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Impact of North Atlantic Oscillation and drought conditions on summer urban heat load - a case study for Zagreb

Frequent heat waves in the latest decade have drawn attention due to large mortality and economic losses. Urban areas are in additional danger of extreme heat due to the well- known urban heat island (UHI) phenomena. Here, we investigate the effect of the North Atlantic Oscillation (NAO) in modifying urban heat in Zagreb, Croatia. Both winter (wNAO) and summer (sNAO) NAO components are analysed in terms of indirect (lagged) and direct effects on the urban heat load. The strongest increase in heat load was found when positive wNAO was followed by negative sNAO, while the opposite situation led to the strongest decrease. Situations with the same wNAO and sNAO polarity resulted in much weaker amplitude of changes due to opposing effects on climate parameters over these areas. These findings were confirmed using ground measurements from meteorological station, as well as an urban climate model. However, not only changes in the total heat load, but also differences regarding UHI intensity are detected. It is found that soil moisture, at least partially, drives such behaviour. The combination of positive wNAO followed by negative sNAO leads to a longer drier and warmer period over the Zagreb area and vice versa. Hence, in situations with an extended drier period the increase in heat load over green areas is found to be stronger compared to the densely built-up regions resulting in lower spatial gradient. This was confirmed by additional modelling experiments where the standardized precipitation evapotranspiration index (SPEI) was used to analyse the effect of drought conditions on UHI. These findings are additionally confirmed using satellite land surface temperature data. The potential applicability of this study includes UHI mitigation by irrigation of green urban areas in situations when seasonal forecasts indicate that warmer and drier conditions are expected.

urban heat island (UHI), North Atlantic Oscillation index (NAO), standardized precipitation evapotranspiration index (SPEI), land surface temperature (LST)

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