POLISH POLAR RESEARCH 20 3 203-220 1999

Marek KEJNA1,2 and Kamil LASKA3

Department of Climatology, Institute of Geography Nicholas Copernicus University Danielewskigo 6 87-100 Toruń, POLAND e-mail: makej@geo.uni.torun.pl

Department of Antarctic Biology Polish Academy of Sciences Ustrzycka 10 02-141 Warszawa, POLAND

Department of Geography and Cartography Faculty of Natural Sciences, Masaryk University Kotlarska 2 611 37 Brno, CZECH REPUBLIC e-mail: laska@porthos.geogr.muni.cz

Weather conditions at Arctowski Station, King George Island, South Shetland Islands,

Antarctica in 1996

ABSTRACT: This paper deals with the frequency of circulation types in the South Shetland Is­lands region during the year 1996 and with the analysis of local causes of possible flow deforma­tions and typical values of meteorological features characteristics at the Polish Antarctic Station Arctowski during the time of an occurrence of individual circulation types.

Key words: Antarctica, King George Island, meteorological conditions.

Introduction

In December 1994, the meteorological station Arctowski was rebuilt and a complete set of meteorological measurements has started (Kruszewski 1996). In­vestigations were continued during the XX Polish Antarctic Expedition (from 17 December 1995 to 14 December 1996). Based on the scientific literature data (Kowalski and Wielbińska 1989; Styszyńska 1990; Kejna 1993, 1995 a,) and on personal observations, the authors analysed the circulation conditions in the King George Island region and the nearby surroundings of the Arctowski Station. As might have been expected, the atmospheric circulation greatly affects the regime of basic meteorological characteristics, which can be proved by a number of exam­ples.

204 Marek Kejna and Kamil Laska

59°00' 5 8° 00'

os_-— 1. > ^ 2. V1. 3. 4g£ 4. «SS> 5. ^ 0 ^ _ 6.

• 7. H e . • 9- "SC 10- O u . © 12.

Fig. 1. Location of measurement stands at the Arctowski Station in 1996: 1 - seashore, 2 - contours, 3 - altitude points, 4 - rocks, 5 - lakes, 6 - streams, 7 - buildings, 8 - meteorological screen, 9 - stands of ground temperature measurement: O - meteorological garden, B - beach, M - moss, D -Deschampsiaantarctica, 10-anemometer, 11-heliograph, 12-stand of snow cover measurement.

Meteorological observations at Arctowski Station

Meteorological measurements and observations were acquired at the Polish Antarctic Station Arctowski ((p = 62°10'S, X = 58°28'W, h = 2 m a.s.l.) situated on the west shore of Admiralty Bay (Fig. 1), on unglaciated surface at the foot of Pt. Thomas massive. Admiralty Bay is surrounded by elevations of 300 to 600 metres

Weather conditions at Arctowski Station 205

a.s.l., only from the south-east is it open towards the Bransfield Strait. From the other sides it is surrounded by the following glaciers: by Arctowski Icefield (eleva­tion up to 650 m) from the north, by Warszawa Icefield (476 m a.s.l.) from the west, and by Kraków Icefield (400 m a.s.l.) from the east. Air masses flowing over these elevations are affected by pseudoadiabatic processes (Styszyńska 1990), the direction of advection can be further modified by local shapes of the relief, which are shaped as deeply carved fiords (e.g. Ezcurra Inlet).

The meteorological station was situated on the northern part of the vegetation oasis, SE of the Arctowski Station. The northern and eastern part of the oasis flat surface is surrounded by a storm ridge, about 1 metre high, on the south the oasis is limited by an elevation with a height about 70 m a.s.l.

Flows coining to the station from western directions must overcome one of the highest barriers - Pt. Thomas (173 m a.s.l.) and Jardine Peak (288 m a.s.l.). The real horizon of the point is not the most suitable from the point of view of the mea­surement of some meteorological features (Styszyńska 1992). In the time of the winter solstice (26 June), when the Sun culminates 4°23' above the horizon and the day is 4 hours and 31 minutes long, a great part of the Sun's course is shaded by the Kraków Icefield, the Arctowski Icefield, or by the elevations Pt. Thomas or Jardine Peak. Another factor entering as a disturbing element into the results of the mea­surement is the small distance of the station from the sea (about 20 m) and the great number of small lakes in the station surroundings. In the case of strong winds the waves often break on the sea or on nearby lakes and are horizontally transported to­wards the station. The capture of the drops by the raingauge affects its correct func­tion (Kejna and Laska 1997).

