Using ERA-40 reanalysis daily data for the period 1958-2002, this study investigated the effect of tran- sient eddy （TE） on the interannual meridional displacement of summer East Asian subtropical jet （EASJ） by co...Using ERA-40 reanalysis daily data for the period 1958-2002, this study investigated the effect of tran- sient eddy （TE） on the interannual meridional displacement of summer East Asian subtropical jet （EASJ） by conducting a detailed dynamical diagnosis. The summer EASJ axis features a significant interannual coherent meridional displacement. Associated with such a meridional displacement, the TE vorticity forcing anomalies are characterized by a meridional dipole pattern asymmetric about the climatological EASJ axis. The TE vorticity forcing anomalies yield barotropic zonal wind tendencies with a phase meridionally lead- ing the zonal wind anomalies, suggesting that they act to reinforce further meridional displacement of the EASJ and favor a positive feedback in the TE and time-mean flow interaction. However, The TE thermal forcing anomalies induce baroclinic zonal wind tendencies that reduce the vertical shear of zonal wind and atmospheric baroclinicity and eventually suppress the TE activity, favoring a negative feedback in the TE and time-mean flow interaction. Although the two types of TE forcing tend to have opposite feedback roles, the TE vorticity forcing appears to be dominant in the TE effect on the time-mean flow.展开更多
Upper-level jet streams over East Asia simulated by the LASG/IAP coupled climate system model FGOALS-s2 were assessed, and the mean state bias explained in terms of synoptic-scale transient eddy activ- ity （STEA）. T...Upper-level jet streams over East Asia simulated by the LASG/IAP coupled climate system model FGOALS-s2 were assessed, and the mean state bias explained in terms of synoptic-scale transient eddy activ- ity （STEA）. The results showed that the spatial distribution of the seasonal mean jet stream was reproduced well by the model, except that following a weaker meridional temperature gradient （MTG）, the intensity of the jet stream was weaker than in National Centers for Environment Prediction （NCEP）/Department of Energy Atmospheric Model Inter-comparison Project II reanalysis data （NCEP2）. Based on daily mean data, the jet core number was counted to identify the geographical border between the East Asian Sub- tropical Jet （EASJ） and the East Asian Polar-front Jet （EAPJ）. The border is located over the Tibetan Plateau according to NCEP2 data, but was not evident in FGOALS-s2 simulations. The seasonal cycles of the jet streams were found to be reasonably reproduced, except that they shifted northward relative to reanalysis data in boreal summer owing to the northward shift of negative MTGs. To identify the reasons for mean state bias, the dynamical and thermal forcings of STEA on mean flow were examined with a focus on boreal winter. The dynamical and thermal forcings were estimated by extended Eliassen-Palm flux （E） and transient heat flux, respectively. The results showed that the failure to reproduce the tripolar-pattern of the divergence of E over the jet regions led to an unsuccessful separation of the EASJ and EAPJ, while dynamical forcing contributed less to the weaker EASJ. In contrast, the weaker transient heat flux partly explained the weaker EASJ over the ocean.展开更多
An observational study focusing on the contribution of tropical cyclones(TCs)that form over the western North Pacific(WNP)to the synoptic-scale transient eddy activity(STEA)over the North Pacific during the boreal aut...An observational study focusing on the contribution of tropical cyclones(TCs)that form over the western North Pacific(WNP)to the synoptic-scale transient eddy activity(STEA)over the North Pacific during the boreal autumn and early winter in the period 1979–2019 is presented in this paper.Statistical results show that WNP TCs entering the midlatitudinal North Pacific provide significant positive effects on the pentad mean strength of STEA,which is primarily concentrated over the Kuroshio/Oyashio Extensions(KOE)and regions from east of Japan to 160°W in the lower and midto-upper troposphere,respectively.TC intensity is highly indicative of the subsequent STEA with a correlation coefficient of 0.37/0.33/0.45 at 300 hPa/500 hPa/850 hPa exceeding the 99%confidence level for the period 1979–2019.The strength of STEA in the upper troposphere associated with TCs presents a more significant linear growth with TC intensity than that at the mid-to-lower levels after the cyclones enter the KOE region,suggesting that the impact of TCs on STEA gradually increases with height.Further analyses reveal that the contribution of TCs accounts for 4%–6%of the total STEA change over the KOE region during the late autumn and early winter.In addition,the influence of TCs on STEA experienced an interdecadal decrease from the early 2000 s through the early 2010 s.展开更多
Diagnostic comparison of the East Asian subtropical jet （EASJ） and polar-front jet （EAPJ） in winter season is carried out by using the ERA-40 dataset. The large-scale circulation characteristics and synopticscale ...Diagnostic comparison of the East Asian subtropical jet （EASJ） and polar-front jet （EAPJ） in winter season is carried out by using the ERA-40 dataset. The large-scale circulation characteristics and synopticscale transient eddy activities （STEAs） associated with the EASJ and EAPJ are examined. The results show that the EASJ and EAPJ in the upper-level monthly mean data have no clear geographical border, while the distribution of the numbers of jet cores from the daily data exhibits a distinct boundary at the latitudes of the northern Tibetan Plateau. The two areas with large numbers of jet cores correspond to the EASJ and EAPJ regions. The analysis of STEAs over the East Asian region shows a spatial match of STEAs with the EASJ and EAPJ in winter： the strong EASJ is located within the weak southern branch of the STEA while the relatively weak EAPJ appears within the active northern branch of the STEA, indicating that the EAPJ is the jet coexisting with the STEA. Further analysis shows two anomalous modes of the winter