The influence of circulation patterns on extreme thermal resources in the growing season and the period of active plant growth in Poland (1951-2006)

This paper deals with the problem of circulation patterns on extreme thermal resources during the growing season in Poland. Thermal resources were specified using the sums of surpluses of mean daily air temperature values above the thresholds of 5.0 degrees C (growing season) and 10.0 degrees C (the period of active plant growth) - the so called effective accumulated temperatures. The main aim here is to define the structure of circulation patterns over Poland during the years of extreme thermal resources during the growing season and the period of active plant growth. Extreme thermal resources were defined as those seasons in which effective accumulated temperatures were equal to or above the 90th percentile, or were not greater than that specified by the 10th percentile. The research was based on the daily data from years 1951-2006 from ten stations of the Institute of Meteorology and Water Management, situated at less than 300 m a.s.l. in Poland (spatially averaged data series), and on the calendar of circulation patterns developed by T. Litynski. A significance evaluation of differences between the frequency of particular circulation types in the years with either extremely small or large effective accumulated temperatures and during the investigated period was performed. The long-term trends and fluctuations of thermal resources were established.