The Changes In The Relationship Between NAO And Summer Precipitation Over Eastern Tibetan Plateau In Different Phases Of AMO

Based on the data of 96 meteorological stations from 1961 to 2015 in the eastern Tibetan Plateau,the relationship between the leading principal component of summer(JJA)precipitation variability and the North Atlantic Oscillation(NAO)is analyzed,and the possible mechanisms for the unstable relationship were explained.Results show that the leading principal component of summer precipitation over the eastern Tibetan Plateau(in the north-south dipole oscillation)is closely associated with the NAO.However,the relationship between the leading principal component and NAO presents an unstability and a shift in the late 1990 s which may be resulted from the phases transition of the North Atlantic Multidecadal Oscillation(AMO).It is found that th e intensity and dispersion direction of the downstream Rossby wave energy over the eastern Tibetan Plateau caused by NAO are different in the phases of the AMO,which causes the differences of geopotential height anomalies and circulation anomalies over th e eastern Tibetan Plateau.During the cold(warm)AMO phase,the West edge of the NAO-related circulation anomalies is in the northeastern(northwest)Tibetan Plateau,which is conducive to the formation of precipitation anomalies of north-south dipole pattern(monopole pattern)in the eastern Tibetan Plateau.Therefore,the relationship between NAO and the leading principal component of summer precipitation changes in the eastern Tibetan Plateau during AMO different phases.We evaluate the skill of 27 CMIP6 models' ?Historical? simulations in simulating the observational AMO,and then select the model(CESM2-WACCM-FV2)with highest skill relaive to other models.Therefore,CESM2-WACCM-FV2' ?Pre-industrial control?(Picontrol)simulation is used in the feature study of AMO with long-term sequence.The results show that there is a significant positive correlation between AMO and low-pass filtered NAO in summer.In addition,there are obvious differences in temperature gradients between the southern and northern parts of 200 h Pa over the Eurasia during different AMO phases in summer,which may be the key factor of modulation of AMO on NAO.Considering the impact of phases of AMO,it may be necessary to consider more interdecadal background signals when studying the impact of NAO on regional climate to improve prediction levels and model simulation capabilities.