EAPJ： the anomalous anticyclonic/cyclonic circulation and the weakened/strengthened local westerly wind. The large-scale circulation anomalies in the Northern Hemisphere related to the first mode are concentrated in the Eurasian mid to high latitudes, and are also influenced by the anomalous circulation in the upstream area. When the local westerly wind over the EAPJ region is weakened/strengthened, the westerly jet in the eastern part of the EASJ and that in the western Pacific region show opposite variations. The corresponding anomalous atmospheric circulation demonstrates the Eurasian （EU） pattern. The EAPJ anomalies are also closely linked with the STEA anomalies over East Asia. The anomalies in the northern branch of the STEA propagate as a wave train along its axis into the East Asian coastal waters, and then migrate eastward to the oceanic region. However, the ones near the southern branch are trapped over the eastern part of East Asia and its coastal waters at 200 hPa.展开更多
The relationships between the 200-hPa westerly jet stream anomalies over the East Asian coastal waterwestern Pacific （WPJS）, and the oceanic surface heating and synoptic-scale transient eddy （STE） activity anomali...The relationships between the 200-hPa westerly jet stream anomalies over the East Asian coastal waterwestern Pacific （WPJS）, and the oceanic surface heating and synoptic-scale transient eddy （STE） activity anomalies over the North Pacific in wintertime are examined by using ERA-40 and NCEP/NCAR reanalysis data. The analysis demonstrates that the surface heating and the STE anomalies have different patterns, corresponding to the three WPJS anomalous modes, respectively. In the first WPJS anomalous mode, the WPJS main part shows no robust anomaly. The anomalous westerly wind, occurring over the mid-latitude central-eastern Pacific past the date line is associated with the anomalous heating presenting both in the tropical central-eastern Pacific past the date line and the center of the North Pacific basin. Meanwhile, the STE anomaly appears around the region of the anomalous zonal wind. The fluctuation in jet strength shown in the second WPJS mode is strongly related to the heating anomaly in the Kuroshio Current region and the STE anomaly in the jet exit region. The third mode demonstrates a northward/southward shift of the WPJS, which has a statistical connection with a south-north dipolar pattern of the heating anomaly in the western North Pacific separated at 35°N. Meanwhile, the STE spatial displacement is in conjunction with jet shifts in the same direction. The heating anomaly has a close connection with the atmospheric circulation, and thus changes the mid-latitude baroclinicity, leading to the STE anomaly, which then reinforces the WPJS anomaly via internal atmospheric dynamics.展开更多
In this paper, major features of the upper-tropospheric jet streams simulated by a coupled Climate System Model BCC_ CSMI.1 are evaluated through comparison with the NCEP/NCAR reanalysis. The jet streams consist of th...In this paper, major features of the upper-tropospheric jet streams simulated by a coupled Climate System Model BCC_ CSMI.1 are evaluated through comparison with the NCEP/NCAR reanalysis. The jet streams consist of the East Asian subtropical jet （EASJ） and the East Asian polar-front jet （EAPJ）. Associated stationary wave and synoptic-scale transient eddy activities （STEA） are also examined. The results show that the climatological positions of the westerly jet streams are well captured by BCC_CSMI.1, but with slight intensity biases. Statistics from the 6-h model outputs reveal that the jet core number （JCN） of ESPJ is significantly underestimated. Examination of the simulated seasonal evolution of the westerly jet stream indicates that the model has produced a westward movement of the EASJ core in May, one month earlier than that in the reanalysis. Analysis of stationary wave activities shows that the overestimated meridional wind component may have caused considerable enhancement of meridional momentum and heat transport. The stationary Rossby wave represented by the wave activity flux at the southern flank of the Tibetan Plateau is favorable to the growth of asymmetric zonal wind and the multiple-center pattern of JCN. Unlike the stationary wave heat flux transport, the model tends to systematically generate weaker transient heat flux over East Asia. Further analysis of STEA exhibits a general consistent pattern between the simulation and the reanalysis, while the intensity of the northern STEA branch associated with the EAPJ is greatly reduced. The deficiencies of eddy momentum and heat flux transport and accompanied eddy forcing may contribute to the biases of the simulated upper-tropospheric jet streams, suggesting the potential importance of midlatitude internal atmospheric dynamics in shaping the tropospheric general circulation, which is not yet fully and accurately resolved in the current BCC-CSMI.1.展开更多
The electromagnetic concentrative coils are indispensable in the functional magnetic stimulation and have potential applications in nondestructive testing. In this paper, we propose a figure-8-shaped coil being compos...The electromagnetic concentrative coils are indispensable in the functional magnetic stimulation and have potential applications in nondestructive testing. In this paper, we propose a figure-8-shaped coil being composed of two arbitrary oblique elliptical coils, which can change the electromagnetic concentrative region and the magnitude of eddy current density by changing the elliptical shape and/or spread angle between two elliptical coils. Pulsed current is usually the excitation source in the functional magnetic stimulation, so in this paper we derive the analytical solutions of transient pulsed eddy current field in the time domain due to the elliptical concentrative coil placed in an arbitrary position over a half-infinite plane conductor by making use of the scale-transformation, the Laplace transform and the Fourier transform are used in our derivation. Calculation results of field distributions produced by the figure-8-shaped elliptical coil show some behaviours as follows： 1） the eddy currents are focused on the conductor under the geometric symmetric centre of figure-8-shaped coil; 2） the greater the scale factor of ellipse is, the higher the eddy current density is and the wider the concentrative area of eddy current along y axis is; 3） the maximum magnitude of eddy current density increases with the increase of spread angle. When spread angle is 180°, there are two additional reverse concentrative areas on both sides of x axis.展开更多
