North Atlantic Oscillation as a modifcator of urban climate

Climate conditions in urban areas considerably differs from the climate of their rural surroundings manifesting as urban heat island (UHI). In addition to impacts of regional factors (such as latitude, topography, distance to large water bodies etc.), the urban climate is largely determined by effects of urbanization (the structure of the city, like height and spacing of buildings, characteristics of constructing materials etc.). However, interaction between urban climate and large-scale climate variability (e.g. heat-waves, North Atlantic Oscillation, El-Niño Southern Oscillation) may amplify UHI and therefore increase the heat stress leading to more unpleasant urban environment for its inhabitants with potential negative economic, health and social consequences. Here, we investigate the role of North Atlantic Oscillation (NAO) in modifying urban heat island of Zagreb, Croatia. Based on analyses of ground measurements and MUKLIMO_3 modelled data, it is demonstrated that both winter (wNAO) and summer (sNAO) NAO affect the summertime urban heat load. The strongest increase in the heat load is found for those summers when positive wNAO is followed by negative sNAO, while the opposite situation leads to the strongest decrease. Situations associated with wNAO and sNAO of the same polarity resulted in much weaker response due to their cancellation effect. Although the NAO is a large-scale phenomenon, its impact is not spatially uniform over the urban domain. Results indicate soil-moisture and related processes as modifying factors. Additional targeted modelling experiments based on standardized precipitation evapotranspiration index (SPEI) uphold the modifying effect of drought conditions on spatial characteristics of urban heat load. Results also imply that efficiency of green infrastructure considerably depends on soil moisture availability indicating the need of their irrigation in dry conditions together with increased water demands for irrigation in future.