316 c. PERIARD AND P. PETTRE

prevailing during the first decade of the twentieth century. This result is broadly consistent with temperature changes that have been reported for both land and marine regions over the rest of the Southern Hemisphere (Jones et al., 1986).

Table 1 gives the trends of the daily minimum and maximum temperature series which have been computed separating winter and summer as defined Figure 4. Relatively strong warming trends are found for both the seasons. Unfortunately, the minimum temperature trend in winter is statistically not significant, due certainly to missing data. However, these results suggest that since the minimum and maximum temperature have increased equally over the period studied, and the difference between minima and maxima has remained constant, then the summer increase can be assumed to have been compensated by a winter decrease.

CLIMATOLOGY

Method

Each month is split into three 10-day periods, the third having possibly 1 day more or less (or even 2 days less in February if the year is not a leap-year). Consequently, there are 36 10-day periods per year. After collecting together the data of the same 10-day periods, the mean, standard deviation, or the first and fourth quintiles, were computed.

As one of the objectives of this study was to determine homogeneous wind periods favourable for aircraft activities, the ‘hierarchical clustering method’ (Johnson and Wichern, 1982), which is well suited to this kind of research, was used. Another objective was to determine the climatic seasons in the region, for which this method is also suitable.

The hierarchical clustering method uses the ‘Ward method’, also known as the ‘inertia criteria’ method, which is briefly explained below.

Let I be a set of n observations described by p continuous variables. 1 is an Euclidian set of points with a distance function d. The aim of the ‘hierarchical clustering’ method is to create a sequence of partitions (po, pl , pz, p 3 , . . . p , ) of the n available subsets of 1 with n, n-1, n-2, n-3, . . . 1 groups so that:

( i ) po has groups of only one observation; (ii) p , - built, the groups of p , are the ones of pi- except two (chosen with a criteria explained later), that is

A level index is attached to each partition p l indicating its heterogeneity. The sequence ( p o , pl, . . . p,) is then naturally in ascending order of this level index with 0 set for p o and 1 for p n - The sequence ( p , ) can be shown as a ‘tree’ emanating from a ‘node’, which defines the aggregation point obtained by joining each p , with the corresponding aggregation point during the construction of the sequence ( p J . The branches of the tree are the n groups of ( p o ) and the trunk is the group p n - ,.

The method used to build this (p , ) sequence is known as the ‘Ward Algorithm’. The distance between clusters can be evaluated by calculating the between-groups inertia. A large value for this distance indicates a

placed into one new group, called aggregation.

Table I. Trends (based on the period 1957-1989) of the daily minimum and maximum temperature measured at the station for winter, summer, and the year. In summer the maximum temperature wartning trend is two times larger than the minimum. The minimum temperature

trend in winter is statistically not significant

T minima (“C) T maxima (“C)

Winter Summer 0 7 4 Year 1.15

1.21 1.14 1.19

DUMONT DURVILLE, ADELIE LAND, ANTARCTICA 317

real difference between the groups. Huyghens theorem shows that the between-groups inertia of pi is smaller than that of pi-

The sequence (p,) is built by iterations, beginning with po; at each iteration i, the two groups A and B of the pi-, partition are joined so that they minimize the decrease of the between-groups inertia and give the new ( p i ) partition.

RESULTS

In Figures 3 ,6 ,7 and 8 the horizontal axes are labelled with the index number of the 10-day periods as follows: 1 is the first 10-day period of January and 36 is the third 10-day period of December; the value of the third 10-day period of December is given twice as 36 and (0).

Teniperatirre

Mean. The graph of the mean temperature (Figure 3) shows three quasi-linear components: the first one is a cooling down from -0.5"C to - 15T, for 10-day periods 1-14, that is from about January to May; tt.: third one is a warming up from - 15" to -0.5"C beginning at the 10-day period 28 in October; between these components, there is a relatively flat region from June to September where the mean temperature remains between - 15°C and - 17°C. Furthermore, it can be noticed that on average, the warming up is twice as fast as the cooling down.

Schwerdtfeger (1984) gave the mean yearly variation of temperature at the continental station of South Pole. At this location, the amplitude of the variation is larger, at 30°C compared with 15°C at Dumont d'Urville, and the curve is symmetrical with the warming up and the cooling down taking about the same time. The longer time for cooling down at Dumont d'urville can be explained by the proximity of the ocean. As pointed out by Fichefet (1988), during the period of cooling down, the cold water in the surface layer of the ocean sinks before reaching its solidification temperature of - 1.9"C and brings warmer water back to the surface. Consequently, as long as the whole mixed-layer near the surface has not cooled down to - 1.9"C, the ice-pack cannot freeze. On the other hand, during the break-up period, only the surface layer has to be warmed up after which the temperature increases more quickly.

