On complex extremes: flood hazards and combined high spring-time precipitation and temperature in Norway

Abstract

Extreme weather and climatic events can have detrimental effects on society. The coincidence of several factors, themselves not necessarily extreme, can have similar adverse implications, such as a combination of high spring-time temperatures and heavy rainfall. A combination of high temperature and heavy precipitation during spring can produce flooding when run-off due to snow-melt adds to river discharge from the rainfall. Such combined events are often referred to as ‘complex extremes’ (IPCC, Climate change 2001: impact, adaptation and vulnerability. Summary for policymakers. WMO, Geneva, Switzerland, p. 7, 2001) . A likely effect of a climate change is a shift in the frequency of both extremes in traditional sense as well as in complex extreme events. Results from a global climate model were downscaled through a higher-resolution nested regional climate model in order to obtain more realistic descriptions of regional climatic features in Norway. Empirically-based joint frequency distributions (two dimensional histograms) were used to study shifts in the frequency of complex extremes. A slight shift in the joint frequency distributions for spring-time temperature-and-rainfall was detected in downscaled results with HIRHAM from a transient integration with the ECHAM4/OPYC3 climate model following the IS92a emission scenario. The analysis involved values that spanned between ordinary and extreme values of the bivariate distributions complicating the estimation of a representative confidence interval as the data fall in the zone between different types of behaviour. The results from HIRHAM were spatially interpolated and compared with station observations, and substantial biases were revealed, however, the apparent model discrepancy is largely due to great small-scale variability due to a complex physiography. The general temporal trends predicted by the model are nevertheless realistic.