Evolution of observed and modelled temperatures in Finland in 1901–2018 and potential dynamical reasons for the differences

Abstract

Observed monthly and annual mean temperatures in Finland in 1901–2018 were compared with simulations performed with 28 global climate models (GCMs), and dynamical factors behind the emerging differences were studied by regression analysis. Observational temperatures were extracted from high‐quality kriging analyses specifically tailored for Finland. Considering the entire time interval, the increase in the annual multi‐GCM mean temperature agrees well with the observed warming, even though observations exhibit substantial inter‐decadal fluctuations. After 2000, the mean temperatures have been higher than during any period in the 20th century. In the baseline regression model, the 10 leading EOFs of the European—Northeast Atlantic sea‐level pressure (SLP) field were used to explain differences between the GCM‐mean and observed evolution of temperature. The regression model is able to reduce the mean squared difference of the temporally‐smoothed temperature by 58%. The performance is highest in winter and summer and lowest in April. For a sensitivity assessment, multiple alternative regression models were tested, for example, one using the local SLP, geostrophic wind and vorticity as predictors. These models mostly showed somewhat inferior performance. We specifically explored the trends of monthly temperatures during 1961–2018, a period considerably affected by anthropogenic emissions. Compared with the multi‐GCM mean, warming proved to be negligible in June, fairly slow in October and quite rapid in December. All these features were explained rather nicely by dynamical factors. Accordingly, the deviations of the observed regional temperature trends from the multi‐GCM mean largely appear to be related to internal variability.

Strong Warming Rates in the Surface and Bottom Layers of a Boreal Lake: Results From Approximately Six Decades of Measurements (1964–2020)

“…The largest warming trend is observed in the AT averaged from November to April, when the lake is ice‐covered (0.880°C per decade), followed by the annual AT (0.611°C per decade). Prior studies also demonstrated a significant warming in the AT in Finland during recent decades (Räisänen, 2019; Ruosteenoja & Räisänen, 2021; Tuomenvirta, 2004) with maximum warming during cold seasons (Tuomenvirta, 2004). In addition, our study period (1964–2020) covers a time span influenced by anthropogenically driven warming at global scale (Hartmann et al., 2013; Myhre et al., 2013; Ruosteenoja & Räisänen, 2021).…”

“…Prior studies also demonstrated a significant warming in the AT in Finland during recent decades (Räisänen, 2019; Ruosteenoja & Räisänen, 2021; Tuomenvirta, 2004) with maximum warming during cold seasons (Tuomenvirta, 2004). In addition, our study period (1964–2020) covers a time span influenced by anthropogenically driven warming at global scale (Hartmann et al., 2013; Myhre et al., 2013; Ruosteenoja & Räisänen, 2021). However, the warming rate in the AT is near zero in June, in line with the results reported by Räisänen (2019) and Ruosteenoja and Räisänen (2021).…”

“…In addition, our study period (1964–2020) covers a time span influenced by anthropogenically driven warming at global scale (Hartmann et al., 2013; Myhre et al., 2013; Ruosteenoja & Räisänen, 2021). However, the warming rate in the AT is near zero in June, in line with the results reported by Räisänen (2019) and Ruosteenoja and Räisänen (2021).…”

“…The largest warming trend is observed in the AT averaged from November to April, when the lake is ice-covered (0.880°C per decade), followed by the annual AT (0.611°C per decade). Prior studies also demonstrated a significant warming in the AT in Finland during recent decades (Räisänen, 2019;Ruosteenoja & Räisänen, 2021;Tuomenvirta, 2004) with maximum warming during cold seasons (Tuomenvirta, 2004).…”

“…This inconsistency is due to the difference in the time spans used by Korhonen (2006) and our study. The latter spans 1964-2020, a period that is strongly influenced by anthropogenically driven warming at global scale (Hartmann et al, 2013;Myhre et al, 2013;Ruosteenoja & Räisänen, 2021). In general, high-latitude freshwater lakes (e.g., Lake Kallavesi) are exposed to the significant increasing AT and to fast changes in ice phenology (Alexander et al, 2013;Sharma et al, 2019Sharma et al, , 2021.…”