LONG-TERM VARIABILITY OF SNOWFALL AND CONTINUOUS SNOW COVER DURATION IN SOUTHERN FINLAND AND THEIR LINKAGE TO TELECONNECTION INDICES

Masoud Irannezhad, Anna-Kaisa Ronkanen, Bjørn Kløve, University of Oulu, Finland

Abstract

Most climate models are projecting increases in temperature and precipitation for high latitude areas in the northern Hemisphere. This warming would increase rainfall by shifting precipitation falling form from snow to rain, and would accelerate snow cover disappearing by increasing melting and evaporation of snow cover. Despite of these impacts of climate warming, increased precipitation in high latitudes could provide sufficient snowfall for expanding the snow cover. Using temperature and precipitation data in the empirical temperature-index snowmelt model, we evaluated variability of snowfall and snow cover duration in southern Finland. The model calibration and validation was done based on observed precipitation, temperature and snow water equivalent data from 1946 to 2011. Trend analyses at the 5% significance level by Mann-Kendall non-parametric test determined the mean temperature increased (by 0.01 °C/year) for the period from 1858 to 2011 based on the data from Kaisaniemi meteorological station, but there was no clear trend in the precipitation. The rainfall showed an increasing trend (by 1.25 mm/year) at the 5% significance level, while the snowfall decreased (by 1.08 mm/year). Typical continuous snow cover duration over the studied period (1858-2011) was about 139 days from 13 December to 1 May. The continuous snow cover duration in hydrologic years from 1858 to 2011 depicted a decreasing trend (by 0.55 day/year).This decreasing trend was associated with an increasing trend (by 0.23 day/year) in starting day and a decreasing trend (by 0.31 day/year) in ending day of snow coverage. Pearson’s correlation coefficient determined the temperature was strongly associated with Arctic Oscillation (AO), the precipitation with East Atlantic/West Russia pattern (EA/WR), the rainfall with Scandinavia pattern (SCAND), the Snowfall with both EA/WR and AO, the continuous snow cover duration with AO, the starting day of snow coverage with East Pacific/North Pacific pattern (EP/NP), and the ending day with AO. In southern Finland, the warming caused the rainfall to increase, and the snowfall to decrease, even the precipitation was unchanged. This reduction of snowfall decreased continuous snow cover duration in the study area where the most effective climate teleconnection index on the temperature and snowfall was Arctic Oscillation (AO).