NEGATIVE TREND OF ANNUAL PRECIPITATION SUM AT THE ARCTOWSKI STATION

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1 Andrzej A. MARSZ Department of Meteorology and Nautical Oceanography The Gdynia Maritime Academy Al. Zjednoczenia 3, Gdynia POLAND POLISH POLAR STUDIES XXVI Polar Symposium / f t Lublin, June 1999 NEGATIVE TREND OF ANNUAL PRECIPITATION SUM AT THE ARCTOWSKI STATION UJEMNY TREND ROCZNYCH SUM OPADOWYCH NA STACJI IM. H. ARCTOWSKIEGO FORMULATING THE PROBLEM The problems of precipitation in the region of the Arctowski Station (Polish Antarctic Station), have not been discussed in literature in a detailed way so far. The characteristics of sums and distribution of precipitation worked out on the basis of a 4-year limited observational series were shown in a paper by Marsz and Rakusa-Suszczewski (1987). The basic data concerning precipitation at the Arctowski Station in can be found in the by Rakusa-Suszczewski, Miętus et al. (1992). The precipitation analysis reffering to the ice balance at the Arctowski Station based on not continuous series of measurement of precipitation in (continuous annual measurements series covered 11 years only) was made by Marsz (1994). A complete series of annual precipitation sum measurements covers 16 years only, from the time the Polish Antarctic Station, the Arctowski meteorological station was founded (1977) till The following years were covered by complete full measurements series: , , , The observational period was both too short and interrupted so it cannot form the basis to draw further conclusions regarding the nature of the climate there. Nevertheless, the precipitation is one of the essential climatic elements in this area as it has direct and indirect influence on some features of the environment. It affects the water balance in the area of the Admiralty Bay, it influences the amount of heat exchange between land and atmosphere, influences the temperature of the ground and controls, to a great extent, the albedo of the surface, etc. What is most important, precipitation is one of the most essential elements of glaciation mass balance (in the vicinity of the Arctowski Station glaciation areas prevail). The balance of glaciation areas influences many short and long-term physical and

2 Andrzej A. Marsz 162 geographical processes being of primary importance for the development of the environment and landscape and finally these processes are followed by biotic processes. The above mentioned facts are arguments in themselves proving the need to describe at least a few typical features of precipitation at the Arctowski Station regardless of the objections to observational material. The aim of this paper is to pay attention to a negative trend of annual precipitation sum at the Arctowski Station, during the 20 years when the meteorological measurements were carried out. GENERAL VARIABILITY IN ANNUAL PRECIPITATION SUM IN In the years , i.e. during meteorological observational period at the Arctowski Station, the annual precipitation sum is observed as decreasing rapidly by leaps. The course of annual precipitation sum indicates that three periods can be distinguished in the analysed period of The first period covers years , the second period covers years and the third period For the first period the mean annual precipitation sum amounted to about 560 mm (an = 26 mm) and within limits mm. Five times during this period the monthly precipitation sum exceeded 100 mm (see Tab. 1, Fig. 1). Table 1. Annual, maximum and minimum monthly precipitation sum in a given year at the Arctowski Station Year Annual sum (mm) Mean monthly sum (mm) month # Minimum monthly sum (mm) month (?) 05 (?) (?) 12 (?)

3 Negative trend of annual precipitation sum at the Arctowski Station 163 7CO e с I cc cc Э5 19S Э Ш SR. ROCZ Y E A R Fig. 1. The course of annual precipitation sum at the Arctowski Station ( ) The second period lasted 4 years and was characterised by a very large variability in precipitation, the first three years: 1986, 1987 and 1988 had a very small, rapidly decreasing precipitation sum - from 456 mm to 377 mm and the last year 1989, had a very rapid increase in the annual precipitation sum, which amounted to 630 mm and has been the largest annual precipitation sum measured at the Arctowski Station so far. The mean precipitation sum of the second period amounted to mm and the standard deviation is unusually large (an = 95.4 mm). The third period is characterised by the decrease in variability and relative stabilisation of precipitation sum at a lower level. The mean precipitation sum for five observational years amounts to (an = 26.1 mm) and is 100 mm lower than the sum in the first period. The range of variability of annual precipitation sum is limited by mm (maximum from 1995) and 422 mm (minimum from 1993). It should be noted that the maximum annual precipitation sum in the third period is lower than the minimum sum from the first period. What is also typical of the third period is that the monthly precipitation sum, lower than 10.0 mm (!), was more frequent than in the preceding periods. For an extreme oceanic climat it is not much.

