LONGTERM CHANGES IN AIR TEMPERATURE AND PRECIPITATION ON SZCZECIŃSKA LOWLAND

Bożena Michalska¹, Eliza Kalbarczyk^2
1 Department of Meteorology and Climatology, West Pomeranian University of Technology in Szczecin, Poland
² Department of Meteorology and Climatology, Agricultural University of Szczecin, Poland

ABSTRACT

The research has been aimed at studying both seasonal and annual changes in air temperature and precipitation over the past 40 years, i.e. from 1961 till 2000, as well as in 2001-2003 period. Additionally increase/decrease rates for all the parameters under analysis were to be determined, as well as thermal and precipitation rating of months over the studied period. The research was based on data recorded over respective 1961-2003 decades and months for air temperature and precipitation that were collected at Lipki agro- and weather station, near Stargard Szczeciński. Magnitude of the standard deviation (S) from the standard from 1961 to 2000 was applied as a criteria for thermal rating all the months over the studied period. To evaluate deficiency or excess of precipitation in particular months a criteria of percentage of total monthly precipitation over the standard value (75 - 125%) was applied.

Monthly, seasonal and annual changes for both air temperature and precipitation tend to be linear. Over the studied period for all the moths, excluding November, a rising tendency for air temperature was observed. A statistically significant, positive trend was observed for April, May, July and August. The biggest increase for temperature, on average by 0.8°C/10 years, was found for January. No clear, solid trends have been detected for precipitation. At the beginning of a year a rising tendency was noticed, while towards the month end precipitation tend to drop. Still, only in May the trend was statistically significant and found negative.

The year 2003, whose summer season was found exceptionally hot and very dry, was also evaluated in terms of temperature and precipitation.

Key words: air temperature, precipitation, variability, Szczecińska Lowland.

INTRODUCTION

The last few decades brought changes to Earth's natural environment that result not only from natural factors, but were also induced by human activities [3,12,14]. The most striking difference in comparison to the past eras, is the growing rate of occurring changes that no longer can be measured in thousands of years, but rather in decades [2,7,11,13]. It is a consequence of increased population and civilisation progress. Over the last decade, natural environment in Poland, especially in the Polish Lowland area has become progressively drier. Soil has become increasingly water deficient, series of dry years have been recorded, and small water ducts and reservoirs cease to exist. On the other hand, years of high precipitations tend to recur, bringing disastrous floods [7]. As Lorenc [10] claims, apparent results of climate warming have been detected. Precipitations have become more intense, short but heavily violent downpours on local scale tend to reappear more regularly and make up for the monthly average standard precipitation.

It seems that recently we entered the next drought cycle. The latter one began in Poland in 1982 and precipitation-deficiencies, despite incidental breaks, persisted till 1995. Over this period total precipitation were lower than the average by 10%. Year 2003, alike 2002, apart from being a very hot one, was also marked with another feature of being exceptionally dry. Annual precipitation did not even reach 70% of the mean value over years, whereas average air temperature was higher by 1° C.

The aim of the presented research was to study both seasonal and annual changes in air temperature and precipitation over the past 40 years, i.e. from 1961 till 2000, as well as in 2001-2003 period. In addition, increase/decrease rates for all the parameters under analysis were to be determined, as well as thermal and precipitation rating for months over the studied period.

MATERIAL AND METHODOLOGY

The data recorded at agro-meteo station in Lipki near Stargard Szczeciński over the period 1961-2003, for decade and monthly temperature and precipitation measurement have been analysed in the presented research. The station is located in the central part of Szczecińska Lowland (53°21'N, 14°58'E, Hs 30 m above see level) which makes the measurements and results representative not only for the region, but also for the west part of Pomerania [8]. Statistical features for temperature and precipitation are presented in Tables 1 and 4 which contain mean values over 1961-2000 as well as standard deviations, whereas for 2003 excluding mean values for temperature and total precipitation, deviation from the standard one are also given. A linear trend was found for both changes in temperature and precipitation. Slopes of the regression lines, determined in relation to 1961-2000 period, enabled to evaluate increase or decrease rate for the parameters under analysis.

