How would the spell of extreme weather observed over North America and western Europe during the 2013/2014 winter manifest itself in a warmer climate? Here a Forced Sensitivity method is used to calculate optimal model tendency perturbations
which result in a simulation which has its upper atmospheric circulation shifted in the direction of the January 2014 jet stream pattern. We apply this novel technique to a simulation run under present-day conditions which reproduces the main features of the observations of this event, such as the position and strength of the mid-latitude North Atlantic stormtrack, the temperature pattern over North America and the excessive precipitation in parts of Europe. When this method is applied to a future warmer climate, the mid-latitude North Atlantic stormtrack changes to a more zonal orientation, similar to the present-day simulations,
with height but its strength increases less
compared to the present-day simulations. Despite southward advection of polar air into Northeastern America, which has also occurred during the 2013/2014 winter, the associated drop in temperature
is less in the future climate than in the present day climate because of the strong warming at high latitudes in a future climate. The less steep drop in temperatures over Northeastern America leads to
a smaller land-sea temperature contrast, less baroclinic instability and a decrease of mid-latitude storminess for this event. This makes that, despite the increased atmospheric moisture content in the warmer climate, the increase in precipitation related to the equivalent of the 2013/2014 winter in a warmer climate is comparable to the one observed in the present-day climate.
LM Rasmijn, G van der Schrier, J Barkmeijer, A Sterl, W Hazeleger. The extreme 2013/2014 winter in a future climate
published, J. Geophys. Res., 2016, 121