Climate-river flow relationships across montane and lowland environments in northern Europe

To assess hydrological sensitivity to climate variability, improved understanding is needed of the process cascade linking large-scale climate to river flow. For northern Europe such knowledge is limited, and is further complicated by mountainous terrain. This research gap is addressed through composite analysis of large-scale climate drivers of monthly high and low river flow across northern Europe for the period 1968-1997. As an organizational framework for this investigation, the study is performed on a regional basis, using cluster analysis for identification of river flow regions. Results of the composite analysis show that the occurrence of high and low flow across northern Europe is associated with large-scale patterns of temperature and precipitation variation. Especially in winter, these patterns are linked to changes in large-scale geopotential height and wind fields, and in particular variation in the Azores High and Icelandic Low atmospheric centres of action. This is suggestive of a link between the North Atlantic Oscillation (NAO) and river flow, but is too complex to be described solely in terms of the NAO. Outside the winter half-year, an inverse climate-river flow relationship is found between northern and southern Scandinavian river flow regions. This relationship is associated with differences in the relative contribution of meltwater to river flow between these regions, their latitudinal separation and orographic effects on precipitation distribution.