Thermal rating was performed by relating an air temperature deviation from the standard value over the concerned period (a month, season, year) to a specific margins of the standard deviation (S), following the Lorenc's [10] pattern, i.e.:

temperature deviation from the standard varies

To complete such characteristics plots illustrating air temperature standardised deviation from monthly, seasonal, and annual standards (1961-2000) over the years covered by the research were added.

To asses precipitation deficiency and excess in a particular month a criterion of percentage ratio of total monthly precipitation to the standard one [6] was applied with the following margins:

precipitation as percentage of the standard value for

Total decade precipitation against air temperature changes are presented on Walter's and Lieth´s [15] climate diagram. Precipitation and temperature for the warm half of 2003 are marked in the chart in a 2:1 ratio, i.e. 20 mm of precipitation corresponds to 10° C temperature. Analysis of both curves allows to determine precipitation deficiency and drought fields that occurred in 2003.

RESULTS

Annual mean air temperature in Lipki (1961-2000) reached 8.2° C (Table 1). January proved to be the coolest month of the studied period (-1.1° C), while the hottest one was July (17.5° C), but the highest and lowest decade mean values for temperature was recorded respectively in the first decade of August and in the first and second decades of January. A raise in temperature was observed for all months, excluding November. The temperature raise was particularly pronounced in the 90-ies, which is confirmed by regression equations presented in Table 2. Though 2003, when set against the studied period from 1961 till 2000, appeared warm (see Fig. 1) with average temperature higher than the standard one (8.2° C ) by 1° C, it was 2000 that has proven to be the warmest of the studied period (see Fig. 2) for which annual average temperature reached as high a level as 10.3° C. It was higher than the standard one by 2.1° C. The most distinguished feature of 2003, however, was its hot and dry summer. Two months, namely June and August were exceptionally warm, and July very warm (Fig. 3). The greatest decade deviation from the standard was recorded in the first decade of June, and it equalled 6.2° C. Temperature in all the other months was significantly higher then the average, with the only exception of first decade of July (see Table 1 and fig. 1). After the scorching summer, temperature had drastically dropped down in October, especially in its 2^nd and 3^rd decades, which led to rating the month as anomalously cold. Similar effect was observed in 1992 and 1994 (Fig. 2).

The statistical analysis of the standardised temperature deviations from the monthly standard temperature for years between 1961 and 2000, has proven the temperature rise over the studied 43 years to be statistically significant for most months (Fig. 3). The trend was particularly clear in spring. In April and May temperature rise was found highly significant (respective R² of 16.6 and 26.2%), while in March significant (R² - 9.7%). For the entire period between March and May the highest value of determination coefficient R² was obtained, namely 31.8%, which manifests the largest increase in temperature deviation from the standard value (Fig. 4). A relatively clear rising trend for the analysed deviations in spring was observable already at the beginning of 80-ties, though anomalously cold March and cool April and May of 1987 inhibited the growth rate, and a steady rise was clearly observed at the end of 80-ties and it continued to develop consistently until the end of the past decade (Fig. 4).

Summer is another season characterised by a highly significant rising trend for temperature deviations from the standard (R² - 27.4%). Though June is known in general for only rising trends, the last 6 years, excluding 2001, were hotter than the standard one, and June 2003 proved to be the hottest month of all 43 studied, with the overridden value over years by 3.6° C (Table 1, Fig.2). Moreover, July and August over the past years tend to be increasingly hot, with respective rise in temperature above the norm amounting on average over 10 years 0.6 and 0.7° C (Table 2).

However, over the studied 43 years neither statistically significant rise nor decline in temperature in autumn have been observed. Warm years in all the months from September to November have been interchangeably following the cooler ones. In 1993 an anomalously cold autumn occurred, while another warm anomalously was observed in autumn 2000 (Fig. 4). November, is the only month of a year when a negative temperature trend was recorded, though it was statistically insignificant.

For winter, which similarly to spring and summer of 1961-2003 period tend to be increasingly milder, the rising trend in temperature deviation from the standard has proven statistically significant (R² - 11.1%), while an average rise in temperature was equal to 0.3° C/10 years. The biggest temperature increase were recorded in January (0.8° C/10 years) and the trend was statistically significant, while the lowest ones in December with the level of significance about p £ 0.1.

