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

Abstract

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 investigated area. 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 conditions 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.

Spatio‐temporal changes in the mean and extreme temperature indices for Serbia

“…Popov et al, 2018b;Nimac et al, 2022) and local studies (Unkaševi c and Toši c, 2013). For example, the strongest and significant correlation with the NAO index is detected for maximum air temperature in Zagreb (Croatia) during the winter and summer seasons (Nimac et al, 2022). Similar results are obtained by Popov et al (2018b) for four meteorological stations in Bosnia and Herzegovina.…”

Temperature characteristics and heat load in the City of Dubrovnik

“…The trilinear interpolation yields interpolated fields of air temperature, relative humidity, and wind speed which enable the calculation of the spatial distribution of climate indices. A more detailed description of the MUKLIMO_3 model and the cuboid method can be found in several studies considering urban heat load (Früh et al, 2011;Žuvela-Aloise et al, 2014Žuvela-Aloise et al, , 2016Žuvela-Aloise, 2017;Nimac et al, 2022).…”

Očekivano toplinsko opterećenje Dubrovnika, Osijeka, Rijeke, Zadra i Zagreba prema projekcijama regionalnih klimatskih modela

“…Boras et al (2022) have shown rising trends for mean, minimum and maximum temperatures in the city of Dubrovnik for the period 1961-2020, which are highest during summer months, but they did not investigate future trends of heat load. On the other hand, it has been shown that in different locations in Zagreb, the type of climate has already changed due to increasing air temperatures, and additional temperature increases are expected in the future (Nimac, 2022;Nimac et al, 2022). It is obvious that there is a lack of systematic research focused on the expected heat load of cities in Croatia.…”