Concurrent Varications Of East Asian Upper-level Jet Streams And Its Machanisms

East Asian upper-level jet stream including subtropical jet stream and polar-front jet stream is one of the important atmospheric circulation systems over north hemisphere.The concurrent variation of East Asian subtropical and polar-front jet streams shows both the effect of circulation change over mid-high latitude and the abnormal circulation signals over tropical regions.In addition,it can also reflect the correlation with atmospheric low-frequency teleconnection.Therefore,concurrent variation of East Asian jet streams acts as a bridge or link between atmospheric circulation anomalies and climate change over East Asia continent.This study investigates the concurrent variations in the location and intensity of the East Asian winter jet streams,and the corresponding climate effects.The possible mechanisms for the concurrent variations are also explored with respect to the thermal-dynamics and dynamics.With evaluating the simulation ability of AMIP models for the concurrent variations,we find out the effect of Artic amplification on the simultaneous changes of East Asian jets.Finally,a novel metric for measuring the midlatitude wave activity is devised and used to evaluate the possible role of the precipitous Arctic sea ice melt during recent decades in the increasing frequency of midlatitude weather extremes.The main results are as follows:1.Concurrent variations in the location and intensity of East Asian winter jet streamsEast Asian upper-level jet stream includes subtropical jet stream(EASJ)and polar-front jet stream(EAPJ).The distribution of jet core number assures the key regions of two jets.The wintertime upper-level zonal wind variation over landmass is characterized by two principal modes,i.e.,the out-of-face variations in the intensity and meridional displacement of the EASJ/EAPJ.The EASJ and EAPJ are intensified(weakened)when they are located close to(for away from)each other,corresponding to a weakened(intensified)East Asian trough over the northern Pacific region and a decreased(increased)geopotential height in the mid-latitude over central Asia.The spatial pattern of 500hPa geopotential height anomaly shows a clear positive-negative-positive wave train pattern which is similar to the Eurasian teleconnection pattern.Besides,the sea level pressure(SLP)anomalies in the Arctic area are opposite to that in Atlantic and Pacific areas in the northern hemisphere demonstrating a large-scale seesaw between the Arctic basin and parts of the mid-latitude region,which is similar to the Arctic oscillation.2.The mechanisms of concurrent variations of East Asian jet streamsBoth thermal forcing like diabatic heating or SST anomaly and dynamic forcing inter atmosphere have effects on the changes of location and intensity of East Asian jet streams.Therefore,the mechanisms for these concurrent variations are examined from the perspective of thermal-dynamic forcing and the atmospheric internal dynamic processes.From the perspective of thermodynamics,when the EASJ and EAPJ are adjacent,the diabatic heating and horizontal heat transport increase the north-south temperature gradients,leading to the intensified EASJ/EAPJ based on the principle of the thermal wind.When the EASJ and EAPJ stay far away from each other,the effects of the thermodynamics are opposite to that when the EASJ and EAPJ stay close.From the perspective of the dynamics,the anomalies of the lower-level baroclinicity in the mid-latitude region are associated with the anomalies of the STEA in the mid-latitude region.However,the relationship is not robust in the subtropical region.In addition,utilizing the diagnostic tool of E vector,we find that in the NDC,large areas of divergent E vector appear on the northern side of the TP,leading to the intensified EAPJ.However,the EAPJ weakens in the FDC because the divergence region narrows down and the convergent E vector centers appear.Furthermore,the E-P flux of synoptic-scale stationary waves and its divergence are calculated to investigate interactions between the stationary waves and the zonal mean flow.Significant differences in the wave-mean flow interaction are found in the EASJ/EAPJ region.The divergence of the E-P flux of synoptic-scale stationary waves intensifies in the NDC,leading to the intensification of the EASJ and EAPJ.In contrast,the divergence weakens in the FDC,resulting in weakened EASJ and EAPJ.3.The effect of Artie amplification on concurrent variations of East Asian jets.The sea ice forcing is isolated by utilizing the AMIP experiments in order to analyze the effect of sea ice melting on the concurrent variations in the intensity and location of East Asian jets.The model simulations of AMIP ?SIC for the upper-level wind anomaly also readily show two principals indicating the out-of-face variations in the intensity(first principal)and meridional displacement(second principal)of the EASJ/EAPJ.The intensity change of both two jets shows annual variation with significant trend in the context of sea ice melting.However,the location change of polar-front jet presents both annual and inter-annual variations,with 2.5 degree southward shift.The zonal wind and meridional wind anomaly regressed from the time series of first principal indicate interesting features.Since the mid-late 1990 of the 20th century,especial the 21st century,increased sea ice melting accompanied with amplified Arctic warming,leads to weakened polar-front jet streams,with specially weakened zonal wind and increase meridional wind.In addition,the trend of polar-front jet location shows equatorward shift.4.The effect of Arctic amplification on extreme events over mid-latitude region and the mechanismThe novel metric is the finite-amplitude wave activity developed recently from the Geophysical Fluid Dynamics community,which is computed from 500hPa geopotential height and can be readily decomposed into the changes in the mean gradient and eddy stirring scale,thus naturally partitioned into a thermodynamic factor(former)and a dynamical one(latter).Evaluating the trends of the both total and local wave activities(LAWA)from ERA-I reanalysis against those from a set of AMIP type AGCM experiments,it shows that the observed Arctic sea ice loss lead to no midlatitude-wide increase in the total wave activity,leaving its explanation to the atmospheric internal variability.While locally the reduction of the sea ice over Barents-Kara Sea can induce an increase in LAWA over mid-latitude regions,especially over central northern Eurasia.The local anti-cyclonic wave activities(AN)have linked to more frequent and persistent anti-cyclonic circulation over northern Eurasia,inducing cold events to its southeastern flank.As anticipated from the large increase of LAWA,a probability dipole with less frequent cold events and more frequent warm events is found in the 1990-2014 trend of the PDF of the ERA-I surface temperature over the Barents-Kara Sea region,while an opposite dipole is found over the central Asia region,giving a confirmation for the connection between LAWA and cold extremes.According the balanced equation of LAWA,the positive AN trend over central northern Eurasia is at least partially maintained by the large AN convergence,which itself may be traced back to the Arctic region following the FAN vectors backward,hinting at a possible Arctic origin of the AN trend.Meanwhile,the cold advection origin from Arctic region leads to more cold snaps over adjacent continent like Eurasia.