Limnological The influence Review of the North 9, 2-3: Atlantic Oscillation on variability of surface temperature of Lake Raduńskie Górne

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1 Limnological The influence Review of the North 9, 2-3: Atlantic Oscillation on variability of surface temperature of Lake Raduńskie Górne 121 The influence of the North Atlantic Oscillation on variability of surface temperature of Lake Raduńskie Górne Zbigniew Trusewicz, Maciej Markowski, Jacek Barańczuk University of Gdańsk, Department of Limnology, Dmowskiego 16a, Gdańsk, Poland, Abstract: The impact of the North Atlantic Oscillation NAO on the variability of the surface temperature of water of a lake of the Kaszubskie Lakeland was analysed. The analysis was based on data of the period concerning the air temperature in Borucino and surface temperature of water of Lake Raduńskie Górne, and the NAO indices (of Jones, Hurrell, Rogers). The basic research method was correlation analysis. A considerable impact of the atmospheric circulation with the inflow of the air from the west on the shaping of surface temperature of the lake, especially in winter months, was established. Key words: NAO index, lake temperature, correlation, Introduction The influence of the atmospheric circulation on the thermal conditions in Poland has been the subject of numerous climatologic studies. They covered the determination of the relationship between air temperature and circulation types (Niedźwiedź 1993), the influence of the configuration of the pressure field above Europe and the Atlantic on the shaping of the variability of thermal conditions (Miętus 1996, 1999), analyses of the correlation of temperature with circulation indices pressure gradients above the North Atlantic (NAO) (Marsz 1999; Marsz and Styszyńska 2001; Marsz and Żmudzka 1999) and on the northern hemisphere between 35 N and 65 N (Kożuchowski 1993). A characteristic feature of circulation influences on the thermal conditions in Poland are seasonally changing relationships between air temperature and circulation indices (Kożuchowski and Degirmendźić 2002 ) and a decrease of this relationship from the north-west of the country towards the south east (Ustrnul and Czekierda 2007). A significant relationship between air temperature in Poland and values of the NAO indices is observed especially in winter months. In winter, due to a considerable weakening of the inflow of solar radiation to the surface, an increase in the intensity of western circulation must cause a substantial increase in air temperature (Marsz and Żmudzka 1999), whereas in summer these relationships are weaker (Ustrnul and Czekierda 2007). In the warm season (from April to October) the air temperature depends mainly on changes in the longitudinal component of the atmospheric circulation. The maximum of its impact falls on May and September. In this period, the shaping of the baric field in areas situated north and east of Poland (especially in the profile crossing the Southern Baltic) becomes the decisive factor determining the direction and size of changes in the mean monthly value of air temperature in Poland (Kożuchowski and Degirmendźić 2002). Hence, the measure of the circulation influences on air temperature in this period are longitudinal circulation indices. In summer, the role of the mean level of pressure in the determination of thermal conditions increases as well - high pressure situations are favourable to insolation, and thus to temperature increase. The analysis of the contribution of the mean monthly air temperature to the determination of the value of the trend of mean annual temperature indicates that the positive trend is a result of a substantial increase in the air temperature of winter months. Due to that it seems that it is the easiest to estimate the observed in-

2 122 Zbigniew Trusewicz, Maciej Markowski, Jacek Barańczuk crease in air temperature above the area of Poland on the basis of the Hurrell index, i.e. the winter North Atlantic Oscillation index (Marsz and Styszyńska 2002). Methods The documentation material comes from the Limnological Station of Gdańsk University in Borucino. It covers the measurement data of water temperature of the years These measurements (including an evaluation of ice phenomena) are preformed in the shore zone of the lake using a scoop thermometer. The phase and intensity of the North Atlantic Oscillation (NAO) are characterised using numerous indices. The paper applies three of them: the NAO indices by Rogers, Hurrell and Jones. The Rogers NAO index is a normalised mean monthly difference in atmospheric pressure between stations Ponta Delgada (the Azores) and Akureyri (Iceland) (Rogers 1984). The Hurrell NAO index, i.e. mean winter index of the North Atlantic Oscillation, is determined like the Rogers index but only for the period December-March and quantifies the mean difference in atmospheric pressure between Lisbon and Stykkisholmur and Reykjavik (Hurrell and Van Loon 1997). The Jones index is the difference in atmospheric pressure between Gibraltar, situated within the zone of impact of the Azores anticyclone, and southwestern Iceland (Stykkisholmur/Reykjavik) (Jones et al. 1997). Values of the Jones index are the monthly, seasonal and mean annual values of the NAO index and, as a supplementary measure, values of the winter NAO index giving mean values from the same period for which the Hurrell and Rogers NAO indices are calculated. For the purposes of evaluation of relationships and connections between hydrometeorological parameters (surface water temperature and air temperature) and values of the NAO indices devised by Rogers and Jones, data of the years were used, employing the correlation analysis and the linear regression method. Using the same statistical tools, the relationships between the characteristics of ice cover of the lake (beginning of ice cover, its duration and disappearance) and the winter Hurrell NAO index were established. The strength of the established relationship was determined using the value of the linear correlation coefficient by Pearson. Fig. 1. Location of Lake Raduńskie Górne