4 Andrzej A. Marsz 164 PERIODICITY OF PRECIPITATION SUM The spectral analysis of the course of precipitation sum indicates the occurrence of clearly marked periodicity. The course of annual precipitation sum of five largest values on the periodogram shows the following frequencies: (6-yearperiodicity), (2-year-periodicity), (4.5-year-periodicity), (2.57-year-periodicity) and (9.0-year-periodicity). The spectral analysis of the 6-year-periodicity proves to be most dense, and the 4.5-year-periodicity turns to be only a bit less dense (see Fig. 2). 660 i о e 540 «zt Я I SCO «: о Ó cc о 420 у = A x j > YEAR Fig. 2. Periodicity of the course of annual precipitation sum at the Arctowski Station (spectral density) The course of monthly precipitation sum from March (precipitation maximum) and for August (precipitation minimum) was also analysed using Fourier (spectral) analysis. The largest values on the periodogram for March are the following: (3.5-year-periodicity), (6-year-periodicity), (2.57-year-periodicity), (3-year-periodicity) and (4.5-year-periodicity). The largest spectral density have a 3.6-year-periodicity and 4.5-year-periodicity. The course of precipitation sum for August shows the following frequencies (3.6-yearperiodicity), (6-year-periodicity), (2-year-periodicity) and (18-year-periodicity), wheras the 4.5-year-periodicity and 3.6-year-periodicity show largest spectral density. The carried out analysis shows, that the 2.0-year-periodicity, 3.6-year-periodicity and 6-year-periodicity are basic elements common for the entire precipita-

5 Negative trend of annual precipitation sum at the Arctowski Station 165 tion variability cycle. The sum of harmonics of a 3.6-year-periodicity and 6-yearperiodicity results in 2.25-year-periodicity (not mentioned before), whereas the result of substraction of these two harmonics results in a 9-year-periodicity. The result of substraction of frequencies (2-year-periodicity) and (3.6-year-periodicity) results in a 4.5-year-periodicity. It is difficult to find out the reasons for periodicity in the precipitation occurring so explicitly. The observational period was not long enough to draw conclusions of greater importance and especially as it is impossible to carry out long-term periodicity analysis (longer than 10 years). Still the occurrence of 2.6-year-periodicity is worth being noted as it is also observed in many hydrometeorological phenomena of the North Atlantic and its vicinity (Sukhovej 1977). LONG-TERM TREND The analysis of amplitudes and phases of periodicity does not explain so explicit a decrease in precipitation sum in the analysed period. The analysis shows that the series of measurements of precipitation sum record the existence of a long term decrease trend (see Fig. 3), which can be described as: RR (annual sum, mm) = x, [1] x - number of years since Hamming weghł: Period (years) Fig 3. The trend of annual precipitation sum at the Arctowski Station. The 95% confidence band marked

