This study investigates the changes and variability of the number of days with snow cover of ≥1 cm depth and the seasonal maximum snow cover depth at 66 meteorological stations throughout Poland, spanning a period of 50 (1948-49 to 1997-98) to 108 (1895-96 to 2002-03) winter seasons. A slight decreasing trend was observed with snow cover characteristics in most of Poland during the second half of the 20th century, but no change was distinguished for the longer periods. The snow cover changes are related to circulation changes, and particularly to the increased frequency of western advection over Poland. However, the scale of the snow cover change is not proportional to the air temperature changes, which have been found in many previous studies to display an increasing statistically significant trend in winter. An increasing trend of snow cover in the last 50 winter seasons is observed in areas with abundant snow cover only, i.e. in northeastern Poland (snow cover depth) and in the mountains (snow cover duration). In lowland areas of Poland the snow cover duration indicates an 8 year periodicity. The year-to-year variability of snow cover has been increasing, particularly during the second half of the 20th century.

Abstract:The main aim of the study is to investigate the atmospheric circulation impact on snow cover duration (SCD) and maximum seasonal depth of snow cover (MDS) in Poland in the twentieth century. The most important is to answer four questions:(1) which of the two, macro-or mesoscale atmospheric circulation, dominates in influencing the snow cover variability and changes in Poland area? (2) which of the two, meridional (S-N/N-S) or zonal (W-E/E-W) advection, dominates in influencing the snow cover variability and changes? (3) are the atmospheric circulation-snow cover relationships stable with time or did they change in circulation epochs? (4) does any strong relation exist between the snow cover in Poland and the atmospheric circulation enabling the possibility to forecast the nival conditions for the next winter season?The principal results of the study are as follows: (1) the macroscale circulation connected to atmospheric patterns over the Atlantic Ocean impacts stronger on the SCD variability in Poland than the circulation patterns located directly over the Poland area; (2) during the period 1966/67-1995/96 the zonal circulation dominated over the meridional one in influencing the SCD variability and change (the exceptions were areas with the most intensive foehn effect); (3) the decreasing trend of the dependence strength of the SCD on the meridional circulation with respect to the increasing tendency for zonal circulation-SCD impact was observed in the twentieth century; (4) the meridional circulation dominated over the zonal one in influencing the SCD till the 1930s of the twentieth century only (5) the snow cover in Poland is positively correlated with the North Atlantic Oscillation (NAO) in October prior to the winter season.The instability of the snow cover dependence on the atmospheric circulation in the twentieth century corresponds roughly to the circulation epochs and periods of intensity changes of the zonal/meridional advection over Europe.

An attempt to reconstruct the seasonal snow cover data in Cracow (for 26 winter seasons) and Zakopane (for 19 winter seasons) at the turn of the 20th century based on climatic data was made by using multiple regression. The results of the reconstruction were more successful for the station located at lower altitude (Cracow, 206 m above sea level) and allowed an analysis of nival conditions to be extended over 104 winter seasons. The reconstructed data obtained for Zakopane (857 m above sea level) appear to be overestimated for the number of snow cover days and sum of daily snow cover depth. Thus, they were excluded from further calculations. The mean snow cover duration at Zakopane (128 d yr -1 ) is twice as long as that at Cracow. An analysis of long-term variability of seasonal snow cover duration, seasonal maximum snow depth and sum of daily snow cover depth in Cracow for the period 1895/96-1998/99 and in Zakopane for the periods 1895/96-1998/99 and 1914/15-1998/99 showed no statistically significant trend in any case. The statistically significant (p < 0.05) decreasing trend of snow cover duration at both stations (-13 d/10 yr in Cracow, -8 d/10 yr in Zakopane) in the period 1961-1990 could be selected for the whole period investigated. The variation coefficient for snow cover duration is over twice as large for Cracow (37%) as for Zakopane (15%). KEY WORDS: Snow cover · Reconstruction · Climate variability · Time series analysis · Southern PolandResale or republication not permitted without written consent of the publisher

The seasonal stability of snow cover (ISS) was defined as a percentage ratio of the real and the potential snow cover duration in a winter season. Main results of the study are as follows: (1) alternately occurring periods of high and low values of the index of snow cover stability did not appeared simultaneously in mountainous and nonmountainous areas; (2) in the majority of Poland area both zonal and meridional components of the atmospheric circulation influence the ISS; however, in south the meridional air flow reveals the stronger impact, mostly due to the intensification of the southern advection by the foehn effect; and (3) changes of two or three indices describing atmospheric circulation explain up to 50 % of the ISS in Poland. The diminishing stability of snow cover in Poland corresponds with an increasing intensity of the advection from the western sector in winter in the second half of the twentieth century in Europe.