In Table 2 regression equations for a monthly, seasonal and annual mean air temperature dependencies on the studied years are presented. Slopes of regression lines for 1961-2000 period indicate that the temperature was raised in all months excluding November. Annual average temperature has been rising at a rate of 0.45° C/10 years for the last 40 years.

Careful analysis of temperature over the specific decades of the studied period between 1961 and 2000 revealed a steady rise in the annual average temperature from 7.6 in 1961-1970 decade to 9.0° C in 1991-2000 decade (Table 3). The highest temperature rise of 0.6° C was observed between the 80-ties and 90-ties, and the highest value of standard deviation for annual mean temperature was recorded in 80-ties, which most likely resulted from vastly varying winter temperatures. Percentage ratio of month with temperatures higher than the average value to the overall number of months in 1961-2000 period, was found similar for 60-ties and 70-ties, i.e. of the values of 5.4% and 5.2%, respectively, only to start picketing in the 80-ties to a value of 7.9% and reached the highest level of 12.9% in the 90-ties. The biggest number of months with temperatures lower than normal fell to 60-ties and 70-ties with respective percentage ratio of 8.9 and 9.2%, while the lowest one, namely 6.0%, occurred during the closing decade of the past century - see Figure 3.

Average total annual precipitation in Lipki amounted to 539.1 mm. The lowest and highest total precipitation values over years were recorded respectively for February and July, reaching the level of 24.1 mm and 66.6 mm respectively (see Table 4). Analysis of monthly total precipitation changing over the studied years has proven precipitation to drop in May, with a significance level not lower than p £ 0.05, and to rise in March (significance level p £  0.1). For the remaining months at the beginning of a year an ascending trend for total precipitation was observed, while towards the end of a year it tend to descend (see Table 5 and Fig. 7).

The highest total annual precipitation were recorded in the period 1961-1970 and from then on it started to decrease, though the difference between 80-ties and 90-ties was really small - only 0.5 mm. (Table 3). It tend to vary greatly in winter, while in summer the changes are found the smallest. Overall precipitation rating in 1961-2000 gave 177 dry, very dry and extremely dry months, whereas 140 wet, very wet and particularly wet months, which contributed respectively to 37% and 29% of the overall number of moths in the concerned period (Fig. 7). The biggest number of months with precipitation below average was observed in the 70-ties, namely 49, while precipitation in 37 months in 60-ties and 90-ties was found higher than the average.

Year 2003 was rated as very dry, since total of 371.2 mm accounted for nearly 70% of the average standard (Table 4). Drought had started in December 2002 and lasted continuously till the end of June. Standard in terms of precipitation July, was followed by a very dry August, with only 16 mm of rainfall that accounted to scarce 28% of the average standard. The lowest total precipitation was recorded only in 1969 and 1982, but in none of them such a long lasting rainfall deficiency accompanied by so high temperatures as in 2003 occurred (Fig. 2 and 7).

Cumulative values of total precipitation over decades presented in Fig. 6, since the 1^st  decade of December 2002 reproduce more and more pronounced difference between values over years and values for 2003 that towards the year's end reached nearly 200 mm. Such low precipitation, in majority of months falling well below the standard value, consequently resulted in a drop of post-winter water resources in soil, soil drought and lowering water table level.

As from April to September temperature curves tend to overpass precipitation curve, Walter's and Lieth's climate diagrams. It indicates drought occurrence, especially at May-June turnover and in August. The most extensive deviations were detected in the 1^st June decade as well as in 1^st and 3^rd August decade, so these months were rated as very dry (Fig. 8).