3 The influence of the North Atlantic Oscillation on variability of surface temperature of Lake Raduńskie Górne 123 Study site description Lake Raduńskie Górne is one of the main lakes of the river-lake system of the upper River Radunia, draining the central part of the Cashubian Lakeland (Fig. 1). It is situated at the bottom of a postglacial channel at an altitude of 162 m. The lake area is ha, its maximum depth reaches 43 m, mean depth is 15.5 m. This reservoir stores 60.2 hm 3 of water. Lake Raduńskie Górce is a flow-through lake, situated on the course of the River Radunia. Despite that, the lake has a passive hydrological regime, which is indicated by the intensity of water exchange in the lake (0.46) and retention time of 2.14 (Borowiak 2000). This is a dimictic reservoir (Maślanka et al. 2005) with a four-phase cycle of annual temperature distribution. A special feature of Lake Raduńskie Górne is the structure of its water balance, in which the dominant role in input is played by the underground alimentation component, accounting for over 50% of the total value of lake alimentation (Okulanis 1981). Results The formation of thermal relationships of lakes depends on the mutual influence of numerous factors and processes which occur in the geographical environment. The main role, however, is played by meteorological elements. The distribution of water temperature in the surface layer corresponds mostly to the seasonally variable volume of the supply of radiation energy. This concerns mainly the water layer of a thickness about 2 m, where the most intense absorption of the infrared part of radiation occurs. Water is characterised by a poor thermal conductivity, hence it does not intensely transmit heat to the inside. The warming of deeper parts of water occurs due to convection and mixing of water by wind, waves and currents. With a weak impact of wind, turbulence transmission of heat to the inside of the reservoir is clearly limited, resulting in its increased accumulation close to the surface of the water table. In an average year of the multi-year period , a clear relationship between the mean monthly temperature of surface waters of Lake Raduńskie Górne and mean monthly air temperature (Fig. 2) was observed. This is a positive correlation. The mean annual value of water surface temperature of Lake Raduńskie Górne reaches 9.4 C and is by about 2 C higher than the local air temperature. In view of the results of the measurements performed since 1960 on other lakes of the region, the thermal conditions of the surface layer of water of Lake Raduńskie Górne can be regarded as typical of deep reservoirs of the Pomerania. The annual variability of the surface water temperature is generally close to the course of air temperature but has a more regular character. A comparison of the annual courses of the temperature of surface water and the air reveals a clear domination of periods of lasting higher values of water temperature (Fig. 3). These periods usually last 10 months. They can be observed most clearly in the phase of full summer and winter stagnation and during autumn circulation. Only in spring (April-May), during the disappearance of ice Fig. 2. Relationship between mean monthly surface water temperature of Lake Raduńskie Górne and mean monthly air temperature in the Limnological Station of Gdańsk University in Borucino (mean values of years )

4 124 Zbigniew Trusewicz, Maciej Markowski, Jacek Barańczuk Fig. 3. Changes in surface water temperature (dotted line) and air temperatures (continuous line) in the cycle of an average year of the period cover, are the values of air temperature close to water temperature in the lake. The phase of spring circulation is a period of weaker thermal activity of the lake. Statistical relationships between mean monthly, maximum and minimum surface water temperature of Lake Raduńskie Górne and the values of the Rogers and Jones NAO indices, determined as mean monthly values and mean of three subsequent months are presented in tables 1-5. Significant correlations of the Rogers NAO index with water temperature occur only in January, February and March (Table 1). The correlation coefficient reaches its maximum value in January with respect to maximum temperature and is From October to December, correlation coefficients do not exceed the value of r = 0.3 indicating a lack of significant relationship between the variables. The weakest correlations were observed between the Rogers NAO index and minimum water temperature (Table 1). This is understandable as the minimum temperature in winter months is the least variable parameter of water thermal conditions in the multi-year period (from -1 C to -2.5 C). Similar results were obtained when evaluating the correlation between lake water temperature and the Jones NAO index (Table 2). Only in January, February and March were the strongest correlations established between this index and minimum, mean and maximum Table 1. Matrix of correlation coefficients between mean monthly Rogers NAO index and minimum, mean and maximum temperature in the period October-March Characteristics X XI XII I II III Minimum temperature Mean temperature Maximum temperature Table 2. Matrix of correlation coefficients between mean monthly Jones NAO index and minimum, mean and maximum temperature in the period October-March Characteristics X XI XII I II III Minimum temperature Mean temperature Maximum temperature