The two main objectives of this study are: 1. to assess the scale of extreme nival conditions in Poland; 2. to provide a fragmentary, regional verification of the IPCC thesis of the increasing frequency of extreme atmospheric phenomena during recent decades, by the estimation of extreme nival conditions tendencies in the second half of the 20 th century. The daily data of snow cover depth in 12 meteorological stations in Poland, for the period 1954-2001, were analysed. A winter season with extreme nival conditions is the season, when the snow cover duration value and/or the maximum seasonal snow cover depth value, was of the empirical probability of < 10 % (a season of extremely short duration and/or thin snow cover) or > 90 % (a season of extremely long duration and/or thick snow cover). A slight negative, usually statistically insignificant, trend was found for characteristics concerning extremely abundant snow cover, i.e.: the seasonal number of days with snow cover of a considerable depth; the 90th percentile value of the daily snow cover depth for the period from December 1 to February 28; the seasonal maximum of a 24-hour increase in the snow cover depth (except for northern Poland); the seasonal number of days with a 24-hour increase in the snow cover depth of ≥ 10 cm. At the same time, since the 1970's the scarce snow cover (i.e. of the empirical probability < 10 %) has been observed much more frequently than before. The above mentioned IPCC thesis has been confirmed towards the extremely low snow cover values in Poland. Zusammenfassung Die beiden Hauptziele dieser Veröffentlichung sind: 1. Untersuchung der Größenordnung extremer Schneeverhältnisse in Polen, 2. Verifizierung einer These vom IPCC über die Zunahme der Häufigkeit von extremen meteorologischen Phänomenen in den letzten Jahrzehnten anhand der Tendenz zu extremen Schneebedingungen in der zweiten Hälfte des 20. Jahrhunderts. Es wurden Tageswerte der Schneedeckenhöhe an 12 meteorologischen Stationen für das Gebiet Polens in der Zeit von 1954-2001 analysiert. Als Winterhalbjahr mit extremen Schneebedingungen wurde ein Halbjahr bezeichnet, in dem die Schneebedeckung und/oder die maximale Höhe der Schneedecke eine empirische Wahrscheinlichkeit von < 10 % aufweist (Winterhalbjahr mit extrem kurzer Schneebedeckung und/oder extrem geringer Schneedecke) bzw. > 90 % erreicht (Winterhalbjahr mit extrem langer Schneebedeckung und/oder hoher Schneedecke). Ein schwach negativer-in den meisten Fällen statistisch nicht signifikanter-Trend wurde bei folgenden charakteristischen Größen der Schneebedeckung festgestellt: Anzahl der Tage pro Winterhalbjahr mit relevanten Schneedeckenhöhen (≥ 10 cm, ≥ 20 cm, ≥ 50 cm), Tageswerte der Schneedeckenhöhe mit empirischer Wahrscheinlichkeit von ≥ 90 % in der Zeit vom 1. Dezember bis 28. Februar, Maximum des Tageszuwachses der Schneedecke im Winterhalbjahr und Anzahl der Tage mit Tageszuwachs der Schneedecke ≥ 10 cm. Winterhalbjahre mit extrem geringer Schneedecke treten vor allem in der zweiten Hälfte der untersuchten Peri...

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The paper focuses on investigation of 'twin' subtropical oceanic highs of the Northern Hemisphere, i.e. the Azores High (AH) and the Hawaiian High (HH) in January and July based on gridded 2.5°× 2.5°data of Reanalysis Project of the National Center for Atmospheric Research for the period 1948-2018. The aim is to answer three questions: (1) Are there any connections between AH and HH (both within and between the systems)? (2) What is the long-term variability and trends of the basic characteristics of AH and HH? (3) Do the AH and HH move, and if so, in what directions? The most important results are as follows: (1) the longterm trend of sea level air pressure in the AH centre in January is positive, statistically significant with the increase of 0.63 hPa/ 10 years, (2) pressure in both centres significantly relates with the latitude of each system; variables characterising the HH in January explain 11% of variation of the variables of the AH in July, (3) the NE-SW/SW-NE index proves the shifting of the AH in January from the southwest to the northeast from the 1990s of the twentieth century and again to the southwest in the twentyfirst century, (4) the HH in January and July moved generally from the northeast to the southwest until the end of the twentieth century and shifted again to northeast during the twenty-first century, (5) the AH in July was characterised by complicated displacement system with the prevalence of the shifting from the northeast to the southwest with the exception for the period 1980-1990. In winter, the AH moves towards the land area of Europe in the second half of the twentieth century, while the HH moves towards the open Pacific. The statistically significant increase of pressure in the centre of the AH in January is closely related to the shifting of the system to the northeast. The positive pressure trend in the centre of the AH in January combined with the zero trend in July is the cause of diminishing difference between summer and winter air pressure value of the high. Due to increased sea surface temperature of the Atlantic, the AH does not lose its strength in winter as it used to a few decades ago.