The Mechanism Of The Seasonal Asymmetric Warming Over Mid-high Latitude Of Northern Hemisphere

By analyzing global temperature in the observations and CMIP5 simulations under different scenarios,we examined the seasonal asymmetric warming dominated by the enhanced cold-season warming over mid-high latitude of Northern Hemisphere.It was found that the projected warming in future is always dominated by the cold-season warming and the asymmetry between seasons will increase.From this perspective,we investigated the changes of land-sea thermal contrast during global warming,and further investigated the impact of winter and summer land-sea thermal contrast on the frequency and duration of blocking and its role in the seasonal asymmetric warming.It found that another important factor for enhanced local warming is the expansion of Siberia evergreen forest,which could induce the decreased albedo and further accelerated the regional warming.Because there is a significant winter cooling trend in Siberia during warmig hiatus period,it is a challenge for the expansion of Siberia evergreen forest due to the damages of cold temperature.However,we verified this phenomenon in a large scale during the wamring hiatus period by using satellite observations.Finally,the simulations were done by the individual-based forest gap model(UVAFME)to investigate the sensitivity of expansion of Siberia evergreen forest to temperature changes induced by interdecadal oscillations.The main results are summarized:(1)The projected asymmetric warming between cold-season and warm-season is still dominated by cold-season warming and mainly focuses on the mid-high latitude of Northern Hemisphere.The defined asymmetric warming index(AWI)shows the asymmetry between cold-season and warm-season is increasing under RCP4.5 and RCP8.5,although it is underestimated in the historical simulations.The spatial distribution of asymmetric warming trend has a large uncertainty in simulations because the warming trend in high latitude is overestimated and the warming trend in middle latitude is underestimated.(2)A land-sea index(LSI)was defined to measure the asymmetric zonal thermal forcing by the linear combination of the average surface temperature anomalies in the key areas of the “COWL” pattern.The index changed from a negative to a positive anomaly in the 1980 s.According to the Charny-DeVore equilibrium theory,the changes of zonal asymmetric thermal forcing can induce the transition between high index and low index of atmospheric circulation.In the positive phase of the LSI,the 500 hPa geopotential height decreased in the polar regions and increased in the mid-latitudes.The tropospheric planetary wave activity became weaker and exerted less easterly forcing on the westerly wind.These circulation changes were favorable for westerly acceleration and reduced blocking.In the Atlantic,the duration of blocking decreased by 38% during the positive LSI phase compared with that during the negative phase;in Europe,the number of blocking persisting for longer than 10 days during the positive LSI phase was only half of the number during the negative phase.The response of the westerly and blocking could further enhance continental warming,which would strengthen the land-sea thermal contrast.The positive feedback amplified regional warming in the context of global warming.(3)The land-sea index in summer was defined to measure the changes of zonal asymmetric thermal forcing.The summer LSI showed a slower increasing trend,which is only half of winter LSI trend.Different with winter feedback,we found a p ositive feedback between summer blocking,land-sea thermal contrast and double-jet structure.In the positive phase of summer LSI,the EP flux convergence induced by the land-sea thermal forcing in the high latitude became weaker than normal,which induced positive anomaly of zonal-mean westerly and double-jet structure.Based on the quasiresonance amplification mechanism,the narrow and reduced westerly tunnel between two jet centers provided a favor environment for more frequent blocking.Composite analysis demonstrated that summer blocking showed an increasing trend of blocking events and a decreasing trend of durations.The short blocking persisting for 5-9 days significantly increased and long blocking persisting for longer than 10 days had a slightly increase with decreasing duration than that in negative phase of summer LSI.The increasing blocking due to summer LSI can further intense the continent warming and increased the summer LSI.This positive feedback between summer blocking and LSI were weaker than winter feedback due to weaker stationary wave forcing and limitations of double-jet structure occurrence.(4)Through the accelerated warming period in the second half of the 20 th century,field observations and simulations both support that the evergreen forest in Siberia is expanding.However,there is no direct evidence for this phenomenon in the large scale.The satellite observation used here verified that the apparent area of evergreen forest has increased by 10%,while that of the deciduous forest has decreased by 40% during 2001-2012,although there is a large uncertainty because of the unstable classification algorithm.During warming hiatus,the Siberia experienced a significant cooling in winter season,which potentially stresses evergreen forest because of the cold-temperature damage and mortality.A gap forest model(UVAFME)was used to verify the response of evergreen forest to the decadal oscillation in the temperature.Simulations demonstrated that the cooling trend induced by the decadal oscillations can not stop the evergreen expansion in Siberia due to the hysteresis response of forest changes.The temperature at which the abrupt change in dominance occurs depends on whether the temperature is increasing or decreasing.Therefore,the changes in forest composition dynamics under temperature oscillations induced by internal climate variability may not reverse this composition change,which is consistent with observations.