Impact of the North Atlantic Oscillation on Swedish Winter Climate and Nutrient Leaching

The winter climate in northwestern Europe is commonly influenced by the North Atlantic Oscillation (NAO). Its intensity, expressed as an index (NAO), has been suggested for use in assessing nutrient leaching from arable land to water and the effects of mitigation measures. We found significant ( < 0.05) positive linear relationships between NAO and an air freezing-thawing index in central and southern Sweden for 2004 to 2016. This period covered winters with both extreme low and high NAO. There were significant negative linear relationships between NAO and a snow depth index. Management and nutrient leaching were studied simultaneously in two agricultural catchments (20.7 ha, code 11M; 788 ha, code M36) in southwestern Sweden. Catchments 11M (silty-clay soil) and M36 (sandy hills with a central, heavy clay plain) are both artificially drained. Total N and total P leaching increased significantly with winter (November-April) NAO in both catchments. In contrast, leaching of dissolved reactive P (DRP) was not related to NAO. The highest DRP concentrations were observed in connection with specific agricultural practices, whereas moderately elevated DRP concentrations were linked to snowmelt events. Concentrations of P in other forms (other P) were even more elevated (1.02 mg L) in 11M in winter 2014-2015, probably due to a large (32% of area) internal buffer (ley-fallow) in a central ravine being plowed down in autumn 2014. No general trend in P or N fertilization was found in catchment M36. Thus NAO may be appropriate for use in trend analyses of nutrient load in the study region.