The vertical bars give the first and fourth quintiles respectively at their lower and upper extremities. In winter the variability is large but almost equally distributed from the mean. On the contrary, during the warming up and about the first half of cooling down, the variability is beneath the mean.

These results suggest that during the cooling down and warming up periods, the variability is mainly due to the effect of katabatic flow, which drains the very cold air from the Antarctic plateau to the coast producing strong cooling down during a relatively short time of the order of 2 or 3 days.

1 0 - d a y p e r t o d s

Figure 3. Annual variation of mean 10-day temperatures. The bars give the first and fourth quintiles, below and above respectively. The cooling down is twice as long as the warming up period.

318 c. PERIARD A N D P. PETTRE

On the contrary, from April to September, the variability should be rather interannual resulting from a cold

SeasclnaE cycte. Cluster analysis was used to determine the seasonal cycle from the temperature as shown on Figure 4. With a large index, two well-marked seasons are evident, one is a warm period from November to March, the other is a cool period from April to October.

With a small index, six pseudo-seasons appear chronologically as summer, winter, pre-summer, post- summer, pre-winter, and post-winter. It would be irrelevant to increase the number of clusters as the index would be too small. The post-winter and the pre-summer periods can be grouped to form springs and the post-summer and the pre-winter periods can be combined to form an autumn period. In good agreement with the mean and standard-deviation graphs, the summer is short with 3 months and a 10-day period compared with a long winter of 16, 10-day periods from May to October. The autumn is relatively long with 2 months from March to April, while the spring is only four 10-day periods, two in October and two in November.

Figure 5 shows the mean daily cycle of temperature at Dumont d'Urville during summer, winter, and the interseasons. The amplitude of the temperature variation remains weak in all seasons owing to the fact that the trajectory of the sun stays very low above the horizon, or even under it in winter. In summer the temperature oscillates between -4°C and O'C, but in winter the curve is almost flat. During the interseasons, the amplitude is reduced between -9°C and - 12"C, the maximum occurs from 1200 to 1500 hours and the minimum from 2400 to 0600 hours.

or warm winter season.

Pressure

Mean. The mean pressure (see Figure 6) shows anticyclonic conditions in summer and winter but cyclonic

Figure 4. Seasonalization from the 10-day period means of temperature. The summer is short from November to January, while the winter is very long, from May to October, and they are separated by very short interseasons

DUMONT DURVILLE, A D E L I E LAND, ANTARCTICA 319

Inter- Summer seasons Winter - ---- - - -

0 3 6 9 12 15 18 21

Hours

Figure 5. Diurnal variation of the mean temperature in summer, winter, and interseasons. The amplitude of the variation remains small in all seasons because the sun stays very low above the horizon.

I

l-T

I 1 0 - d a y p e r l o d

Figure 6. Annual variation of mean 10-day pressure and standard deviation. One observes a period of half-year due to the geographical configuration of the Antarctic.

conditions during the interseasons, with pressure values lower in spring than in autumn. The standard deviation changes little between 5 hPa and 8 hPa.

There is, however, a variation of the mean pressure with a period of half-year. This well known phenomenon has been explained by Schwerdtfeger (1 967) as being due to the geographical configuration of the Antarctic. On average, during January and July, Dumont d’Urville is in a region of high geopotential (see Schwerdtfeger, 1984, p. 122). The lower values of pressure coincide approximately with the interseasons, as determined before with temperature and with strong wind periods, as will be seen later.

Sunshine fraction

As Dumont d’Urville is near the polar circle, the daylight time varies from 2 to 22 h, so for this reason, it is more accurate to use the sunshine fraction, which is the ratio of the sunshine hours to maximum daylight time.

320 C. PERIARU AND P. PETTRE

As the population has two modes, the mean is not significant and consequently only the median and two quintiles have been computed.

The sunshine fraction (see Figure 7) is large during October, November, and December with, respectively, 50 per cent and 20 per cent of values above 65 per cent and 90 per cent sunshine. The low values are found from May to August with 50 per cent of zeros.