The spatial distribution of eddy diffusivity,basic characteristics of coherent mesoscale eddies and their relationship are analyzed from numerical model outputs in the Southern Ocean.Mesoscale fluctuation information ...The spatial distribution of eddy diffusivity,basic characteristics of coherent mesoscale eddies and their relationship are analyzed from numerical model outputs in the Southern Ocean.Mesoscale fluctuation information is obtained by a temporal-spatial filtering method,and the eddy diffusivity is calculated using a linear regression analysis between isoneutral thickness flux and large-scale isoneutral thickness gradient.The eddy diffusivity is on the order of O(103 m2/s)with a significant spatial variation,and it is larger in the area with strong coherent mesoscale eddy activity.The mesoscale eddies are mainly located in the upper ocean layer,with the average intensity no larger than 0.2.The mean radius of the coherent mesoscale cyclonic(anticyclonic)eddy gradually decays from(121.2±10.4)km((117.8±9.6)km)at 30°S to(43.9±5.3)km((44.7±4.9)km)at 65°S.Their vertical penetration depths(lifespans)are deeper(longer)between the northern side of the Subpolar Antarctic Front and 48°S.The normalized eddy diffusivity and coherent mesoscale eddy activity show a significant positive correlation,indicating that coherent mesoscale eddy plays an important role in eddy diffusivity.展开更多
Previous studies have indicated a global reversal of near-surface wind speeds from a declining trend to an increasing trend around 2010;however,it remains unclear whether upper-air wind speeds exhibit a similar revers...Previous studies have indicated a global reversal of near-surface wind speeds from a declining trend to an increasing trend around 2010;however,it remains unclear whether upper-air wind speeds exhibit a similar reversal.This study evaluates reanalysis products using surface and radiosonde observations to analyze upper-air wind speed variations in the Northern Hemisphere,focusing on their seasonal and latitudinal differences.Results demonstrate that JRA-55 effectively captures wind speed variations in the Northern Hemisphere.Notably,upper-air wind speeds over land experienced a reversal in winter 2010 with significant latitudinal differences.The trend reversal of upper wind speed between the midlatitudes and subtropics presents a dipole pattern.From 1990 to 2010,upper-air wind speeds in the midlatitudes(40°-70°N)significantly declined,while the subtropical zone(20°-40°N)displayed an opposite trend.However,during 2010-2020,wind speeds in the midlatitudes shifted to a significant positive trend,whereas the subtropics experienced a significant negative trend.The variations in Northern Hemisphere winter wind speeds can be attributed to changes in low-level baroclinicity driven by tropical diabatic heating and midlatitude transient eddy feedback.Enhanced diabatic heating and weakened eddy feedback during 1990-2010 contributed to reduced wind speeds in the midlatitudes and increased speeds in the subtropics,while reduced diabatic heating and strengthened eddy feedback during 2010-2020 resulted in increased wind speeds in the midlatitudes and decreased speeds in the subtropics.The reversal of upper-air wind speeds could affect surface wind speeds by downward momentum transfer,which could contribute to the reversal of surface wind speeds.展开更多
In this paper, the boundary value problem (BVP) of 3 D transient eddy current field in the end region in the case that the generator is affected by impact load is specified. Besides, ways to implement discrete method...In this paper, the boundary value problem (BVP) of 3 D transient eddy current field in the end region in the case that the generator is affected by impact load is specified. Besides, ways to implement discrete methods in both time domain and space domain during the solution of the problem are investigated. The Crank Nicolson scheme is utilized to attain the iterative format of time differential, after taking factors that can ensure both computation precision and stability into consideration. In this paper, the magnetic distribution in the end region of a turbogenerator in the case that the generator is affected by impact load is specified. As a result, it provides foundation for further study of electromagnetic force and electromagnetic vibration in the end region of the turbogenerator.展开更多
Based on diagnostic analysis of reanalysis data for 58-year,the distribution characteristics of decadal variability in diabatic heating,transient eddy heating and transient eddy vorticity forcing related to the sea su...Based on diagnostic analysis of reanalysis data for 58-year,the distribution characteristics of decadal variability in diabatic heating,transient eddy heating and transient eddy vorticity forcing related to the sea surface temperature(SST)anomalies over the North Pacific,as well as their relationship with anomalous atmospheric circulation have been investigated in this paper.A linear baroclinic model(LBM)was used to investigate atmospheric responses to idealized and realistic heat and vorticity forcing anomalies,and then to compare relative roles of different kinds of forcing in terms of geopotential height responses.The results illustrate that the responses of atmospheric height fields to the mid-latitude heating can be either baroclinic or barotropic.The response structure is sensitive to the relative horizontal location of heating with respect to the background jet flow,as well as to the vertical profile of heating.The response to the idealized deep heating over the eastern North Pacific,mimicking the observed heating anomaly,is baroclinic.The atmospheric response to the mid-latitude vorticity forcing is always barotropic,resulting in a geopotential low that is in phase with the forcing.The atmospheric responses to the realistic heat and vorticity forcing show the similar results,suggesting that diabatic heating,transient eddy heating and transient eddy vorticity forcing can all cause atmospheric anomalies and that the vorticity forcing plays a relatively more important role in maintaining the equivalent-barotropic structure of geopotential height anomalies.展开更多