Results and discussion

Atmospheric circulation and circulation types The northern part of the Antarctic Peninsula, together with the South Shetlands

archipelago, are situated in the maritime Antarctic region. Atmospheric circulation in this region is driven by the stationary lows existing nearby in the Antarctic Pen­insula and above the Bellingshausen and Weddell Seas. Together with the migra­tory cyclones, which are under a permanent influence of cyclogenesis and cyclone termination, they create a low pressure trough around Antarctica (Schwerdtfeger 1970). Migratory cyclones, respectively frontal cyclones are moving from W to E through the Drake Passage in various trajectories (Figs 2 and 3), very often con­tinue towards the south, where they cross the mountain ranges of the Antarctic Peninsula (Schwerdtfeger 1984). In the South Shetlands region, the zonal atmo­spheric circulation is often blocked by high pressure, which is connected to the central Antarctic anticyclone (Fig. 4F) or one of the subtropical anticyclones (Stępko and Wielbińska 1981, Kejna 1993). The South Shetlands region is cut

206 Marek Kejna and Kamil Liska

90° W 80° 70° 60° 50° 40° W

Fig. 2. NOAA satellite image (channel 2 and 4) of the area between the West Antarctica and South America with the cyclonic circulation "Nc" (see text) affecting weather conditions at the Arctowski

Station on 21 December 1996 (23:30 UTC). The image is not geometrically corrected.

through by a seasonal ice-extents boundary. In dependence on the advection direc­tion, the oceanic air masses flow to the Arctowski Station (from W and N) or much colder air comes from the frozen waters of Weddell Sea or from Antarctica. These circulation factors, together with other climatogenetic factors cause the variability of the climatic conditions on the Arctowski Station (Wielbinska and Skrzypczak 1992; Kejna 1995 a, b; Styszyńska 1996).

According to the classification of the synoptical situations by Kejna (1993), derived from the typification by Niedźwiedź (1981), it is possible to divide the syn­optical situations into 20 types. According to the ordering of the baric field, which determines the advection in the South Shetland region, the individual types are marked by the abbreviations of directions of air mass advection, anticyclone with index "a", cyclone with index "c", ridge of high pressure "Ka" and low pressure trough "Be". "Cc" means cyclonic center. Synoptical situations which cannot be easily classified are marked "X". For the classification ground synoptic charts were used, published as facsimiles by the Chilean Meteorological Service in Valparaiso, and satellite imagery was available at the Brazilian Antarctic Station Ferraz from a receiver which collected the 4 km horizontal resolution Automatic

Weather conditions at Arctowski Station 207

Fig. 3. NOAA satellite image (channel 2 and 4) of the area between the West Antarctica and South America with the cyclonic circulation "NWc" (see text) affecting weather conditions at the Arctow­

ski Station on 22 December 1996 (19:30 UTC). The image is not geometrically corrected.

Picture Transmission (APT) data from the NOAA 14 of polar orbiting meteorolog­ical satellite (Figs 2 and 3).

In the year 1996 the oceanic type of weather prevailed at the Arctowski Station. In the course of the year, there were 79.0% of days with cyclonic situations and only in 20.7% the weather conditions were formed by anticyclones (Table 1). The most frequent type NWc influenced the weather at the station for 23.1% of days in the year. During the NW flow King George Island was most often at the northern or north-eastern edge of the primary depression above the Antarctic Peninsula (Figs 3 or 4A). The clouds of frontal systems, tied with the cyclone above the SE Pacific, re­spectively above Weddell Sea, sometimes only partially reach the South Shetlands region. Significant frequencies were reached by circulation types Wc (13.0%) and SWc (11.5%). Their advection is conditioned by the existence of at least two primary depressions, one of which is situated above Bellingshausen Sea and the other one above the northern part of Weddell Sea (Fig. 4B). The particular circulation type (de­termined by a flow on the north and north-western edges of those cyclones) is condi­tioned by the interrelation of their location and their connection by a trough of low pressure (Figs 4B, C). Frontal systems tied with the mentioned lows usually come to King George Island already occluded and at the Arctowski Station their presence is

