Change in Atmospheric Circulation Patterns

Abstract

This chapter examines both variability and trends in atmospheric circulation favouring the occurrence of flood precipitation defined as daily totals ≥30, ≥50 and ≥100 mm in warm half-year (May–Oct) and in summer (JJA). We used a catalogue of circulation types created for the Upper Vistula Basin, and related circulation indices (zonal circulation index, meridional circulation index, cyclonicity index and NAO) covering the 1874–2015 period. Climatology of atmospheric circulation over the Upper Vistula Basin is discussed as a basis for further investigations. In order to select circulation types and indices impacting both the occurrence and long-term variability of flood precipitation, we calculated the frequency and conditional probability of high precipitation (≥50 mm) in circulation types and correlation between selected circulation characteristics and high precipitation frequency (≥30 mm). Trends in the frequency of circulation types and indices favouring the occurrence of high precipitation were calculated to assess current and possible future flood conditions. In summer and warm half-year the Upper Vistula Basin was usually under an influence of anticyclonic wedge (Ka circulation type) and cyclonic trough (Bc circulation type). Circulation types with the air flow from the west (Wa and Wc) were the most frequent of all advectional types. The occurrence and long-term variability in flood precipitation over the Upper Vistula Basin were strongly linked to the frequency of air advection from the north and north-east under an influence of low pressure system (Nc and NEc circulation types) and to Wi—zonal circulation index at both stations in summer (JJA) and to Ci—cyclonicity index at Kasprowy Wierch station in warm half-year (May–Oct). Trends in majority of circulation characteristics favouring the occurrence of high precipitation and impacting its long-term variability were not statistically significant with an exception of the frequency of Nc type and Ci index in the warm half-year and Wi index in summer. Significant increase in the number of days with Nc circulation type and the cyclonic situations (Ci index) in warm half-year and intensification of air advection from the north-east in summer may lead to increase in the frequency of flood conditions in these seasons. Regardless insignificant trends, the variability in the NEc frequency considerably determines the occurrence and long-term variability of high precipitation thus was recognized as indicator of flood conditions. There was intensification of the air advection from the north (Nc) in the warm half-year (May–Oct) in the 50-year period between 1930 and 1980. High frequency of NEc type was found in seventies and eighties which coincided with the high frequency of floods in those decades in Poland and at the turn of the first and second decades of the 21st century. Low frequencies of these circulation types during 1982–1995 were in phase with relatively dry conditions in Southern Poland without the floods within the Upper Vistula Basin. In the further more wet years the great floods happened in July 1997 and May 2010.