The most severe snowstorm and freezing-rain event in the past 50 years hit central and southern China in January 2008. One of the main reasons for the anomalous climate event was the occurrence of atmospheric circulat...The most severe snowstorm and freezing-rain event in the past 50 years hit central and southern China in January 2008. One of the main reasons for the anomalous climate event was the occurrence of atmospheric circulation anomalies over middle and high latitudes, particularly the persistent blocking that occurred over the Ural Mountains. Along with atmospheric anomalies, a strong La Nia event in the Pacific and warm sea surface temperature anomalies （SSTAs） in the North Atlantic were the most significant in the lower boundary. Since a brief analysis suggests that La Nia exerts no significant impact on the Urals, the key point of focus in this study is on the role of the warmer SSTAs in the North Atlantic. Based on an observational composite, North Atlantic SSTAs pattern when the height anomaly over the Urals is strongly positive is found similar to that in January 2008, but no significant SSTAs occurred elsewhere, such as the Pacific. Using an atmospheric general circulation model, ECHAM5, the impact of North Atlantic SSTAs on the extratropical atmosphere circulation in the event was investigated. The results show that the warm SSTAs strengthened the blocking high over the Urals, through anomalous transient eddies. The consistency between the study model and the observational composite indicates that the warm SSTAs in the North Atlantic were indeed an important factor in the formation of the snowstorm disaster of January 2008.展开更多
During recent decades, the tropical Indo-Pacific Ocean has become increasingly warmer. Meanwhile, both the northern and southern hemispheric polar vortices （NPV and SPV） have exhibited a deepening trend in boreal wi...During recent decades, the tropical Indo-Pacific Ocean has become increasingly warmer. Meanwhile, both the northern and southern hemispheric polar vortices （NPV and SPV） have exhibited a deepening trend in boreal winter. Although previous studies have revealed that the tropical Indian Ocean warming （IOW） favors an intensifying NPV and a weakening SPV, how the tropical Pacific Ocean warming （POW） influences the NPV and SPV remains unclear. In this study, a comparative analysis has been conducted through ensemble atmospheric general circulation model （AGCM） experiments. The results show that, for the Northern Hemisphere, the two warmings exerted opposite impacts in boreal winter, in that the IOW intensified the NPV while the POW weakened the NPV. For the Southern Hemisphere, both the IOW and POW warmed the southern polar atmosphere and weakened the SPV. A diagnostic analysis based on the vorticity budget revealed that such an interhemispheric difference in influences from the IOW and POW in boreal winter was associated with different roles of transient eddy momentum flux convergence between the hemispheres. Furthermore, this difference may have been linked to different strengths of stationary wave activity between the hemispheres in boreal winter.展开更多
The 1998 summer-time floods at the Yangtze River basin of China, the severest in last 50 years or so, directly resulted from the abnormal extension of Meiyu (rainy season), which was related to a weak East Asian summe...The 1998 summer-time floods at the Yangtze River basin of China, the severest in last 50 years or so, directly resulted from the abnormal extension of Meiyu (rainy season), which was related to a weak East Asian summer monsoon and persistent anomalies of extratropical circulation. The long persistence of blocking over the Ural Mountains is a conspicuous feature. The physical processes responsible for the prolonged maintenance of this key system are investigated in terms of internal forcing (transient eddy upon basic flow) and external forcing (tropical heating forcing) via diagnosis and numerical experiments in the paper. Using the adjoint method, the location and structure of optimal perturbations favorable for the development and maintenance of Ural blocking are identified, which shows an apparent coincidence with the observed storm track at the eastern Atlantic to Europe sector. The diagnosis of E-vector and the response of baroclinic stationary wave to transient forcing both suggest further that the enhanced transient eddy activity favors the occurrence and maintenance of positive anomalies. The upper-level jet and heat sources (sinks) during that period are calculated, and the results indicate that the anomaly of upper jet and tropical heating is evident. The ensemble forecasting experiments by a GCM, IAP T42L9 show that the anomalous heating over the tropics, especially over the central-western Pacific and Atlantic, favors the formation of positive anomalies of height at the Ural region. Finally, a self-sustain mechanism of positive anomalies through two-way interaction between planetary stationary wave and transient eddy under the stimulation of anomalous tropical heating is proposed.展开更多