208 Marek Kejna and Kamil Laska

? &*\j

X

.A

(i©

f i j | ! f ; /

'X/if/ i XźkfHA

~>~\^y/

saw\ / X / " B

7jA%

5/> ^— /

r ^ ^ v ^ \ / X V

• King George Island 20.2. 1996, ISUTCfNWc] * King Gcurgc Island 3.7.19%, mrrcfswc]

° . /"^"OOJgM km

/ / / \ J S ^ <

-* / V l ( Ąf

|M$r\ •€W$v P~~~\ ł^Ytf * >v-

- ' • ^ ^ ^ / TA -»•

• King ficorRC Islwyl 10,4. 1996. l8U'l'C[NWa]

• Kjng Cjeorge Island 4.1. 1996.ISUTC [SWj,] • King (jwigi; bland 16.9. 1996. 18UTC|Ka]

Fig. 4. A-F. Surface synoptic charts of the area between the West Antarctica and South America with circulation types most often affecting weather conditions at the Arctowski Station in 1996 (see text

and Table 1).

Weather conditions at Arctowski Station 209

Table 1 Frequency [n and %] of atmospheric circulation types in the region of the Arctowski Station

in the period 1986-89 and in 1996.

Type

Nc

NEc

Ec

SEc

Sc

SWc

We

NWc

Cc

Be

Na

NEa

Ea

SEa

Sa

SWa

Wa

NWa

Ca

Ka

X

cyclonic

anticyclonic

1986-1989

[n]

67

51

10

21

68

253

195

187

81

57

26

46

16

27

14

68

38

46

28

80

40

990

389

[%]

4.7

3.6

0.7

1.5

4.8

17.8

13.7

13.2

5.7

4.0

1.8

3.2

1.1

1.9

1.0

4.8

2.7

3.2

2.0

5.6

2.8

69.8

27.4

1996

[n]

33

11

3

13

12

40

45

80

23

14

5

5

0

1

2

9

5

21

2

22

1

274

72

[%]

9.5

3.2

0.9

3.7

3.5

11.5

13.0

23.1

6.6

4.0

1.4

1.4

0.0

0.3

0.6

2.6

1.4

6.1

0.6

6.3

0.3

79.0

20.7

recognized due to reduced cloudiness (Prosek and Janouch 1997). The cyclone types from the western sector represented up to 47.6% of all situations.

The least frequent were the cold air masses coming from the southern (Sc -3.5%, Sa - 0.6%) and eastern (Ec - 0.9% and Ea- 0.0%) directions. During the year, the conditions for the main cyclone formation between South Orkney Islands and South Georgia, which, together with the cyclone situated west and south-west of the Antarctic Peninsula, are major participants in the advection of air from the southern sector, were not created frequently. The close neighbourhood of frontal systems of this cyclone also causes greater cloudiness in the South Shetlands region.

From comparison of the frequency of circulation types in 1996 with the 1986-89 period, analysed by Kejna (1993), the increase of circulation types in the cyclone period in 1996 (9.2%) against anticyclonic types (6.7%) is clear (Fig. 5). Cyclonic situations supporting advection of air masses from the west and from the north, mostly influenced the climatic conditions on the Arctowski Station in 1996.

210 Marek Kejna and Kamil Laska

• 1996

D1986 - 1989

types of atmospheric circulation

Fig. 5. Frequency [%] of atmospheric circulation types at the Arctowski Station in 1996 and in the pe­riod 1986-1989.

Air pressure

The high frequency of atmospheric circulation caused the average air pressure in 1996 to reach 989.3 hPa, which was lower than the long-term average for period 1978-1989 (990.8 hPa) (Rakusa-Suszczewski et al. 1992). The highest monthly values were reached in April (996.5 hPa), the lowest in November (979.6 hPa) -Table 2. The result of the rapid movement and continuous changing of deep cy­clones was great, causing intensive changes in atmospheric pressure in intervals shorter than one day, or interdiumal changes. This is reflected in the occurrence of high variabilities and extreme values. The highest air pressure was measured on 25 June (1029.9 hPa) and the lowest on 24 February (953.2 hPa).