5 The influence of the North Atlantic Oscillation on variability of surface temperature of Lake Raduńskie Górne 125 temperature (Table 2). The strongest correlation (0.68) was observed in February. In October, November and December, the correlation coefficient was not significant. The analysis of relationships between mean monthly, maximum and minimum surface water temperature of Lake Raduńskie Górne and mean 3-month values of the Jones and Rogers NAO indices reveals slightly stronger correlations. The strongest statistical correlations with minimum water temperature were observed in January, February and March. The correlation coefficients reached their maximum value in February 0.64 (Rogers index) and 0.73 (Jones index) (Table 3). Strong correlations of the Rogers and Jones NAO indices with mean surface water temperature also occur in January, February and March (Table 4). It can be observed that the correlation coefficients of oscillation indices with mean water temperature are slightly higher than with minimum water temperature. The maximum value of the correlation coefficient is characteristic of February 0.80 (Jones index) and 0.73 (Rogers index). Weak and strong correlations in the first three months of the year can also be observed between the Rogers and Jones NAO indices and maximum water temperature (Table 5). The correlation coefficients range from 0.40 to 0.80, and they reach their maximum value of 0.80 (Jones index) and 0.78 (Rogers index) in February. In October, November and December, the correlation coefficients do not exceed the value of r = Changes in surface water temperature in winter are manifested in ice phenomena. Ice cover starts forming when the temperature of surface waters reaches about 0 C and to a greater or smaller degree, depending on its thickness, insulates the water body from external factors, such as air temperature or wind. The relationships between the characteristics of the ice cover of Lake Raduńskie Górne, such as: formation of ice cover, its duration and disappearance, were analysed with reference to the Hurrell winter NAO index. The data concerning the NAO index as well as ice cover come from the winter half-years of the period The Hurrell NAO index determines the duration of ice cover most strongly. The correlation coefficient is 0.79 (Fig. 4). The duration of ice cover is inversely proportional to the value of the Hurrell NAO index. Slightly weaker statistical relationships occur between the Hurrell NAO index and the date of disappearance of ice cover (Fig. 5). The NAO index accounts for the variability of the date of ice cover disappearance only in 50%. Table 3. Matrix of correlation coefficients between mean 3-month Rogers and Jones NAO indices and minimum surface temperature of water of Lake Raduńskie Górne in the period October-March NAO index X (Sep. - Nov.) XI (Oct. - Dec.) XII (Nov. - Jan.) I (Dec. - Feb.) II (Jan. - Mar.) III (Feb. Apr.) Rogers NAO Jones NAO Table 4. Matrix of correlation coefficients between mean 3-month Rogers and Jones NAO indices and mean surface water temperature of Lake Raduńskie Górne in the period October-March NAO index X (Sep. - Nov.) XI (Oct. - Dec.) XII (Nov. - Jan.) I (Dec. - Feb.) II (Jan. - Mar.) III (Feb. Apr.) Rogers NAO Jones NAO Table 5. Matrix of correlation coefficients between mean 3-month Rogers and Jones NAO indices and maximum surface temperature of water of Lake Raduńskie Górne in the period October-March NAO index X (Sep. - Nov.) XI (Oct. - Dec.) XII (Nov. - Jan.) I (Dec. - Feb.) II (Jan. - Mar.) III (Feb. Apr.) Rogers NAO Jones NAO

6 126 Zbigniew Trusewicz, Maciej Markowski, Jacek Barańczuk Fig. 4. Relationship between the winter Hurrell NAO index and duration of ice cover (duration of ice cover given in days) Fig. 5. Relationship between Hurrell NAO index and date of disappearance of ice cover (date of disappearance of ice cover is given in days counting from 1 st January) Fig. 6. Relationship between NAO index and date of the beginning of ice cover (date of the beginning of ice cover is given in days counting from 1 st December)

