How does North Atlantic Oscillation modify summer urban heat load in Zagreb (Croatia)?

Combined with global warming, urban areas are in additional danger of extreme heat due to the well-known urban heat island (UHI) phenomenon. In this study, the effect of the North Atlantic Oscillation (NAO) on urban heat load in Zagreb (Croatia) is investigated using ground measurements from meteorological station Zagreb-Maksimir, as well as an urban climate model MUKLIMO_3. NAO impact in both winter (wNAO) and summer (sNAO) seasons are analysed in terms of indirect (lagged) and direct effects on the urban heat load. The strongest increase in heat load is detected when positive wNAO is followed by negative sNAO, while the opposite situation is associated with heat load decrease. NAO impact is the weakest for situations with the same wNAO and sNAO polarity due to their opposing effects on climate parameters over investigated area. Besides changes in the total heat load, differences in UHI intensity are also found. Results indicate soil moisture as one of potential physical links between NAO and the heat load. The combination of positive wNAO and negative sNAO supports dry and warm conditions over the Zagreb area and vice versa. In situations with extended dry period, green areas experience stronger increase in heat load than densely built-up regions. Therefore, cooling efficiency of vegetation can be modified with NAO through the processes that include precipitation, temperature and soil moisture. This was confirmed by additional modelling experiments considering standardized precipitation evapotranspiration index (SPEI). These findings are additionally confirmed using land surface temperature data from Landsat-8 satellite. Results of this study demonstrate that irrigation of green urban areas should be included in UHI mitigation measures, particularly for situations when seasonal forecasts indicate long-lasting warm and dry conditions.

Nimac, I., Herceg-Bulić, I., Žuvela-Aloise, M. and Žgela, M. (2022), Impact of NAO and SPEI conditions on summer urban heat load – a case study for Zagreb. Int J Climatol. Accepted Author Manuscript. https://doi.org/10.1002/joc.7507