Wind direction and velocity

The direction of wind at the Arctowski Station is strongly modified by the local orography of the two main fiord systems - Ezcurra Inlet and Martel Inlet (Styszyń-ska 1990). The main axis of those two fiords is oriented in NNW-SSW direction and affects the flow of dominant western circulation. Wind from the west overflowing the Arctowski Icefield falls into the Ezcurra Inlet and after leaving this inlet turns southwards. NNW winds are characterised by high velocity and gustiness (Kowalski 1985) and their frequency in 1996 reached 13.2% of all days (Fig. 6 A). Frequency was also significant for the winds coming from WSW (11.3%), SW (9.4%) or SE (8.7%). The least frequent were winds from WNW (3.5 %), which were prevented from the entry to the station by Pt. Thomas massive (173 m a.s.l.) or by advection from the east (E - 3.5 %, ENE - 2.3%), which was characterised by a low frequency of circulation from that direction. A calm occurred only 0.6% of the days.

Weather conditions at Arctowski Station 211

Table 2 Monthly values of meteorological conditions at the Arctowski Station in 1996

Element

P[hPa]

v [m/s]

C [0-10]

Sd [hour]

Tmean Tmax Tmax abs Tmin Tmin abs

e[hPa]

r[%]

R[mm]

Rs [cm]

I

990.8

6.3

8.7

126.6

2.4 4.3 8.9 1.0

-1.1

6.2

84

48.7

2

II

989.6

6.5

8.1

122.0

3.4 5.2 8.9 1.5

-2.3

6.4

81

32.6

2

in 985.7

6.4

8.4

78.2

2.0 4.1 8.3 0.1

-7.5

6.0

84

44.9

2

rv 996.5

5.3

7.5

51.7

-1.1 1.0 5.9

-3.6 -8.9

4.8

83

22.1

5

V

986.4

7.3

8.5

6.8

-2.3 0.4 7.1

-4.6 -10.9

4.5

84

15.3

8

VI

992.9

6.7

8.7

7.9

-4.9 -1.9 4.2

-7.9 -14.7

3.8

84

23.8

23

VII

990.4

9.8

8.2

13.8

-3.1 0.4 5.9

-6.2 -15.1

4.3

83

62.6

27

vni 989.9

6.9

8.1

26.2

-3.4 0.9 6.6

-5.9 -11.7

4.1

82

33.9

68

DC

995.7

7.9

8.9

21.5

-1.0 1.2 6.8

-3.5 -13.1

5.1

87

59.9

73

X

984.9

8.0

7.8

95.0

-0.9 1.1

7.3 -3.3 -8.3

4.9

83

41.9

38

XI

979.6

8.3

8.4

95.3

0.3 1.8 5.5

-1.4 -6.5

5.3

83

49.3

9

XII

988.7

6.1

8.3

115.3

1.7

3.6 7.2

-0.2 -2.9

5.9

86

44.2

15

i-xn 989.3

7.1

8.3

760.3

-0.6 1.6 8.9

-2.8 -15.1

5.1

84

479.2

73

Comments: P - air pressure, V - wind velocity, C - cloudiness, Sd - sunshine duration, Tmean - air temper­ature - mean ['C], Tmax - air temperature - mean daily maximum ['C], Tmax abs - air temperature - abso­lute maximum ['C], Tmin - air temperature - mean daily minimum ['C], T min abs - air temperature - ab­solute minimum ['C], e - water vapour pressure, r - relative humidity, R - precipitation, Rs - maximal

snow cover.

At the Arctowski Station baric fields with great pressure gradients cause very strong winds and their enormous variability during the year, with frequency of changes often shorter than one day (Fig. 8). The average wind velocity in 1996 was 7.1 m/s and was higher by 0.7 m/s than the long-term average (Table 3). The lowest monthly mean values were in April (5.3 m/s) and in the time of the polar summer in December (6.1 m/s) and in February (6.5 m/s). In contrast, the winter and spring months were the most windy, with the average monthly mean of wind velocity in July reaching 9.8 m/s and in November 8.3 m/s (Table 2). On 25-30 July a hurri­cane occurred at the Arctowski Station. The stable baric field ordering with the se­ries of deep cyclones at the edge of Antarctica caused extremely high wind veloci­ties (10 minutes average values were about 30 m/s) in those days and gusts of wind exceeded the value of 50 m/s.

