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Trends and irregular variation of spatial temperature differences in the high-latitude coastal city of Turku, Finland

Juuso Suomi*, Miika Meretoja

*Corresponding author:

ABSTRACT: Spatio-temporal dynamics of urban heat island (UHI) and other kind of spatial temperature variation was studied during the 17 year (2002-2018) period in the middle-sized (193 000 inhabitants in 2019) coastal city of Turku in south-western Finland. The study was based on geographic information systems (GIS) data and 30 minute interval air temperature observations in altogether 52 observation sites in the city and its surroundings. Correlation and regression analyses, image interpretation and various GIS methods were used to unravel the temperature dynamics and affecting factors on a diurnal, monthly and seasonal basis. The results showed relative cooling and warming trends of certain areas as well as fluctuating and random variation of coldest and warmest regions, respectively. Relative cooling trends were observed in semi-urban and urban areas that had kept unchangeable during the study period. Relative warming trends were most common in semi-urban areas where urbanization and construction of new buildings were fast and remarkable. Fluctuating variation was mainly related to the variation of weather conditions, especially to the severity of winter and related sea-ice extent on the archipelago to south-west of the city. The UHI intensity showed a statistically significant (p ≤ 0.01) warming trend of 0.11 °C per decade. The findings of this study demonstrate the dynamic nature of spatio-temporal intra-urban temperature differences, the fact that should more actively be taken into account in climate related urban planning in general.