Impact And Mechanism Of Spring Land Surface Thermal Anomaly In West Asia On Early Summer Climate In Northern Chin

Under the background of global warming,the phenomenon of land surface warming over West Asia(35°-75°E,12°-45°N)during the past decades has drawn considerable attention.As the entrance of subtropical upper-tropospheric jet stream is over West Asia,the thermal anomalies there cloud not only induce the atmospheric disturbance above,but also impact on the circulation and climate over East Asia by the forced wave along jet stream.Results from statistical analysis show that the spring(March-April-May)anomalous warming over West Asia is associated with the early summer(June)circulation anomalies over northern China.However,the physical mechanisms behind such a teleconnection and its impacts on climate need further explorations.In this study,the influences of spring land surface thermal anomalies on early summer precipitation and air temperature over northern China(West Asia thermal anomaliesnorthern China climate teleconnection)and its mechanisms are explored based on statistical diagnosis with multi-source datasets and numerical experiments.The main conclusions are as follows:(1)The abnormal spring land surface warming over West Asia tends to increase precipitation over North China and decrease(increase)precipitation(air temperature)over Northeast China in early summer.This teleconnection is most significant on the interannual scale.The land surface anomalous warming can heat the atmosphere above by enhancing the upward sensible heat flux and longwave radiation,leading to the anomalous cyclonic(anticyclonic)circulation in the lower(middle-upper)troposphere through thermal adaptation processes.This further trigger eastward-propagating Rossby wave train along the westerly jet stream via enhanced atmospheric baroclinic instability,forming low-frequency wave train around the Northern Hemisphere.From spring to early summer,the abnormal wave train gradually strengthens and evolves due to the land surface abnormal thermal forcing and the interaction between the basic flow(especially the westerly jet stream)and forced waves.In early summer,the abnormal wave train is similar to the CGT(Circumglobal Teleconnection)pattern,whose wave center over East Asia affects the circulation over northern China.The anomalous water transport and vertical motion are responsible for the precipitation increases(decreases)over North(Northeast)China.Meanwhile,the enhanced descending motion,radiation heating and warm advection result in the evident warming over Northeast China.(2)The spring land surface anomalous warming over West Asia can also impact on the climate over northern China by adjusting the Indian monsoon activity in early summer.The anomalously warm land surface over West Asia can advance the transition of atmospheric conditions towards warm season.The increased land-sea meridional thermal contrast favors the intensification of the low-level jet and monsoon trough,further inducing anomalous moisture convergence and ascending motion over northern Indian peninsula.Additionally,the heat-driven anomalous upper-tropospheric anticyclone over West Asia favors the intensification of tropical easterly jet and the north-westward development of South Asian high.The enhanced South Asian high dynamically couples with the lower-to middle-level cyclonic circulation over northern Indian peninsula,resulting in a stronger monsoon and increased precipitation.At the same time,the high-level anomalous anticyclone induced by precipitation heating can extend upstream due to the vorticity advection,further propagate across the critical latitude by background southerly wind and then continue downstream along the westerly jet stream.That forms anomalous wave train,which further strengthens the Eurasian part of CGT and affects the circulation over northern China.In addition,the enhanced Indian monsoon circulations and CGT action center over East Asia are also beneficial to the East Asian summer monsoon activity.Finally,the precipitation(air temperature)abnormally increases(decreases)over North China,and the precipitation(air temperature)abnormally decreases(increases)over Northeast China.(3)There are two possible pathways via which spring land surface anomalous warming over West Asia influences the early summer climate over northern China.On the one hand,the anomalous warming over West Asia triggers the low-frequency circumglobal wave train,which strengths the early summer CGT pattern,whose action center over East Asia effects on the circulation,precipitation and air temperature over northern China.On the other hand,the abnormal warming over West Asia enhances the CGT and East Asian monsoon in early summer by promoting the Indian monsoon precipitation and latent heating,which also leads to the climate anomalies over northern China.In a word,the Indian monsoon activity plays an important role in the West Asia thermal anomalies-northern China climate teleconnection.The latent heating of Indian monsoon can help the baroclinic disturbance caused by land thermal forcing over West Asia last from spring to early summer,further strengthening the action center downstream.Quantitatively,the contribution of Indian latent heating in the teleconnection is about 35% based on the simulated circulation anomalies.This research deepens the scientific understanding of the climate effects of spring land surface thermal anomalies over West Asia,and provides a scientific reference for improving the prediction of early summer climate over northern China.