The strongest winds were tied with the northern flow (NNW -11.0 m/s, NW -9.5 m/s and N - 7.9 m/s) (Fig. 6 B). These flows are falling, orographically strongly influenced winds, which come to the station as horizontal whirls (with diameters up to 20 m) that evolve at the Ezcurra Inlet mouth (Kowalski 1985). Permanent winds with the average velocity of about 7.0 m/s appearing during cold air advection from SE were not prevented in their path to the Arctowski Station by any orographic bar­rier. The mildest winds were during the southern (S and SW - 5.3 m/s) and eastern (ENE - 4.7 m/s) flows.

212 Marek Kejna and Kamil Laska

[%] N

NNW,;. - l -S r - . - . ^NNE

[m/s]

WNW,' .

w;

wsw •. •

.ENE

SW*-. / ' :" \ ,.-'''SE

SSV\K- —..]...-—VSSE S

calm = 0.6%

'.<. ENE

•'E

Fig. 6. Frequency [%] of wind directions (A) and mean wind velocity (B) according to wind direction at the Arctowski Station in 1996.

Cloudiness

The high cyclone activity in the South Shetlands region and the prevalence of maritime air masses cause an increased cloudiness at the Arctowski Station. The great variability in time determined by changes of baric field, location and by fast development of atmospheric fronts caused the average cloudiness in 1996 to reach 8.3/10, which was higher by 0.4 than the long-term average (Table 3). Cloudiness, in most cases of frontal origin, showed a small yearly amplitude (Fig. 8). The high­est mean monthly cloudiness occurred in September (8.9), January and in June (8.7) - Table 2. In the annual cycle there were only 3 days of clear sky (cloudiness C<2), 137 cloudy days (2<C<8) and 226 overcast days (C>8), of which 49 days had cloudiness C - 10 (Kejna and Laska 1997).

Sunshine duration

The potential length of sunshine is dependent on the length of the day and oc­curs in the interval from 4h 31m in June to 19h 22m in December. The real number of hours with sunshine depends on cloudiness, type of clouds, atmospheric features reducing visibility (fog, haze, snow storm) or on the physical horizon of the partic­ular place. In the summer period, when the relative height of the horizon around the sunrise and sunset azimuth is negligible, the sunshine duration is influenced mostly by cloudiness (Prosek et al. 1996).

In 1996 only 760.3 hours with sunshine were registered (Table 2), which is about 60 hours less than the long-term average (823.9 hours). The relative sunshine duration reached 19.8% with regard to that theoretically possible. The highest monthly totals were measured in the summer period, in January 126.6 hours, in

Weather conditions at Arctowski Station 213

WSW'

NNW •-

[%]

90 N ,NNE

NW-.' j

WNW/. lf\

w( f

wswV" \ SV\k',

SSW"''

. ?9-U. (M^-[ • ' ; - - : - - * ; i

i i , " " - - - i . - - " " ' * i

^S-\NE

/ \ \'>.,ENE

1-\-4E

' • • / / - ; E S E

v^>SE

"ŚSE

Fig. 7. Frequencies [%] of cloudiness (A), air temperature (B) and relative humidity (C) according to wind direction at the Arctowski Station in 1996.

February 122.0 hours and in December 115.3 hours. On the other hand the least sunshine duration, registered in May (6.8 hours) and in June (7.9 hours) is related to the short days around winter solstice, great cloudiness, and eclipsed horizon. There were up to 24 days without sunshine in May.

Air temperature The air temperature at the Arctowski Station is heavily dependent on the type of

advected air mass (Kejna 1995a). During West and North advection a relatively warm air comes from the non-frozen areas of Bellingshausen Sea and Drake Pas­sage to the station. Flow from southern and eastern directions brings significant chilling related to Antarctic air masses from Weddell Sea or from Antarctica (Kowalski and Wielbińska 1989). The regime of air temperature is very calm due

214 Marek Kej na and Kamil Lśska

Table 3 Comparison of mean annual values of meteorological conditions at the Arctowski Station

in 1996 and in the period 1978-1989.

Period

1996

1978-89

Air pressure

[hPa]

989.3

990.8

Wind velocity [m/s]

7.1

6.4

Cloudiness [0-10]

8.3

7.9

Sunshine duration [hour]

760.3

823.9

Air temperature ['C]

mean

-0.6

-1.6

max

8.9

16.7

min

-15.1

-32.3

Precipitation [mm]

479.2

527.9

to the significant advection and small temperature changes of the ocean during the year.