DISCUSSION

The performed analysis revealed that in the central part of Szczecińska Lowland a positive trend for air temperature prevails nearly all year long. The highest rise in observed temperature over 1961-2003 period were found for January, August and July, namely from 0.76 to 0.6° C/10 years. The research on the global warning has proved that the mean global temperature estimated from atmospheric and ocean surface measurements performed from 1861 to 1991 rose on average by 0.5° C. In Poland, winters tend to be warmer, but the increase in temperature in not uniform [4]. In Warsaw temperature rises by 1° C/100 years, while in Cracow by 1.4° C and in Wrocław be a mere 0.1° C/100 years. Summer temperature rise is not that distinctive and amounts to 0.1 and 0.3 in Warsaw and in Cracow, respectively, while in Wrocław even a drop temperature by 0.7° C/100 years was recorded. The Values for temperature averaged over 5 weather station in Pomerania region show that over the past 50-years (1951-2000) 2 highest temperature increments were recorded for February and March, reaching respectively 0.6° C/10 years and 0.5° C/10 years (statistical significant at the level p £ 0.05), and undoubtedly the 90-ties have contributed to such a result that was the heaviest one. Less drastic temperature increase of only 0.3° C/10 years was observed for April and May. For the remaining months year-temperature correlation were not statistically significant, but it is worth noting that for June and November a negative tendency was determined (see [1]). Consistent over years temperature trends are also related to the duration of the thermal seasons. In Szczecin, from the data collected in years 1931-1998, a definite, statistically significant shortening of the winter season can be concluded, while springs and autumns tend to last longer and a clear trend for prolonged early spring and early winter were detectable, in contrast to shrinking summers [9].

Precipitation is not that clearly affected as air temperature. The data of the presented research prove neither for monthly nor seasonal total precipitation any statistically significant increase or decrease to occur, excluding May. It is known from Boryczko et all research. [3] that in Warsaw the highest anthropogenic tendency for monthly total precipitation decrease over 1813-1980 years is observed in August and June, namely -4.8 mm/100 years and -4.6 mm/100 years, respectively, while an increase of 6.3 mm/100 years was found in January. The results of Żmudzka research [16] on the precipitation variability in years 1951-2000 in the lowland areas of Poland revealed higher precipitation in spring and autumn, while lower ones for summer and winter, though monthly total precipitation trends were not consistent in terms neither rate, nor the direction of change. A growing trends in annual total precipitation - higher than 1 mm/year - were detected by this author in the northern slope of Pomeranian Lake District, however, it was statistically insignificant. Still, the 3^rd IPCC report, published in September 2001, predicted precipitation to intensify in a global scale in the 21^st century. For mean and high latitudes precipitation phenomena will tend to intensify, thus more frequent droughts and floods shall be expected.

SUMMARY

1. A growing trend in air temperature over the studied years was observed for all months, excluding November. A statistically highly significant, positive trend occurred for April, May, July, and August. The greatest increase in temperature - on average by 0.8° C/10 years - was found in January.

2. Temperature seasonal changes demonstrate a growing trend - the most significant of the value of 0.4° C/10 years in spring and summer, the smallest, statistically insignificant, in autumn.

3. The biggest number of months with a positive deviation from the standard temperature (>0.5 S) was found in 90-ties. The number accounted for 12.9% of the overall number of months of 1961-2000 period. Months that were anomalously and extremely warm (>2.0 S) were recorded in 90-ties and at the beginning of the present decade, particularly in spring and summer.

4. Such a consistent trend was not detected for precipitation. At the beginning of all years, a growing tendency was observed, while towards their ends a drop tend to occur. Only for May the detected negative tendency was statistically significant.

5. Over the studied 1961-2000 period the highest total annual precipitations were recorded in the 60-ties, while the lowest ones in 80-ties and 90-ties. A precipitation below 75% and above 125% of the standard value, was found respectively at the percentage of 37% and 29% of the overall number of months in the studied period.

6. Year 2003 was rated as warm and very dry, with an average annual temperature of 9.2° C that was higher than the standard over years by one standard deviation. The summer was the hottest of all in the researched 43-year period, June and August were extremely hot, and July was found to have been anomalously hot in terms of temperature. The average summer temperature was higher than the standard one by 3.3° C. Precipitation deficiency was first observed in December 2002 and it persist to continue by the end of August. Average precipitation in July did not manage to balance a deep water deficit due to high temperature recorded in the second and third decade of the month.

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