7 The influence of the North Atlantic Oscillation on variability of surface temperature of Lake Raduńskie Górne 127 Statistically insignificant relationships were observed between the Hurrell index and the date of ice cover formation (Fig. 6). The correlation coefficient was 0.30, which can indicate a variability of thermal conditions of the winters in this period or a decisive role of the quantity of heat accumulated in the lake during summer. Conclusion 1. The North Atlantic Oscillation index does not constitute a satisfactory tool for evaluating the shaping of surface water temperature in large channel lakes of the Cashubian Lakeland. 2. Only in the winter months (from January to March, inclusive), in which the western flow of air masses dominates, is a strong relationship observed between surface water temperature in the lake and the values of the NAO indices. 3. The statistically strongest correlations of the Rogers NAO index (r = 0.70) were observed with maximum water temperature in January; whereas the weakest correlations with minimum water temperature. 4. The Jones NAO index is most strongly correlated (r = 0.68) with maximum and minimum water temperature in February. 5. Slightly stronger correlations can be observed between 3-month values of the NAO indices and mean monthly, maximum and minimum monthly values of surface water temperature of Lake Raduńskie Górne. 6. The strongest statistical correlations between the Rogers NAO index of 3-month periods were obtained with maximum water temperature in February (r=0.78) and with mean water temperature also in February (r=0.73). The index reveals the weakest correlation with minimum water temperature; the highest value of r=0.64 was also obtained in February. 7. The Jones NAO index is most strongly correlated with mean and maximum water temperature in February (r = 0.80). 8. The Hurrell winter NAO index most strongly influences the duration of ice cover (r = 0.79) with an increase in the NAO index, the duration of ice cover decreases. 9. The Hurrell NAO index accounts for the variability of the date of ice cover disappearance on the lake only in 50%. 10. No significant correlation was determined between the Hurrell winter NAO index and the date of ice cover formation (r=0.30). 11. In the summer period, the NAO was not observed to influence the shaping of surface temperature of water in the studied lake. The obtained correlation coefficients between the NAO indices and (mean, maximum and minimum) surface water temperature of the lake do not reach beyond the threshold of statistical significance. References Borowiak D., 2000, Reżimy wodne i funkcje hydrologiczne jezior Niżu Polskiego (Water regimes and hydrological functions of Polish Lowland lakes), Bad. Limnol. 2, Wyd. KLUG, Gdańsk, p.164 (in Polish, English summary). Chabior M., Korpalska-Chabior G., 2002, Wpływ cyrkulacji atmosferycznej i NAO na warunki lodowe Zalewu Szczecińskiego i Zatoki Pomorskiej (Effect of atmospheric circulation and the NAO on ice conditions of the Szczecin Lagoon and Pomeranian Bay), [in:] Marsz A., Styszyńska A. (eds.), Oscylacja Północnego Atlantyku i jego rola w kształtowaniu zmienności warunków klimatycznych i hydrologicznych Polski (The North Atlantic Oscillation and Its Role in Climate and Hydrology in Poland), Akademia Morska, Gdynia: (in Polish). Girjatowicz J., Świątek M., Olechwir T., 2002, Związki Oscylacji Północnego Atlantyku z warunkami lodowymi polskiego wybrzeża (The Relationships Between the North Atlantic Oscillation and Southern Baltic Coast Ice Conditions), [in:] Marsz A., Styszyńska A. (eds.), Oscylacja Północnego Atlantyku i jego rola w kształtowaniu zmienności warunków klimatycznych i hydrologicznych Polski (The North Atlantic Oscillation and Its Role in Climate and Hydrology in Poland), Akademia Morska, Gdynia, (in Polish). Hurrell J.W., Van Loon H., 1997, Decadal variations in climate associated with the North Atlantic Oscillation, Climatic Change 36: Jones P. D., Jonsson T., Wheeler D., 1997, Extension to the North Atlantic Oscillation using early instrumental pressure observations from Gibraltar and South-West Iceland. Int J. Climatol. 17: Kożuchowski K., Degirmendźić J., 2002, Wskaźniki cyrkulacji a temperatura w Polsce (The interaction between circulation indicators and temperature in Poland), [in:] Marsz A., Styszyńska A. (eds), Oscylacja Północnego Atlantyku i jego rola w kształtowaniu zmienności warunków klimatycznych i hydrologicznych Polski (The North Atlantic Oscillation and Its Role in Climate and Hydrology in Poland), Akademia Morska, Gdynia: (in Polish). Marsz A., 1999, Oscylacja Północnoatlantycka a reżim termiczny zim na obszarze północno-zachodniej Polski i na polskim wybrzeżu Bałtyku (The North Atlantic Oscillation and the thermal regime in the area of northwest Poland and the Polish coast of the Baltic Sea), Prz. Geogr.71(3): (in Polish, English summary).

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