Impacts of Two Types of Northward Jumps of the East Asian Upper-tropospheric Jet Stream in Midsummer on Rainfall in Eastern China

The East Asian upper-tropospheric jet stream （EAJS） typically jumps north of 45~N in midsummer. These annual northward jumps are mostly classified into two dominant types： the first type corresponds to the enhanced westerly to the north of the EAJS＇s axis （type A）, while the second type is related to the weakened westerly within the EAJS＇s axis （type B）. In this study, the impacts of these two types of northward jumps on rainfall in eastern China are investigated. Our results show that rainfall significantly increases in northern Northeast China and decreases in the Yellow River-Huaihe River valleys, as well as in North China, during the type A jump. As a result of the type B jump, rainfall is enhanced in North China and suppressed in the Yangtze River valley. The changes in rainfall in eastern China during these two types of northward jumps are mainly caused by the northward shifts of the ascending air flow that is directly related to the EAJS. Concurrent with the type A （B） jump, the EAJS-related ascending branch moves from the Yangtze-Huai River valley to northern Northeast （North） China when the EAJS＇s axis jumps from 40~N to 55~N （50~N）. Meanwhile, the type A jump also strengthens the Northeast Asian low in the lower troposphere, leading to more moisture transport to northern Northeast China. The type B jump, however, induces a northwestward extension of the lower-tropospheric western North Pacific subtropical high and more moisture transport to North China.

Upper-tropospheric potential vorticity anomaly before the rapid intensification of Typhoon Mujigae(2015)and its response to reduced SST

As the strongest typhoon in 2015,the rapid intensification(RI)process of Typhoon Mujigae is simulated at the cloud-resolving scale by utilizing the Weather Research and Forecasting model,coupled with observed sea surface temperature(SST)and reduced SST as the control and sensitivity experiments,respectively.The spatiotemporal distribution characteristics of uppertropospheric potential vorticity(PV)and its response to SST changes are analyzed simultaneously.The results show a significant upper-tropospheric PV anomaly 6 h before RI,which indicates the approaching RI of Typhoon Mujigae.Deep convection overlies the strong signals of upper-tropospheric PV anomalies.More insight into the PV budget analysis verifies that the diabatic heating effect and vertical advection terms associated with deep convection dominate the upper-tropospheric PV anomaly,inferring that deep convection plays a crucial role in the upper-tropospheric PV anomaly.As SST decreases,diabatic heating and vertical advection effects are weakened due to reduced deep convection;therefore,the PV anomaly in the model run is not stronger than that in the control run,and the RI process of Mujigae disappears.Therefore,from the viewpoint of upper and lower-level interactions,how SST impacts upper-tropospheric PV anomalies and their interactions with RI are clarified.Thus,in addition to SST,the upper-tropospheric PV anomaly might be a significant factor in differentiating between RI and non-RI tropical cyclones.

A Simulation of the Upper-Tropospheric Temperature Pattern in BCC_CSM1.1

A simulation of the upper-tropospheric temperature (UTT) by the Beijing Climate Center Climate System Model version 1.1 (BCC_CSM1.1) model is evaluated through a comparison with NCEP/NCAR reanalysis data. It is shown that this model has the ability to simulate the climate pattern of the UTT in all four seasons. The spatial correlation on the climatological distribution between the simulation and the observation is 0.92, 0.93, 0.90, and 0.93 for spring, summer, autumn, and winter, respectively. The first leading mode of the UTT in the simulation agrees with that in the observation, except that the simulated second leading mode corresponds to the observed first leading mode in spring. The standard deviation distribution of the simulation is also roughly consistent with the observation, with a pattern coefficient of 0.82, 0.78, 0.82, and 0.82 in spring, summer, autumn, and winter, respectively. The potential UTT change in the second half of the 21st century under the Representative Concentration Pathway 8.5 (RCP8.5) scenario is examined. The prominent change is that the summer UTT will increase over Eurasia and decrease over the North Pacific compared with the present, indicating that the zonal thermal contrast between Asia and the North Pacific will be strengthened within the context of future global warming. The intensity of the interannual variability of the UTT over the Asian-Pacific region is also generally increased. The zonal thermal contrast between Asia and the North Pacific will tend to be enhanced in winter, concurrent with the intensified interannual variability.

Does a monsoon circulation exist in the upper troposphere over the central and eastern tropical Pacific?

Considering the central and eastern tropical Pacific （CETP） has important climate impacts, and its seasonal variability is also thought to be important, the authors used the monsoon investigation method named ＇dynamical normalized seasonality＇, which can precisely describe the wind vector direction over time, to analyze the upper-tropospheric circulation over the region. The authors discovered that there is a clear reversal of seasonal changes between winter and summer wind, just like the classic monsoon. Accordingly, the authors propose the new concept of the upper- troposphere monsoon over the CETP. The results extend the classical lower-troposphere monsoon region into the upper troposphere.

Characteristics of Atmospheric Circulation Associated with Cold Surge Occurrences in East Asia:A Case Study During 2005/06 Winter

The characteristics of the upper-level circulation and thermodynamical properties for the period when two distinct cold surges broke out over East Asia during the 2005/06 winter are investigated. From early December 2005 to early January 2006, exceptionally cold weather lasted for approximately one month due to two successive cold surges that took place on 2 December 2005 and 2 January 2006, respectively. This study reveals that both involve the upper-tropospheric circulation, which induces the amplification and expansion of the surface Siberian high toward East Asia, but arose from different causes： the former is caused by the upper-level blocking originated from the North Pacific and the latter is caused by the upper-level wave train across the Eurasian Continent. In particular, it is suggested that the lower-tropospheric anomalous wind caused by upper-level circulation anomalies and a steep meridional temperature gradient amplified by phase-locked annual cycle combined to induce very strong cold advection in East Asia, which resulted in exceptionally cold weather that lasted for several weeks. The present results emphasize that the characteristics of the upper-tropospheric circulation can be considered as important precursors to cold surge occurrences in East Asia.

A comparison study of atmospheric circulations and potential vorticity anomaly between the two rapid-intensified typhoons

For two rapid-intensification typhoons-Mujigae(2015)and Vicente(2012)-the atmospheric circulation conditions and potential vorticity(PV)anomaly are compared.Although similar in their rapid-intensification(RI)rate,their atmospheric circulation conditions differ considerably,with the absence or presence of an upper-tropospheric inverted trough(IT)being their main difference.The IT provides useful clues for the onset of RI,by estimating the interaction between the environmental upper-tropospheric IT and the typhoon based on eddy momentum flux convergence calculation.The trough–typhoon interaction is examined by comparing the PV transport process for the two cases.An isolated positive PV column develops vertically near Mujigae’s onset of RI,which is not influenced by synoptic-scale PV advection.However,for Vicente,another source-advection from a high-latitude PV reservoir along the upper-tropospheric IT-joins the built-up high-PV anomaly in favor of RI.