In this paper, we report on the results of an investigation into inter-decadal changes in moisture transport and divergence in East Asia for the two periods 1980-2001 and 1958 1979. The aim is to explore the mechanism...In this paper, we report on the results of an investigation into inter-decadal changes in moisture transport and divergence in East Asia for the two periods 1980-2001 and 1958 1979. The aim is to explore the mechanism of summer rainfall change in the region after abrupt changes. The relevant changes are calculated using ERA-40 daily reanalysis datasets. The results show that both stationary and transient eddy moisture transports to the Chinese mainland have declined since the abrupt change in atmospheric general circulation in the late 1970s, leading to more rainfall in South China and less in the North. The anomalous rainfall pattern coincides well with anomalous large-scale moisture divergence in the troposphere, of which stationary-wave or monsoon transport is dominant, in comparison with the contribution of the transient eddies. F^rthermore, their divergences are found to be in opposite phases. In addition, meridional divergence is more important than its zonal counterpart, with an opposite phase in East Asia. Abnormal zonal moisture convergences appear in northwestern and northeastern parts of China, and are related to the excess rainfalls in these regions. An increase in transient eddy activity is one of the major mechanisms for excess rainfall in northern Xinjiang. Consequently, the anomalous rainfall pattern in East Asia results from a decline of the East Asian monsoon after the abrupt change, while the rainfall increase in northwestern China involves anomalies of both stationary waves and transient eddies on boreal westerly over the mid- and high latitudes.展开更多
Based on the climatological reanalysis data of the European Center for Medium-Range Weather Forecasts and the Arctic sea ice data of the National Snow and Ice Data Center, the relationship between the Arctic sea ice a...Based on the climatological reanalysis data of the European Center for Medium-Range Weather Forecasts and the Arctic sea ice data of the National Snow and Ice Data Center, the relationship between the Arctic sea ice area(SIA)and the interannual variation of atmospheric meridional heat transport(AMHT) was analyzed. The results show that the atmospheric meridional heat transported by transient eddy(TAMHT) dominates the June AMHT in midhigh latitudes of the Northern Hemisphere, while the western Baffin Bay(B) and the eastern Greenland(G) are two gates for TAMHT entering the Arctic. TAMHT in the western Baffin Bay(B-TAMHT) and eastern Greenland(G-TAMHT) has a concurrent variation of reverse phase, which is closely related to the summer Arctic SIA.Possible mechanism is that the three Arctic atmospheric circulation patterns(AD, AO and NAO) in June can cause the concurrent variation of TAMHT in the B and G regions. This concurrent variation helps to maintain AD anomaly in summer through wave action and changes the polar air temperature, thus affecting the summer Arctic SIA. Calling the heat entering the Arctic as warm transport and the heat leaving Arctic as cold transport, then the results are classified into three situations based on B-TAMHT and G-TAMHT: warm B corresponding to cold G(WC), cold B corresponding to warm G(CW), cold B corresponding to cold G(CC), while warm B corresponding to warm G is virtually non-existent. During the WC situation, the SIA in the Pacific Arctic sediments and Kara Sea decreases;during the CW situation, the SIA in the Laptev Sea and Kara Sea decreases;during the CC situation, the SIA in the Kara Sea, Laptev Sea and southern Beaufort Sea increases.展开更多
This study concerns atmospheric responses to the North Pacific subtropical front （NPSTF） in boreal spring over the period 1982-2014. Statistical results show that a strong NPSTF in spring can significantly enhance t...This study concerns atmospheric responses to the North Pacific subtropical front （NPSTF） in boreal spring over the period 1982-2014. Statistical results show that a strong NPSTF in spring can significantly enhance the East Asian jet stream （EAJS）. Both transient eddy activity and the atmospheric heat source play important roles in this process. The enhanced atmospheric temperature gradient due to a strong NPSTF increases atmospheric baroclinicity, resulting in an intensification of transient eddy and convection activities. On the one hand, the enhanced transient eddy activities can excite an anomalous cyclonic circulation with a quasi-baraotropical structure in the troposphere to the north of the NPSTF. Accordingly, the related westerly wind anomalies around 30°N can intensify the component of the EAJS over the Northeast Pacific. On the other hand, an enhanced atmospheric heat source over the NPSTF, which is related to increased rainfall, acts to excite an anomalous cyclonic circulation system in the troposphere to the northwest of the NPSTF, which can explain the enhanced component of the EAJS over the Northwest Pacific. The two mechanisms may combine to enhance the EAJS.展开更多
In this paper,the dynamical evolutions of two types of Arctic Oscillation (AO),the stratospheric (S) and tropospheric (T) types,have been investigated on an intermediate time scale in terms of transient eddy feedback ...In this paper,the dynamical evolutions of two types of Arctic Oscillation (AO),the stratospheric (S) and tropospheric (T) types,have been investigated on an intermediate time scale in terms of transient eddy feedback forcing and three-dimensional Rossby wave propagation.S-Type (T-type) events are characterized by an anomalous stratospheric polar vortex that is in phase (out of phase) with its tropospheric counterpart.Approximately onethird of AO events,both positive and negative,are T-type events.For the positive phase of a T-type event,the formation and maintenance of stratospheric positive anomalies over the polar cap are associated with an upward propagation of Rossby wave packets originating from the near-tropopause altitude over northeastern Asia.However,such upward propagating features are not found for S-type events.In the troposphere,transient eddy feedback forcing is primarily responsible for the meridional seesaw structure of both the S-and T-type events,with an additional contribution from Rossby wave propagation.展开更多
