In spite of global warming, northern midlatitudes, over central Eurasia in particular, have frequently experienced severe winter in recent decades (Fig.1a). Remote influence from the Arctic sea-ice loss (Fig. 1c) on the Eurasian cooling has been suggested, but significance of this counterintuitive climatic impact of the sea-ice loss is still under debate due to discrepancy among modelling and observational studies.
A new study published in Nature Climate Change in this month has tackled this difficult issue. To clarify the reason for discrepancy and to reconcile differing estimate, authors synthetically analysed observations and outputs from large-ensemble simulations with seven atmospheric general circulation models. This novel approach enables to extract forced signal common to the observation and simulations. All seven models are found to capture observed structure of forced surface temperature response featuring the Eurasian cooling against Barents-Kara sea-ice loss, but its magnitude is systematically underestimated. Owing to the varying degrees of this underestimation of sea-ice-forced signal, the signal-to-noise ratio differs markedly. These mean that the influence of sea-ice loss is easily masked by other factors unless the numerical experiments and analysis are conducted with special care, which is considered to be a potential cause of discrepant conclusions. Correcting this underestimation reconciles the discrepancy between models and observations, leading to the conclusion that ~44% of the central Eurasian cooling trend for 1995–2014 is attributable to sea-ice loss in the Barents-Kara Seas. Provided that sea-ice melt is a manifestation of anthropogenic climate change, these results indicate that the recent frequent occurrence of severe winters in central Eurasia is in part a transient response to human-caused radiative forcing under increasing greenhouse gases.
The results of this research is very important to make the more reliable estimation of the change in extreme event due to anthropogenic climate change and to provide suggestion to future climate projection and policy decision reflecting it.
The article “A reconciled estimate of the influence of Arctic sea-ice loss on recent Eurasian cooling” is published in Nature Climate Change at doi: 10.1038/s41558-018-0379-3 .
Masato Mori (the University of Tokyo, a member of theme 5)
Fig. 1: Trend of winter (DJF) mean (a) surface air temperature (ºC), (b) sea level pressure (hPa), and (c) sea ice concentration (%) for 1980–2014. The value shows the amount of change per 10 years. The values show Stippling indicates regions exceeding 90% and 95% statistical confidence in a-b and c, respectively. Only the trends exceeding the 95% statistical confidence are shown in c. Areas enclosed by a black line in a and c are referred to as “central Eurasia” and “Barents Kara Seas”, respectively.