In 1996 the average annual temperature was -0.6°C and was higher by 1 °C than the long-term average (Table 3). In every month there were temperatures above zero (in August the temperature was higher by 3.2°C than the long-term average). The warmest month was February (3.4°C) and the coldest one was June (-4.9°C) (Table 2). The annual amplitude (8.3°C) proves the prevailing maritime climate (Marsz and Rakusa-Suszczewski 1987, Styszyńska 1995). Daily average maxi­mum temperatures reached 1.6°C. The highest mean daily maximum was mea­sured in February (5.2°C), the minimum in June (-1.9°C). Even if the year 1996 was extremely warm, the absolute maximum temperatures reached only 8.9°C (22 Jan­uary and 19 February). The average daily minimum air temperature was -2.8°C and the lowest temperature -15.TC was measured on 20 July.

An extraordinary variability of the advection in time causes a very irregular air temperature regime, with isolated signs of the temperature wave with interdiumal period. The change of the air temperature from day to day is related to the change of the advection direction and to the type of air mass. The largest variability was mea­sured in the winter time (Fig. 8). For example, from 15 August to 16 August average daily temperature fell about 8.8°C, so as to rise by 8.9°C from 20 -21 August.

The warm year 1996 confirms progressive warming in the region of the Ant­arctic Peninsula (Martianov et al. 1990, King 1994, Morris et al. 1994, Ackley et al. 1996, Rodriguez etal. 1996, and others).

Humidity The prevalence of the maritime air masses in the South Shetlands region keeps

the surrounding atmosphere relatively humid. The water vapour pressure corre­sponds to the air temperature (Fig. 8). In 1996 the average water vapour pressure was 5.1 hPa and was highest in the summer months (February - 6.4 hPa) and low­est in winter (June - 3.8 hPa). The monthly mean relative humidity at the Arctowski Station did not fall below 80% and in 1996 reached 84% (Table 2). During the year it was possible to observe sudden falls of relative humidity (below 50%), probably caused by pseudoadiabatic processes transforming western flows.

Weather conditions at Arctowski Station 215

[hPa]1040 -,

1020 1000 980 960 940

[m/s] 25 20 15 10 5

air pressure

V VI VI V i

wind velocity

10

0 [°C]10

llnUlilUwi. sunshine duration

4'-, ,-'<• i - ^ r » r ->«-M-l^, •, l|J.,Ml.

mean, max imum and minimum air temperature

« IV V VI VII V* IX X XI XII

Fig. 8. Annual course of selected meteorological elements at the Arctowski Station in 1996.

216 Marek Kej na and Kamil Laska

Precipitation

Precipitation is one of the most variable meteorological characteristics. As a result of low temperatures the clouds usually contain little water. Individual pre­cipitation periods are often not rich, but they are more frequent. Unlike the centre of the Antarctica continent its edges, including the Antarctic Peninsula and South Shetland Islands, are relatively rich in precipitation. At the Arctowski Station, the liquid precipitation (rain, drizzle) and even the solid precipitation (snow, small hail, snow pellets) were present in every season of the year.

The annual precipitation in the year 1996 amounted to 479.2 mm, which was lower than the long-term value - 527.9 mm (Marsz 1994). The highest precipitation amounts were measured in July (62.6 mm), in September (59.9 mm) and in January (56.0 mm) - Table 2. Very low precipitation was measured in the period from April to June (e.g. in May it was 15.5 mm), and in February. Those months did not reach even 50% of the long-term average precipitation amount. The low annual precipita­tion amount was, on the other hand, compensated by its high occurrence in each month. In 1996 precipitation over 0.1 mm (including immeasurable precipitation) was recorded on 301 days (82.3%) The highest daily amount occurred on 19 August and reached 19.4 mm. It can be generally said that the precipitations with the highest intensity are related to NW advection. The real precipitation amounts are often over­valued by spray from Admiralty Bay (Kejna and Laska 1997).

Snow cover

Solid precipitation at the Arctowski Station can occur during any season of the year. Sunshine and temperatures above zero in the summer cause its quick melting. In the year 1996, the first snow cover appeared at the end of March. Solid snow cover was created in the surroundings of the station in June and lasted till the end of October (Fig. 8).