The Kuroshio extension(KE)exhibits interdecadal variability,oscillating from a stable state to an unstable state.In this paper,ERA-Interim reanalysis data are used to discuss the possible reasons for the asymmetric re...The Kuroshio extension(KE)exhibits interdecadal variability,oscillating from a stable state to an unstable state.In this paper,ERA-Interim reanalysis data are used to discuss the possible reasons for the asymmetric response of the atmosphere to symmetric sea surface temperature anomaly(SSTA)during periods of differential KE states.The analysis has the following results:the SSTA presents a nearly symmetrical distribution with opposite signs during the KE stable and unstable periods.During the KE stable period,the storm track is located north of 40°N and is significantly enhanced in the northeast Pacific Ocean.The atmospheric response is similar to the West Pacific/North Pacific Oscillation teleconnection(WP/NPO like pattern)and presents a barotropic structure.The inversion results of the potential vorticity equation show that the feedback of transient eddy vorticity manifests a WP/NPO like pattern and presents a barotropic structure,which is the main reason for bringing about the response of the WP/NPO like pattern.The magnitude of the feedbacks of both diabatic heating and transient eddy heating is small,which can offset one another.During the KE unstable period,the main body of the storm track is located to the south of 40°N,and there is no significant response signal in the atmosphere,except near the west coast of North America.Compared with the KE stable period,the asymmetry of response of the transient eddy vorticity is the main reason for the asymmetric response of the atmosphere.展开更多
Since the 2000 s,extratropical extremes have been more frequent,which are closely related to anomalies of planetary-scale and synoptic-scale systems.This study focuses on a key synoptic system,the extratropical cyclon...Since the 2000 s,extratropical extremes have been more frequent,which are closely related to anomalies of planetary-scale and synoptic-scale systems.This study focuses on a key synoptic system,the extratropical cyclonic vortex(ECV)over land,to investigate its relations with extreme precipitation.It was found that ECVs have been more active post-2000,which has induced more extreme precipitation,and such variation is projected to persist along with increasing temperature within 1.5℃of global warming.An enhanced quasi-stationary vortex(QSV)primarily contributes to the ECV,rather than inactive synoptic-scale transient eddies(STEs).Inactive STEs respond to a decline in baroclinicity due to the tendency of the homogeneous temperature gradient.However,such conditions are helpful to widening the westerly jet belt,favoring strong dynamic processes of quasi-resonant amplification and interaction of STEs with the quasi-stationary wave,and the result favors an increasing frequency and persistence of QSVs,contributing to extreme precipitation.展开更多
基金supported by the National Natural Science Foundation of China (Grant Nos. 40730953 and 40805025)the 973 program (Grant No. 2010CB428504)+1 种基金the National Public Benefit Research Foundation of China (Grant No.GYHY200806004)the Jiangsu Natural Science Foun-dation (Grant No.BK2008027)
文摘Using ERA-40 reanalysis daily data for the period 1958-2002, this study investigated the effect of tran- sient eddy （TE） on the interannual meridional displacement of summer East Asian subtropical jet （EASJ） by conducting a detailed dynamical diagnosis. The summer EASJ axis features a significant interannual coherent meridional displacement. Associated with such a meridional displacement, the TE vorticity forcing anomalies are characterized by a meridional dipole pattern asymmetric about the climatological EASJ axis. The TE vorticity forcing anomalies yield barotropic zonal wind tendencies with a phase meridionally lead- ing the zonal wind anomalies, suggesting that they act to reinforce further meridional displacement of the EASJ and favor a positive feedback in the TE and time-mean flow interaction. However, The TE thermal forcing anomalies induce baroclinic zonal wind tendencies that reduce the vertical shear of zonal wind and atmospheric baroclinicity and eventually suppress the TE activity, favoring a negative feedback in the TE and time-mean flow interaction. Although the two types of TE forcing tend to have opposite feedback roles, the TE vorticity forcing appears to be dominant in the TE effect on the time-mean flow.
基金supported by the National High Technology Research and Development Program of China(Grant No.2010AA012304)the National Program on Key Basic Research Project of China(Grant No.2010CB951904)+1 种基金the National Natural Science Foundation of China project(Grant No.41125017)the"Strategic Priority Research Program-Climate Change:Carbon Budget and RelatedIssues"of the Chinese Academy of Sciences(Grant No.XDA05110301)
文摘Upper-level jet streams over East Asia simulated by the LASG/IAP coupled climate system model FGOALS-s2 were assessed, and the mean state bias explained in terms of synoptic-scale transient eddy activ- ity （STEA）. The results showed that the spatial distribution of the seasonal mean jet stream was reproduced well by the model, except that following a weaker meridional temperature gradient （MTG）, the intensity of the jet stream was weaker than in National Centers for Environment Prediction （NCEP）/Department of Energy Atmospheric Model Inter-comparison Project II reanalysis data （NCEP2）. Based on daily mean data, the jet core number was counted to identify the geographical border between the East Asian Sub- tropical Jet （EASJ） and the East Asian Polar-front Jet （EAPJ）. The border is located over the Tibetan Plateau according to NCEP2 data, but was not evident in FGOALS-s2 simulations. The seasonal cycles of the jet streams were found to be reasonably reproduced, except that they shifted northward relative to reanalysis data in boreal summer owing to the northward shift of negative MTGs. To identify the reasons for mean state bias, the dynamical and thermal forcings of STEA on mean flow were examined with a focus on boreal winter. The dynamical and thermal forcings were estimated by extended Eliassen-Palm flux （E） and transient heat flux, respectively. The results showed that the failure to reproduce the tripolar-pattern of the divergence of E over the jet regions led to an unsuccessful separation of the EASJ and EAPJ, while dynamical forcing contributed less to the weaker EASJ. In contrast, the weaker transient heat flux partly explained the weaker EASJ over the ocean.
