Cold extremes enduring in a much warmer world

It is the combination of internally induced oscillations and externally forced climate change signals that we observe and feel every day as climate conditions. External forcing can change not only the mean state, but also the internal variability. One of the most important and impactful aspects of variability is the frequency and magnitude of extremes. Even though the cold extremes are expected to warm, they can still have severe impacts on society and ecosystems, which have adapted to a warmer climate. We investigate how the internal variability of winter temperature might change under stronger radiative forcing. For this purpose, we utilize two different datasets: a set of LongRunMIP simulations, analyzing near-equilibrium conditions under preindustrial and abrupt 4xCO2 forcings, and transient large ensemble simulations comparing the historical and scenario periods (the end of the 21st century under RCP8.5/SSP5-8.5 socio-economic pathways). We focus on northern middle latitudes (40 – 70 ° of latitude). In this region, the near-surface climate is largely influenced by atmospheric circulation, including various large-scale modes of variability. A change in the shape of the temperature distribution can then point to a fundamental change in climate-governing processes. It has been argued that increasing winter mean temperatures would be accompanied by a decrease in variance, as day-to-day temperature variations are induced by the occurrence of synoptic-scale weather systems, and in warmer climates, this is expected to decline. Our study provides new insights, showing that the variance shrinking is spatially heterogeneous. We further concentrate on the skewness of the temperature distribution and investigate the changes in the lengths of the cold and hot tails, which are related to the changes in variance. In many mid-latitude regions, the skewness is decreasing, and the cold tail is shrinking at a slower rate than the hot tail, implying enduring cold extremes, even in climatic states much warmer than those we are familiar with.