6 166 Andrzej A. Marsz This trend is significant from a statistical point of view (p < 0.023, F (1; 14), r = -0.56, and standard estimation error (BSE) of RR (annual sum) estimated with the help of [1] correlation equals ±63.2 mm. The analysis of the courses of maximum and minimum monthly precipitation sum in a given year shows the presence of negative trends in both cases. In case of maximum monthly precipitation sum in a given year, the value of a yearly decrease amounts to 2.3 mm and in case of minimum sum it amounts to -0.4 mm. Similar negative trends in the years show days with measurable precipitation during the year (-1.22 days per year) and mean annual day and night precipitation sum during days with measurable precipitation ( mm per year). The negative trends are noted in the course of 8 monthly sums, where 4 months show the negative trend values exceeding 1 mm per year. These are values of precipitation sum from February (-2.2 mm per year - significant trend from the statistics point of view - 3%) from March (2.7 mm, insignificant), April (-1.8 mm per year, insignificant) and December (-1.3 mm, insignificant). The negative trend of the precipitation sum has not been described in literature for the region of the Antarctic Sea. The fact is that even works devoted to the analysis of precipitation in that region are few as the greatest attention is paid to temperatures. The positive trend of precipitation sum is found to be on the western coast of the Antarctic Peninsula (Ackley, Bentley et al. 1996; Turner, Colwell, Harangozo 1997). Many scientists (see: Smith and Sterns 1993 and review article by Ackley, Bentley et al 1996) relate the occurrence of certain atmospheric processes in the Subantarctic and Antarctic areas to the formation of the SOI (South Oscillation Index) value. Such relations between SOI and the values of atmospheric pressure and between SOI and the air temperature have been indicated in different periods of time. The analysis of relation between annual precipitation sum at the Arctowski Station and annual values of SOI (according to Ropelewski and Jones 1987) did not prove to be significant from the statistical point of view. The reason for such results might be found in too short and not continuous series of precipitation measurements at the Arctowski Station. Still the present relations indicate that: a) both the trend of annual precipitation sum and the trend of annual values of SOI in is negative (the same signs), b) the strongest link between annual precipitation sum and values of annual SOI is removed in time. The course of changes in SOI advances the course of changes in annual precipitation sum and the maximum precipitation amount in the same year (the same signs) by one year and by three years it advances the amount of minimum precipitation in a given year (see Tab. 2). The values of SOI index have been obtained from NASA (Climate Research Unit Southern Oscillation Index; Lee Kyle).

7 Negative trend of annual precipitation sum at the Arctowski Station 167 Table 2. The values of correlation coefficients between the values of SOI in the present year and the preceding years and the values of annual precipitation sum and the monthly minimum and maximum in the same year at the Arctowski Station Precipitation sum at the Arctowski Station the same year annual sum annual max. annual min. Annual SOI Years ago Present year In spite of the fact, that none of the correlation coefficients included in Tab. 2 exceeds p < 0.05 the distribution of correlation coefficients seems to indicate a certain order. c) In shorter periods of time (the mean monthly values of SOI Index, the monthly precipitation sum) the synchronic relations have not been noted. There are significant relations from the statistical point of view (p < 0.05) between the precipitation sum in January and February and the values of SOI Index at the end of winter and in spring months (August-December). This relation has an opposite character for January (negative indexes of correlation), for February - the same (positive indexes of correlation, see Fig. 4). For precipitation occurring during the remaining months such relation was not found. The precipitation from January (RRqj) can be aproximated as a function of SOI: RR 0 1 = S O I I I p y S O I ^ [2] 2 (R = 0.82, adj. R = 0.62, F (2;14) = 13.99, p < , BSE = ± 9.9 mm) The precipitation from February (RRo2): RR 0 2 = S O I 0 9 p y S O I 0 8 p y [3] 2 (R = 0.81, adj. R = 0.59, F (2;13) = 12.04, p < , BSE = ±14.2 mm), where: S O I l l p y, SOI 0 4 p y, SOI 0 9 p y, SOI o g p y - indexes of SOI are respectively for months: November, April, September and August of the previous year. The above described facts seem to indicate, that relations between the amount of precipitation on King George Island and the formation of atmospheric circulation in the equatorial zone of the Pacific should seriously be considered. The occurrence of asynchronic relations indicates the presence of indirect links of inertial character. The thermic state of the ocean may be one of those indirect links as it is indicated by relatively strong asynchronic relations between the temperature of some parts of the Southern Ocean and the course of mean monthly air temperature in the area of the Admiralty Bay. That phenomenon has recently been discovered and noted by A. Styszyńska (in print). There are also many other papers (not Practically between atmospheric circulation above this area. Changes in the precipitation sum and changes in their distribution in time from one of the effects of changes in atmospheric circulation.