基金sponsored jointly by the National Key Basic Research Program(2018YFC1505905)National Natural Science Foundation of China(Grant Nos.41975090,41922033,41675077,and 42005025)+2 种基金Scientific Research Program of National University of Defense Technology(18/19-QNCXJ)the Jiangsu Collaborative Innovation Center for Climate Change in Nanjing Universitythe Jiangsu Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters。
文摘An observational study focusing on the contribution of tropical cyclones(TCs)that form over the western North Pacific(WNP)to the synoptic-scale transient eddy activity(STEA)over the North Pacific during the boreal autumn and early winter in the period 1979–2019 is presented in this paper.Statistical results show that WNP TCs entering the midlatitudinal North Pacific provide significant positive effects on the pentad mean strength of STEA,which is primarily concentrated over the Kuroshio/Oyashio Extensions(KOE)and regions from east of Japan to 160°W in the lower and midto-upper troposphere,respectively.TC intensity is highly indicative of the subsequent STEA with a correlation coefficient of 0.37/0.33/0.45 at 300 hPa/500 hPa/850 hPa exceeding the 99%confidence level for the period 1979–2019.The strength of STEA in the upper troposphere associated with TCs presents a more significant linear growth with TC intensity than that at the mid-to-lower levels after the cyclones enter the KOE region,suggesting that the impact of TCs on STEA gradually increases with height.Further analyses reveal that the contribution of TCs accounts for 4%–6%of the total STEA change over the KOE region during the late autumn and early winter.In addition,the influence of TCs on STEA experienced an interdecadal decrease from the early 2000 s through the early 2010 s.
基金Supported by the National Natural Science Foundation of China under Grant Nos. 40775044 and 40730953the Natural Science Foundation of Jiangsu Province under Grant No. BK2008027
文摘Diagnostic comparison of the East Asian subtropical jet （EASJ） and polar-front jet （EAPJ） in winter season is carried out by using the ERA-40 dataset. The large-scale circulation characteristics and synopticscale transient eddy activities （STEAs） associated with the EASJ and EAPJ are examined. The results show that the EASJ and EAPJ in the upper-level monthly mean data have no clear geographical border, while the distribution of the numbers of jet cores from the daily data exhibits a distinct boundary at the latitudes of the northern Tibetan Plateau. The two areas with large numbers of jet cores correspond to the EASJ and EAPJ regions. The analysis of STEAs over the East Asian region shows a spatial match of STEAs with the EASJ and EAPJ in winter： the strong EASJ is located within the weak southern branch of the STEA while the relatively weak EAPJ appears within the active northern branch of the STEA, indicating that the EAPJ is the jet coexisting with the STEA. Further analysis shows two anomalous modes of the winter EAPJ： the anomalous anticyclonic/cyclonic circulation and the weakened/strengthened local westerly wind. The large-scale circulation anomalies in the Northern Hemisphere related to the first mode are concentrated in the Eurasian mid to high latitudes, and are also influenced by the anomalous circulation in the upstream area. When the local westerly wind over the EAPJ region is weakened/strengthened, the westerly jet in the eastern part of the EASJ and that in the western Pacific region show opposite variations. The corresponding anomalous atmospheric circulation demonstrates the Eurasian （EU） pattern. The EAPJ anomalies are also closely linked with the STEA anomalies over East Asia. The anomalies in the northern branch of the STEA propagate as a wave train along its axis into the East Asian coastal waters, and then migrate eastward to the oceanic region. However, the ones near the southern branch are trapped over the eastern part of East Asia and its coastal waters at 200 hPa.
基金the National Natural Science Foundation of China under Grant Nos.40675041,40333026,and 40425009.
文摘The relationships between the 200-hPa westerly jet stream anomalies over the East Asian coastal waterwestern Pacific （WPJS）, and the oceanic surface heating and synoptic-scale transient eddy （STE） activity anomalies over the North Pacific in wintertime are examined by using ERA-40 and NCEP/NCAR reanalysis data. The analysis demonstrates that the surface heating and the STE anomalies have different patterns, corresponding to the three WPJS anomalous modes, respectively. In the first WPJS anomalous mode, the WPJS main part shows no robust anomaly. The anomalous westerly wind, occurring over the mid-latitude central-eastern Pacific past the date line is associated with the anomalous heating presenting both in the tropical central-eastern Pacific past the date line and the center of the North Pacific basin. Meanwhile, the STE anomaly appears around the region of the anomalous zonal wind. The fluctuation in jet strength shown in the second WPJS mode is strongly related to the heating anomaly in the Kuroshio Current region and the STE anomaly in the jet exit region. The third mode demonstrates a northward/southward shift of the WPJS, which has a statistical connection with a south-north dipolar pattern of the heating anomaly in the western North Pacific separated at 35°N. Meanwhile, the STE spatial displacement is in conjunction with jet shifts in the same direction. The heating anomaly has a close connection with the atmospheric circulation, and thus changes the mid-latitude baroclinicity, leading to the STE anomaly, which then reinforces the WPJS anomaly via internal atmospheric dynamics.
