Studies On The Concurrent Variability Between Wintertime East Asian Polar-front Jet And Subtropical Jet

The characteristic of East Asian upper-level jet streams (EAJS) is brought by the unique mechanical and thermodynamic conditions in East Asia. Two splitted jets are located at the southern and northern flank of the Tibetan Plateau (TP), namely, the East Asian subtropical jet (EASJ) and East Asian polar-front jet (EAPJ). The former is driven by the thermal Hadley cell, while the latter by the baroclinic eddy. The leading interannual variability modes of EAJS are diagnosed from the NCEP/NCAR reanalysis. Then, the concurrent variability between EASJ and EAPJ is studied from the perspective of simultaneous correlations and climatological effects. Based on the reanalyzed results, the simulations of climatology, occurrence and interannual variability of EAJS by the Coupled Model Intercomparison Project phase-5(CMIP5) models are evaluated. The projections of EAJS are further studied with emphasis on transient eddy kinetic energy (TEKE). The main results are as follows:1Diagnostic Analysis from Observational DataIn the climatological distribution of wind field at300hPa, two westerly jet axes are found at the southern and northern flank of the TP, corresponding to the locations of EASJ and EAPJ respectively. However, EAPJ can hardly be distinguished from the climatological wind field, since the wind speed decreases gradually from the subtropic area to midlatitude. From the statistic features of jet occurrence percentage (JOP) and jet core number (JCN) calculated from daily data, two distinct belts of jet occurrence are detected. Two splitted jets are associated with two branches of TEKE. The strong EASJ (weak EAPJ) over the landmass is accompanied by a weak (strong) TEKE at the southern (northern) side of the TP. The Empirical Orthogonal Function (EOF) analysis is applied to the vertical zonal wind weighted by grid mass and then the leading modes are extracted. The meridional dipole pattern of EOF1is analogue of the spatial pattern of zonal index (ZI) and annular mode (AM), corresponding to the centers of EASJ and EAPJ with out-of-phase variability of intensity. The EOF2represents the meridional displacement of the two jet axes. The first principal component (PC1), characterized by north-south shifts in atmospheric mass between the polar regions and the middle latitudes, can be regarded as the intensity of East Asian winter monsoon.The key region of EAPJ (70-100°E,50-60°) is then defined from the distributions of JCN and TEKE in winter. EASJ is divided into two parts, the land portion EASJ-L (70-100°E,25-30°N) and the ocean portion EASJ-O (120-150°E,25-35°N). The five-days leading correlation between EAPJ and EASJ-O is characterized by the synoptic wave propagating along the East Asia deep trough towards the westerly jet center over the ocean. The most significant leading correlation between EAPJ and EASJ-L happens around10days and this relation becomes stronger since90s.Composited analysis reveals that the concurrent variability defined as the decreased (increased) EASJ (EAPJ) is associated with the intensified polar vortex, the upper-level anticyclonic anomaly in East Asia-North Pacific and low-level warming in the East Asia land area. The four pairs (positive-positive, negative-negative, positive-negative, and negative-positive) are selected based on the time series of area-mean EAPJ and EASJ zonal wind. Then, using daily gauged observations, the temperature and precipitation anomaly is composited based on the four configurations. The results show that the concurrent variability of upper-level jets can be served as a remarkable indicator of surface weather and climate anomaly.2Evaluation of historical simulations from CSMsReasonable performance of CSMs in reproducing the current state is fundamental to credible projection of future climate change. Before the study of upper-level jets projected by CMIP5in future scenario, it is necessary to evaluate the model’s capability to reproduce the20th century climate.Firstly, historical simulations of upper-level jets by two CSMs developed by the Chinese communities are evaluated. BCC-CSM-1.1is studied from the perspective of stationary and transient eddies. The results show that the climatological EASJ is well reproduced whereas notable biases happen to EAPJ, underestimated intensity and reduced JCN. The deficiencies of eddy momentum and heat flux transport and accompanied eddy forcing may remarkably contribute to the biases of the simulated EAPJ, suggesting the potential importance of midlatitude internal atmospheric dynamics in shaping the tropospheric general circulation. The major features of the westerly jets in boreal winter, simulated by a newly developed climate system model FGOALS-g2, were evaluated with an emphasis on the meridional location of the westerly jet axis. Compared with the intensity bias, the southward shift of the westerly jet axis seems to be a more remarkable deficiency. The southward shift of westerly jet axis in FGOALS-g2is accompanied an equatorward displacement of the dynamic tropopause and associated climatology. As the westerly jet axis is closely related to the locations of the Hadley cell, tropopause and transient eddy activity, the accurate simulation of westerly jets will greatly improve the atmospheric general circulation and associated climatology in the model.The climatological distributions of upper-level jets over East Asia are generally reproduced in CMIP5models, albeit much differences among the models’framework and resolution. However, a large spread of both JCN and TEKE is caused across CMIP5models. Multi-model ensembles (MME) can effectually eliminate the intermodel discrepancy, leading to the significantly improvement the simulation of TEKE. Meanwhile, much of the reduced JCN over EAPJ area can be resulted from the limited performance of reproducing TEKE in CMIP5models.3Projection of EAJS from CMIP5modelsGenerally, in the future scenario of Representative Concentration Pathways4.5(RCP4.5), changes of jet mean states are smaller than models’bias (the difference of climatological zonal wind between historical simulation and reanalysis). The westerly jet axes almost remain still without distinct northward shift. From the perspective of interannual variability, the leading two modes, EOF1and EOF2, of wintertime upper-level jets are reproduced by CMIP5models and projected to be stabilized in RCP4.5scenario. The relatively change of explained variances for EOF1is smaller than that for EOF2. The response of TEKE to the climate change is much larger than the mean state and projected with significantly hemispheric comparison. The response in southern hemisphere is described as the zonal homogeneous and intermodel consensus, whereas northern hemisphere is characterized by regional specificity and models’diversity, especially in East Asia. In boreal winter, the projected changes of TEKE (AEKE) in the two storm track centers over North Pacific and North Atlantic are relatively small but with a significant poleward shift, whereas the TEKE in EAPJ is projected to keep the in situ but with a significant intensification (some region near20%). In EAPJ, the upper-level AEKE is connected with change of low-level static stability, Brunt-Vaisala frequency (ABVF)(AEKE∝-ABVF). All models project an intensified upper-level TEKE in a reduced low-level BVF. Meanwhile, the future change of EKE is constrained by its current state (AEKE∝-EKE). The negative correlation between AEKE and EKE indicates that transient component of mean state cannot grow free in the model, suggesting the’rich not get richer’internal model mechanism.