Some aspects of the climatology of dumont D'Irville, adélie land, Antarctica

Download Some aspects of the climatology of dumont D'Irville, adélie land, Antarctica

Post on 15-Jun-2016

216 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

  • Page no. 313-314 missing

  • 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 - 15C and - 17C. 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 30C compared with 15C 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 -4C and O'C, but in winter the curve is almost flat. During the interseasons, the amplitude is reduced between -9C 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 dUrville 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 dUrville 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 ma...