Measuring of the snow cover thickness at the Arctowski Station is difficult for two main reasons. The first is the frequent transport of the fallen snow by wind. The second reason is the flow of melting water from the surrounding slopes onto the flat terrain near the station, which causes flooding of the measuring point. After a thaw the water freezes again and creates a new basis for the measurement of snow accu­mulation. The maximal thickness of snow cover (73 cm) was measured in Septem­ber. Melting of the snow in the beginning of spring (the end of October) was very quick. During two major warmings, when the temperature rose to 7.3 °C, the snow melted everywhere in the surroundings of the meteorological station (Fig. 8).

Dependence of climatic conditions on the direction of advection

The specifics of atmospheric circulation in the subantarctic zone, in particular the great activity of cyclonic centres with atmospheric fronts are favourable for the development of cloudiness, which limits direct solar radiation. In the region of the

Weather conditions at Arctowski Station 217

Arctowski Station, the surface is strongly differentiated (Burdecki 1957, Schwerdt-feger 1970,1984). The ice fields of the Weddell Sea lie to the east and south of the Arctowski Station and the open water of Southern Ocean to die north and west. The flowing air masses differ in meteorological elements.

Air masses flowing from the west wind sector to the Arctowski Station are strongly transformed by the orography. Winds from NW and SW directions domi­nating in this region flow over the Arctowski Icefield (650 m a.s.l.) and Warszawa Icefield (476 m a.s.l.), undergoing foehn transformation (Kowalewski and Wiel-bińska 1983). The increase of the air temperature caused by pseudoadiabatic pro­cesses reaches 1.5-2.5°C in the summer and about 0.5-1.6°C in the winter (Sty­szyńska 1990). The question of the forming of thermal conditions at the Arctowski Station in dependence on wind direction has already been analysed by Wielbińska and Skrzypczak (1988) and by Styszyńska (1990), while the influence of atmo­spheric circulation on the air temperature and other meteorological elements has been analysed by Kejna (1995 a, b, 1996).

The direction and quality of flowing air masses strongly affects the weather conditions at the Arctowski Station. High values of cloudiness are connected with eastern and southern winds (SE - 9.4), while in the case of western and southern--western winds the cloudiness is reduced (W - 6.9, WSW - 7.2) - Fig. 7A. This is connected with the foehn phenomena. When the air flows over an elevation like King George Island, a region with reduced cloudiness is formed at the windy (western) side of the Island and behind it a foehn wall is built with lenticular types of clouds (Boniewicz etal. 1981). The occurrence of die foehn phenomena is con­firmed by the analysis of relative humidity according to wind direction. A signifi­cant decrease in the air humidity can be observed in the case of southern-western advection (SW - 79%). Air masses flowing from east (NE, SE, S - 88%) are char­acterized with high water vapour saturation (Fig. 7C).

The highest temperatures appear with winds from the north (NNW 2.1°C, NW 1.6°C, N 1.5°C) and the lowest ones with winds from east and south (ESE -4.9°C, SE -3.2°C) - Fig. 7B. The values of these deviations from the average are compara­ble with the conditions occurring in the warm year 1985 (Styszyńska 1990). The occurrence of extreme temperatures is also conditioned by circulation. The highest air temperature (8.9°C) occurred on 22 January and 19 February with NW wind and the lowest one (-15.1°C) with an eastern advection.

Conclusions

In 1996 meteorological conditions at the Arctowski Station were formed by cy­clonic circulation (79.0% of the days), with frequent advection of warm and humid air masses from the west. The weather conditions at the Arctowski Station are also influenced by local orographic conditions and by wind patterns connected with

218 Marek Kej na and Kamil Laska

them. In 1996 winds from the Ezcurra Inlet (NNW - 13.2%), from WSW (11.3%) from Warszawa Icefield and from SE (8.7%) from the open water of Admiralty Bay were most frequent. Winds from the directions NNW (11.0 m/s) and from SE (7.0 m/s) are the strongest.

In comparison to the long term average (1978-1989), the air pressure in 1996 was lower (989.3 hPa) - Table 3. Higher wind velocities have been observed (7.1 m/s). The insulation conditions were very bad, the cloudiness was very high (8.3 in the scale 0-10) and there were only 3 clear days (C<2). The number of hours with sunshine reached only 760.3 (19.8% of the possible sunshine duration), which was 60 hours less than the average long-term total of sunshine duration. The annual mean air temperature was -0.6°C, which was about 1°C higher than the normal tem­perature. In every month positive thermal anomalies occurred (in July even 4.2°C) and the winter was an especially mild one (kernlose effect). Atmospheric precipita­tion (479.2 mm) was lower than the average long term total. Permanent snow cover was maintained from the beginning of June until the end of October, and its maxi­mal thickness was 73 cm in September.