8 168 Andrzej A. Marsz Fig. 4. The formation of coefficients of correlation between monthly precipitation sum of January (01) and February (02) at the Arctowski Station and monthly SO indexes of the previous year. The 0.05 significant level marked quoted here) treating the ocean as the inertial link which moves in time the effects of earlier changes in atmospheric circulation ( oceanic memory"). It is difficult to evaluate the obtained results seeing that there are so many observational gaps. Another problem not to be solved here and now is how long the present trend is going to last, and whether it is only a local phenomenon, typical of the Arctowski Station area, or if its range is larger and covers all the Southern Shetlands. THE ENVIRONMENTAL EFFECTS There is no doubt that the negative trend of annual precipitation sum is followed by the possibility of very large changes in the environment of this area. Observations of the snow cover limits, especially snow-patches, and other elements of the landscape of the Arctowski Station and vicinity seem to prove the fact of the decrease in the precipitation sum. Year by year in summer the disapperance of small lakes and surface meltwater streams and seasonal streams hastening the ablation of glacier's ice when compared to previous period, the drying of the surface of coastal terrace where the Arctowski Station is situated, can be observed. These statements are based on the author's own observations carried out in summer at the Arctowski Station in 1977/78, 1978/79, 1984/85 and 1987/88. Similar facts were

9 Negative trend of annual precipitation sum at the Arctowski Station 169 observed by G. Kruszewski in 1995 (personalcommun.) and S. Rakusa-Suszczewski in 1997/98 (personal commun.). Strong ablation of marginal parts of glaciers reaching the land area as well as intensive recession of ice cliffs in the area of the Admiralty Bay, mentioned by many authors and easily to be examined on consecutive charts, satellite and airplane pictures (Furmańczyk 1983, Marsz 1996 et al.) reflect to some extent the significant decrease in precipitation sum and not only the rise in air temperature (not significant from the statistics point of view, see: Rodriguez et al. 1996). The masses exchange in the glaciation system on King George Island is very fast (Bryazgin and Govorukha 1986, Zamoruev 1972) that is why the reaction of marginal parts of glaciers to changes in precipitation sum is also very fast. So the changes in climatic conditions which are observed at the Arctowski Station have a much wider spectrum than previously assumed. They cover not only the temperature but they follow an untypical direction (as it was believed till now, see Dolganov 1986) of Subantarctic, i.e. the increase in temperature is not accompanied by the increase, but by the decrease in precipitation. How persistent this tendency is and how typical of that region it is, these and similiar questions could be answered in the course of further research. REFERENCES S., BENTLEY С., FOLDVIK A., CLARKE A., KING J., PRIDDLE J : A summary of Global Change in the Antarctic. ANTARCTIC Global Change Research No 2. Newsletter of the SCAR Global Change Programme: BRYAZGIN N. N GOVORUCHA L. S : Osobiennosti klimata i sovremiennogo oledeneniya o-va King Dzordz (Waterloo). Metorologicheskie issledovaniya v Antarktike. z. II. AANII. Leningrad: DOLGANOV L. V : Atmosfernye usloviya Yużnoj polyarnoj oblasti. Gidrorneteoizdat, Leningrad: pp FURMAŃCZYK К : Prace fotointerpretacyjne w Polskiej Stacji Antarktycznej im. H. Arctowskiego w sezonie badawczym / 7 9. [In] Teledetekcja w badaniach środowiska geograficznego. Prace Naukowe Uniwersytetu Śląskiego, nr : MARSZ A : Opady na Stacji Arctowskiego. Problemy Klimatologii Polarnej, 4 : MARSZ A. 1996: Procesy kształtujące morfologię brzegów współcześnie rozwijających się fiordów (na przykładzie Hornsundu i Zatoki Admiralicji). Prace Wydziału Nawigacyjnego WSM w Gdyni, 3: MARSZ A., RAKUSA-SUSZCZEWSKI S : Charakterystyka ekologiczna rejonu Zatoki Admiralicji (King George Island, South Shetland Islands). I. Klimat i obszary wolne od lodu. Kosmos, 36 (1): 103ACKLEY 127. PIASECKI J. 1992: Pogoda i klimat. [In:] Zatoka Admiralicji. Praca zbiorowa pod red. S.Rakusy-Suszczewskiego. Oficyna Wydawnicza IE PAN, Dziekanów Leśny: 41- RAKUSA-SUSZCZEWSKI S., MIĘTUS M 50. Analysis of the mean and extreme temperature series of the Arctowski Antarctic Base. Problemy Klimatologii Polarnej, 6 : ROPELEWSKI С. F., JONES P. D. 1987: An extension of the Thaiti-Darwin Southern Oscillation Index. Monthly Weather Review, 115: SMITH S. R STEARNS Ch. R. 1993: Antarctic climate anomalies surrounding the minimum in the Southern Oscillation index. Antarctic Research Series, vol. 61. Antarctic Meteorology and Climatology; Studies Based on Automatic Weather Stations. Ed. D. H. Bromwich, Ch. R. Stearns. Washington, D.C.: RODRIGUEZ R LLASAT C. M RAKUSA-SUSZCZEWSKI S :