At Dumont d'urville, the sunshine duration, which can be very long in summer, is measured with a double heliograph Campbell. The horizon is unobstructed by mountains, but another Campbell recorder has been set up in order to eliminate the errors due to shadows from the instrument tower and some antennae. During winter, the glass ball is kept clear and from personal discussions with meteorologists who were in charge of the station, it seems that there are very few problems associated with frost. Nevertheless, when the solar elevation is very low, the intensity reaching the record paper is relatively weak and consequently the record can be doubtful. This is the main difficulty encountered with this instrument. In conclusion, as the sunshine duration is carefully measured even in winter, one can safely assume that the results shown in Figure 7 reflect a real effect rather than an instrumental problem or a statistical artefact.

Wind

The wind is the major climatic phenomenon in Adelie Land. The meteorological station of Dumont d'Urville holds the record of the most windy month on average. Moreover, among the 32 years of data, a maximum wind, averaged over 2 minutes, of 90 m s- ' (324 km h-') has been recorded.

Mean. As shown in Figure 8, two periods are evident: the first one with a relatively weak average wind speed between 8 ms- ' and 9-5 ms- ' from the 28th to the third 10-day period (October to January) and another one during the rest of the year with a stronger mean wind speed between 10 m s - ' and 12 ms-', except during the 19th 10-day period with a wind speed of 9 m s- '. The strongest average winds are observed at the end of autumn, as was the case for temperature, but the strong wind period begins almost 2 months before, in February. Thus the most suitable weather period, especially for aircraft activities, is limited to a short period from October to January.

Quintiles. From March to April, 80 per cent of values of the first quintile (see Figure 8) are larger than 6 m s- ' while during the other months, it remains between 4 m s- ' and 5 m SKI. The fourth quintile is relatively low between 10ms- ' and 14ms- ' from October to January and larger than 15 ms- ' from February to September, except for the 10-day periods 19 and 21. The strongest value of 18 m s- ' is observed

:1 I '"mrrrl 33 36

1 0 - d a y p e r l o d s

Figure 7. Quintiles and median of the sunshine fraction. A marked decrease of the sunshine fraction occurs when the sun is very low above or even under the horizon.

DUMONT DIJRVILLE, ADELIE LAND, ANTARCTICA 321

?Ti \- E

v I

al

1 8 - d a y p e r l o d s

Figure 8. Annual variation of mean 10-day wind speed with first and fourth quintiles as in Figure 3. The strongest average winds occur at the end of autumn

in September at the end of winter, as was the case for temperature. The average wind falls relatively sharply in October, clearly indicating the beginning of the summer.

Occurrence of strong winds. As the local weather depends largely on the wind conditions, it is of interest to know if the increase of the mean wind speed results from an average strengthening of the wind or from an increase in strong wind frequency.

We have counted the episodes of three consecutive days during which the wind speed remained larger than two fixed limits: I2 m s- ', which corresponds to the appearance of blowing snow, and 20 m s - ', which is a strong katabatic wind. The continuity criterion was defined as follows: the wind speed must be larger than the limit for at least six out of the eight periods of 3-hourly data. When this criterion was satisfied for more than 3 days, we counted two occurrences of 4 days, 3 occurrences of 5 days and so on.

The results shown in Figures 9 suggest than the increase in the mean wind during the interseasons from February to April and for August and September is rather due to an increase of the occurrence of very strong wind events. For instance, the sharp fall of the mean wind in October can be explained by the decrease of the occurrence of winds larger than 20 m s-'. In the same way, one observes an increase of the mean wind from February to March as the occurrence of winds greater than 20 m s- ', whereas the winds between 12 m s - ' and 20 m s decrease.

Srusonul cycle. Considering the daily mean wind speed, a separation into three clusters gives the following results (see Fig. 10):

(i) weak wind speed-October, November, December, and January; (ii) strong wind speed-February, end of May, June and July;

(iii) very strong wind speed-March, April, August, and September.

Two interesting facts are shown by the graphs (see Figure 12) of the mean daily variations of the wind speed for each month:

(i) during the night, the wind speed for the period of weaker average winds from October to January is equal to, or even stronger than, the period of strong average winds in March, April, August, and September;

(ii) The daily behaviour of the wind changes markedly from one month to another in a given cluster, except for that of weak wind, which is very homogeneous.