As a result of the maritime character of the weather and the mild winter, the Admiralty Bay and even the sea in the surrounding of King George Island did not freeze in 1996.

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Received April 27, 1998 Accepted September 16, 1998

220 Marek Kejna and Kamil Lśska

Streszczenie W 1996 r. w czasie XX Wyprawy Antarktycznej Polskiej Akademii Nauk na Stację im.

H. Arctowskiego (<p = 62'10'S, X = 58'2S~W, h = 2 m n.p.m., Wyspa Króla Jerzego, Szetlandy Płd., Antarktyka) prowadzono pomiary meteorologiczne (rys. 1). Warunki pogodowe na Stacji im. H. Arctowskiego w 1996 roku były kształtowane przez bardzo ożywioną cyrkulację cyklonalną przynoszącą wilgotne i ciepłe masy powietrza z zachodu (tab. 1, rys. 2-5). Z mniejszą częstością, w porównaniu do okresu 1986-1989, wystąpiły antycyklonalne typy cyrkulacji, a zwłaszcza adwek­cje mroźnych mas powietrza z południa i ze wschodu.

W 1996 najczęściej występowały wiatry z kierunków: NNW -13,2% (znad Ezcurra Inlet), WSW -11,3% (znad Warszawa Icefield) oraz z SE - 8,7% znad otwartych wód Zatoki Admiralicji. Średnia prędkość wiatru była wysoka i wyniosła 7,1 m/s. Największe prędkości wiatru wystąpiły w lipcu (9,8 m/s). Najsilniejsze wiatry notowano z kierunków NNW (11.0 m/s) i SE (7.0 m/s) - rys. 6.

Średnie miesięczne i roczne wartości poszczególnych elementów meteorologicznych podano w tabeli 2, a ich roczny przebieg prezentuje rys. 8. W 1996 r. ciśnienie atmosferyczne było niższe w porównaniu ze średnią wieloletnią (1978-1989) i wyniosło 989,3 hPa (tab. 3). Przy dużym zach­murzeniu (8,3 w skali 0-10) wystąpiły zaledwie 3 dni pogodne (C<2). Zarejestrowano 760,3 godzin ze Słońcem (19,8% usłonecznienia możliwego) i jest to wartość niższa o 60 godzin od średniej sumy wieloletniej. Średnia roczna temperatura powietrza wyniosła -0,6*C i była wyższa o 1"C od średniej z wielolecia. Prawie we wszystkich miesiącach wystąpiły dodatnie anomalie temperatury powietrza (w lipcu temperatura była wyższa aż o 3,2'C). Łagodna zima spowodowała, że wody Zatoki Admiralicji nie zamarzły. Suma opadów atmosferycznych wyniosła 479,2 mm. Stała pokrywa śnieżna utworzyła się dopiero na początku czerwca i trwała do końca października, z maksymalną grubością 73 cm we wrześniu.

Warunki pogodowe na Stacji im. H. Arctowskiego zależą od kierunku adwekcji i rodzaju napływających mas powietrza oraz od czynników lokalnych: orografii, ekspozycji względem domi­nującej zachodniej cyrkulacji oraz ekspozycji względem Słońca. W czasie przepływu mas powietrza ponad wysokimi kopułami lodowcowymi otaczającymi Zatokę Admiralicji dochodzi do ich trans­formacji, występują zjawiska fenowe. W 1996 r. największe zachmurzenie wystąpiło przy wiatrach ze wschodu i południa (SE - 9,4), podczas gdy przy wiatrach z południowego zachodu było ono znacznie mniejsze (W - 6,9, WSW - 7,2) - rys. 7. Także wilgotność względna powietrza była niższa przy wiatrach z SW (79%), powietrze napływające z NE, SE i S było bardziej nasycone (88%). Najwyższe temperatury powietrza na Stacji im. H. Arctowskiego wystąpiły przy wiatrach z północy (NNW 2,rC, NW 1,6"C, N 1,5'C) a najniższe przy wiatrach ze wschodu i południa (ESE -4,9'C, SE -3,2"C).