10 Andrzej A. Marsz 170 J., COLWELL S. R., HARANGOZO S : Variability of precipitation over the coastal western Antarctic Peninsula from synoptic observations. Journal of Geophysical Research, vol. 102, D12, TURNER V. F. 1977: Izmencivost' gidrologiceskikh uslovij Atlanticeskogo okeana. Naukova Dumka, Kiev. pp ZAMORUEV V. V : Rezultaty glaciologiceskikh nabludenij na Stancii Bellingshauzen v 1968 g. Trudy SAE, 55: SUKHOVEJ STRESZCZENIE Artykuł omawia zmienność rocznych sum opadowych zarejestrowanych na Stacji Arctowskiego w latach Roczne sumy opadów wykazują dużą zmienność, można wyróżnić w ich przebiegu trzy okresy. W okresie średnie roczne sumy opadowe wynosiły 560 mm (cm = 26 mm), w następnym okresie ( ) sumy opadowe charakteryzowała bardzo silna zmienność (min = 377, maks. = 630 mm), średnia suma opadowa wynosiła 472 mm przy on = 95.4 mm. W ostatnim, trzecim okresie ( ) średnia suma opadów wynosiła 456 mm (an = 26.1 mm) (tab. 1, fig. 1). W biegu rocznych sum opadowych wykryto występowanie silnej okresowości 6.0, 2.0,4.5, 2.57 i 9.0-letniej (fig. 2). Przeprowadzono również analizę spektralną biegu sum opadów miesięcznych w marcu (maksimum opadowe) i w sierpniu (minimum opadowe). Analiza wykazała istnienie silnych okresowości wspólnych (dla sum rocznych, miesięcznych: minimalnych i maksymalnych): 2.0, 3.6 i 6.0-letniej. Analiza amplitud i faz okresowości nie wyjaśnia występowania tak dużej obserwowanej zmienności sum opadowych. Występuje tu silny trend ujemny rocznych sum opadowych, istotny statystycznie (patrz fig. 3, formuła 1). Ujemne trendy wykryto również w biegu średnich miesięcznych sum opadowych 8 z 12 miesięcy w roku (najsilniejszy i istotny statystycznie - w lutym), w biegu liczby dni z opadem mierzalnym w roku, średnią roczną dobową sumę opadu). Ujemny trend sum opadowych na Stacji Arctowskiego jest niezgodny z sygnalizowanym (Ackley S Bentley С., Foldvik A., Clarke A., King J Priddle J. 1996; Turner, Colwell, Harangozo 1997) dodatnim trendem sum opadowych jaki zaznacza się na zachodnich wybrzeżach Półwyspu Antarktycznego. Badanie związków sum opadowych na Stacji Arctowskiego z wartościami Southern Oscillation Index wskazuje, że najsilniejsze związki między rocznymi i maksymalnymi sumami opadowymi w danym roku zaznaczają się z jednorocznym opóźnieniem (SOI roku poprzedniego -»opady roku bieżącego), zaś w przypadku sum minimalnych z opóźnieniem trzyletnim (tab. 2). Istotne pod względem statystycznym związki między miesięcznymi sumami opadów na Stacji Arctowskiego a wartościami SOI zaznaczają się dla stycznia i lutego (fig. 4, najsilniejsze korelacje z wartościami SOI z miesięcy końca zimy i wiosennych poprzedniego roku). Ujemnemu trendowi SOI w okresie odpowiada ujemny trend rocznych sum opadowych na Stacji Arctowskiego. Omówiono również krótko obserwowane skutki środowiskowe, potwierdzające zmniejszanie się sum opadowych na Stacji Arctowskiego, (zmniejszanie się powierzchni śnieżników, zanik jeziorek i cieków okresowych, osuszanie powierzchni teras nadmorskich, przyspieszanie ablacji czystego lodu na otaczających lodowcach, przyspieszenie procesów deglacjacji, etc.).