Clearly, the regime of wind closely depends on the diurnal evolution of temperature: during winter as the temperature variations are very small, the wind speed is quasi-constant; in contrast, during summer with

322

JAN FEB MAR

c. PERIARD AND P. PETTRE

I L O Z I I I I i 1 I

APR MAY JUN JUL AUG SEP OCT NOV DEC

12<vt<20 vt>=20

$ 150

$ L J 100 0

6 50

0 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Months Figurc 9. Occurrence of three consecutive days with a wind speed greater than 12 m s- ' and 20 m s- '. Strong winds occur preferentially

during the interseasons.

128 29 30 31 32 33 8 9 x) H 12 13 23 24 25 26 14 I5 4 5 6 7 (6 R 18 20 21 22 27

10 days period riurnber

RESULT WITH 3 CLUSTERS

Weak wind Very strong wind = Strong wind

Figure 10. Seasonal cycle from the 10-day period mean of the wind speed. It coincides approximately with the seasonal cycle for temperature.

DUMONT DURVILLE, A D E L I E LAND, ANTARCTICA 323

a large amplitude of temperature (see Figure 5), the wind speed changes between 7 m s- at 1500 hours and 1 1 m s

To understand the effect of the daily temperature variation on the wind speed, we computed the mean daily variation of the wind speed, filtering the days where the temperature amplitude was weak, that is the days where the sunshine fraction remains lower than a fixed threshold.

Figure 11 shows the mean daily wind speed for the two typical months of December in summer and July in winter. Two facts are noticeable:

(i) the wind speed decreases as the sunshine fraction increases;

at 0300 hours.

D ec:emb er

fraction > 70% >90% any fraction fraction

- - - - ----

0 3 6 9 12 15 18 21 (241

Local timr

July

fraction > 70% any fraction fraction

>90% - - - - -I--

11

10 - r

I

e 2 9

-L 2 1 is a

6

5 0 3 6 9 12 15 18 21 ( 2 4

Local time

Figure 1 1 . Mean daily variation of the wind speed as a function of the sunshine fraction. These results suggest that in summer and even in winter, the wind variation depends closely on the diurnal temperature cycle. Nevertheless, the maximum observed in July at midnight is

probably overestimated, as explained in the text

3 24 C. PERIARD AND P. PETTRE

(ii) in summer, the shape of the graph does not change, whereas in winter there is a maximum at midnight and

A three-dimensional representation of the yearly evolution of the mean diurnal variation of the wind speed is shown in Figure 12. With no consideration of the sunshine fraction, a sinewave-like daily variation of the wind speed can be observed from January to April. The sinewave amplitude decreases and the surface becomes quasi-flat from May to August, then during the four last months the sinewave daily variation appears again.

a slight increase of the wind speed at 1500 hours.

Any suiishiiie fraction

Sunshine fraction > 70%

Figure 12. Annual evolution of the mean daily variation of the wind speed as a function of the sunshine fraction as in Figure 11 but for all months of the year

DUMONT DURVILLE, ADELlE LAND, ANTARCTICA 325

When filtered with a sunshine fraction of 70 per cent, the wind speed is reduced globally, except in November, it is even weak at night in June and July, but the wind increases relatively in winter from April to August during the afternoon from 1700 hours to 1900 hours. Nevertheless, the wind speed maximum found at midnight during the winter months can be in doubt. As the solar elevation is very low, the sun can be under cloud cover in such a manner that one can observe insolation and relatively bad weather conditions in the same time. On the other hand, if the selected days are preceded and followed by a windy day, on average, the wind speed at midnight will be overestimated. However, in winter as in summer, strong winds can occur due to the katabatic effect resulting from the intense cooling down during the clear sky nights. As in winter the nights are longer it can be expected that the wind speed maximum occurs earlier than in summer. Taking into account these considerations, one can assume that this maximum exists but that its value is probably overestimated.

Wind direction data are available from 1962. Figure 13 shows the wind roses over 27 years (1988 excluded). Two facts are remarkable: the steadiness of the direction (120"/140") of strong winds coming from the continent and the existence of some winds of direction 300" coming from the sea.

The winds of direction 300" appear in spring and summer when the ocean is free of an ice-pack. This suggests that a sea-breeze can occur at the coast of Adelie Land in summer time. The temperature of the ocean can be assumed constant at about - 1 9 C as there is sea-ice and free water near the coast. The surface temperature of the continent should be generally lower than the ocean as it is covered in ice or snow. However in summer, due to intensive solar radiation, the surface temperature of the continent can be larger than the ocean producing a temperature gradient inferring a sea-breeze. To substantiate that such a gradient can occur, it can be argued that the temperature maximum recorded at Dumont d'Urville over 30 years is 9"C, while a maximum of 14°C has been measured by the Automatic Weather Station at D10 located on the continent at 10 km from Dumont d'urville, as reported by Wendler et al. (1991).

