Simulation And Predictability Of Northern Hemisphere Blocking In Winter By Dynamic Models

Atmospheric blocking('blocking')in the Northern Hemisphere(NH)is a crucial driver of extreme cold spells in winter.Here we investigate the anthropogenic influence on the NH blocking and its impact on surface air temperature(SAT)during the winter 1960/1961-2012/2013 using two HadGEM3-GA6-N216 simulations with 15 ensemble members:(a)with anthropogenic and natural forcing(All-hist)and(b)with natural forcing only(Nat-hist).Compared to the Nat-hist run,the blocking frequency in the All-hist run decreases in the Euro-Atlantic,the Urals and the western Pacific,whereas it increases in the eastern Pacific and Greenland.These responses can be explained by the response of planetary waves and storm tracks.On the other hand,the decrease in SAT downstream of the blocking regions in the All-hist run is more pronounced than the Nat-hist run,especially in Europe and the Urals.Correspondingly,the proportion of cold days during all blocking days in these sectors is higher in the All-hist run than the Nat-hist run.These responses can be explained by the wind response associated with blocking.Overall,the spatiotemporal characteristics of blocking is crucial for evaluating the impact of blocking on extreme weather,and their response to anthropogenic forcing should be investigated by more models.Additionally,we further analysis the dynamic and thermodynamic structure of the persistent blocking event and assessed the predictability of that.The occurrence of a Ural blocking(UB)event is an important precursor of severe cold air outbreaks in Siberia and East Asia,and thus is significant to accurately predict UB events.Using subseasonal to seasonal(S2S)models of the European Centre for Medium-Range Weather Forecasts(ECMWF)and the Environment and Climate Change Canada(ECCC),we evaluated the predictability of a persistent UB event on 18 to 26 January 2012.Results showed that the ECCC model was superior to the ECMWF model in predicting the development stage of the UB event ten days in advance,while the ECMWF model had better predictions than the ECCC model for more than ten days in advance and the decaying stage of the UB event.By comparing the dynamic and thermodynamic evolution of the UB event predicted by the two models via the geostrophic vorticity tendency equation and temperature tendency equation,we found that the ECCC model better predicted the vertical vorticity advection,ageostrophic vorticity tendency,the tilting effect,horizontal temperature advection,and adiabatic heating during the development stage,whereas the ECMWF model better predicted the three dynamic and the two thermodynamic terms during the decaying stage.In addition,during both the development and decaying stages,the two models were good(bad)at predicting the vortex stretching term(horizontal vorticity advection),with the PCC between both the predictions and the observations larger(smaller)than+0.70(+0.10)Thus,we suggest that the prediction of the persistent UB event in the S2S model might be improved by the better prediction of the horizontal vorticity advection.