These results are confirmed by Figure 14, which shows the wind roses for the four typical months of December, February, May, and October. During the warm season, in October and December, the number of winds in the 300" sector is not negligible, with 2 per cent and 2.6 per cent of observations respectively. The winds of sectors 120" and 140°, which include the katabatic flows and also the land-breezes, are much more

1 2

5 % t--i

Figure 13. Wind rose from 1962 to 1989. Note the constahcy of katabatic winds of direction 120"/140" blowing from the continent

326 c. P E R I A R D

1 2

December (a)

AND P PETTRE

3 4 3 6 2 & 3 2

February (b)

1 2

October (d)

Figure 14. Wind roses from 1962 to 1989 for December (a), February (b), May (c), and October (d). The winds of 300" which appear in spring and summer, suggest that sea-breezes can occur at the coast due to the temperature contrast between the ocean and the continent

frequent with respectively 41.5 per cent and 35.2 per cent. During the cold season, with the formation of the ice-pack, the winds of sector 300" disappear as the gradient of temperature between the ocean and the continent vanishes. The winds of sector 120" and 140" increase in February and May with 59.7 per cent and 46.8 per cent of observations respectively. In February, it is principally the winds greater than 15 ms- ' of sector 140" that increase.

CONCLUSTON

The climatology of the station of Dumont durville gives several interesting indications concerning the climate of the Antarctic and the formation of katabatic flows in Adelie Land near the coast.

The study of temperature has shown a warming trend of 0.78"C over 32 years. The evolution of the annual cycle suggests that the formation of the sea-ice cover makes the arrival of the winter later than in the interior of the continent.

DUMONT DURVILLE, ADELIE LAND, ANTARCTICA 327

From the study of winds, it is obvious that during the summer season a coastal breeze can occur due to the temperature contrast between the land and the ocean, which has the effect of limiting locally the influence of katabatic flows coming from the continent.

However, the results clearly show that in fine weather the evolution of wind speed is determined by the diurnal cycle.

These two facts can explain the high occurrence of strong winds observed during the interseasons period during which the sea-ice covers the ocean, reducing the temperature contrasts producing the breeze, whereas the katabatic flows are forced by the radiative cooling inland.

As the sea-ice cover surrounding the Antarctic is very dynamic and shows a large interannual variability, one can conclude that it is important for the present climate of the Antarctic.

REFERENCES

Fichefet, T. 1988. fnteractions Ocean-Glace marine: modeles a une et deux dimensions pour la simulation du climat. Master’s thesis,

Hushcke, R. E. 1959. Glossary of Meteorology, 1980 edn, third printing, American Meteorological Society, Boston, MA. Johnson, R. A. and Wichern, D. W. 1982. Applied Multivariate Statistical Analysis, Prentice-Hall, New Jersey. Jones, P. D. 1990. ‘Antarctic temperatures over the present century-A study of the early expedition record’, J . Climatol., 3, 1193-1203. Jones, P. D.. Raper, S. C. B. and Wigley, T. M. L. 1986. ‘Southern hemisphere surface air temperature variations: 1851-1984, J . Climate

Loewe, F. 1972: ‘The land of storms’, Weather, 27, 110-121. Mawson, D. 1915. The Home of rhe Blizzard. Heinemann, London. Parish, T. and Wendler, G . 1991. ‘The katabatic wind regime at Adelie Land, Antarctica’, Int. J. Climatol., 11(1), 97-107. Pettre, P. and Andre, J. C. 1991. ‘Surface-pressure change through Loewe’s phenomena and katabatic flow jumps: study of two cases in

Schwerdtfeger, W. 1967. Annual and semi-annual changes of atmospheric mass over Antarctica, J . Geophys. Res., 14, 3543-3547. Schwerdtfeger, W. 1984. Weather and Climate of the Antarctic. Developments in Atmospheric Sciences, Elsevier, New York. Wendler, G., Andre, J. C., Pettre, P. and Gosink, J. 1991. ‘Katabatic winds in Adelie Land’, Special A C U Series on Antarctic Meteorology,

Louvain-la-Neuve.

A p p l . Meteorol., 25, 1213-1230.

Adelie Land, Antarctica’, J . Atmos. Sci., 48(4), 557-571.

American Geophysics Union.