基金Supported by the National Basic Research and Development (973) Program of China (2011CB952002 and 2012CB955901)National Natural Science Foundation of China (41130963)+1 种基金Research Fund for the Doctoral Program of Higher Education (20100091110003)China Meteorological Administration Special Public Welfare Research Fund (GYHY200906015 and GYHY201006019)
文摘In this paper, major features of the upper-tropospheric jet streams simulated by a coupled Climate System Model BCC_ CSMI.1 are evaluated through comparison with the NCEP/NCAR reanalysis. The jet streams consist of the East Asian subtropical jet （EASJ） and the East Asian polar-front jet （EAPJ）. Associated stationary wave and synoptic-scale transient eddy activities （STEA） are also examined. The results show that the climatological positions of the westerly jet streams are well captured by BCC_CSMI.1, but with slight intensity biases. Statistics from the 6-h model outputs reveal that the jet core number （JCN） of ESPJ is significantly underestimated. Examination of the simulated seasonal evolution of the westerly jet stream indicates that the model has produced a westward movement of the EASJ core in May, one month earlier than that in the reanalysis. Analysis of stationary wave activities shows that the overestimated meridional wind component may have caused considerable enhancement of meridional momentum and heat transport. The stationary Rossby wave represented by the wave activity flux at the southern flank of the Tibetan Plateau is favorable to the growth of asymmetric zonal wind and the multiple-center pattern of JCN. Unlike the stationary wave heat flux transport, the model tends to systematically generate weaker transient heat flux over East Asia. Further analysis of STEA exhibits a general consistent pattern between the simulation and the reanalysis, while the intensity of the northern STEA branch associated with the EAPJ is greatly reduced. The deficiencies of eddy momentum and heat flux transport and accompanied eddy forcing may contribute to the biases of the simulated upper-tropospheric jet streams, suggesting the potential importance of midlatitude internal atmospheric dynamics in shaping the tropospheric general circulation, which is not yet fully and accurately resolved in the current BCC-CSMI.1.
基金Project supported by the National Natural Science Foundation of China (Grant No. 50807001)
文摘The electromagnetic concentrative coils are indispensable in the functional magnetic stimulation and have potential applications in nondestructive testing. In this paper, we propose a figure-8-shaped coil being composed of two arbitrary oblique elliptical coils, which can change the electromagnetic concentrative region and the magnitude of eddy current density by changing the elliptical shape and/or spread angle between two elliptical coils. Pulsed current is usually the excitation source in the functional magnetic stimulation, so in this paper we derive the analytical solutions of transient pulsed eddy current field in the time domain due to the elliptical concentrative coil placed in an arbitrary position over a half-infinite plane conductor by making use of the scale-transformation, the Laplace transform and the Fourier transform are used in our derivation. Calculation results of field distributions produced by the figure-8-shaped elliptical coil show some behaviours as follows： 1） the eddy currents are focused on the conductor under the geometric symmetric centre of figure-8-shaped coil; 2） the greater the scale factor of ellipse is, the higher the eddy current density is and the wider the concentrative area of eddy current along y axis is; 3） the maximum magnitude of eddy current density increases with the increase of spread angle. When spread angle is 180°, there are two additional reverse concentrative areas on both sides of x axis.
基金The National Key Research Programs of China under contract No.2017YFA0604100the National Natural Science Foundation of China under contract Nos 41906008,41806039 and 41706205+3 种基金the Open Fund of State Key Laboratory of Satellite Ocean Environment Dynamics,Second Institute of Oceanography,MNR under contract No.QNHX2022the Startup Foundation for Introducing Talent of Nanjing University of Information Science&Technology under contract No.2019r049the Startup Foundation for Introducing Talent of Zhejiang Ocean Universitythe Innovation Group Project of Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai)under contract No.311020004。
文摘The spatial distribution of eddy diffusivity,basic characteristics of coherent mesoscale eddies and their relationship are analyzed from numerical model outputs in the Southern Ocean.Mesoscale fluctuation information is obtained by a temporal-spatial filtering method,and the eddy diffusivity is calculated using a linear regression analysis between isoneutral thickness flux and large-scale isoneutral thickness gradient.The eddy diffusivity is on the order of O(103 m2/s)with a significant spatial variation,and it is larger in the area with strong coherent mesoscale eddy activity.The mesoscale eddies are mainly located in the upper ocean layer,with the average intensity no larger than 0.2.The mean radius of the coherent mesoscale cyclonic(anticyclonic)eddy gradually decays from(121.2±10.4)km((117.8±9.6)km)at 30°S to(43.9±5.3)km((44.7±4.9)km)at 65°S.Their vertical penetration depths(lifespans)are deeper(longer)between the northern side of the Subpolar Antarctic Front and 48°S.The normalized eddy diffusivity and coherent mesoscale eddy activity show a significant positive correlation,indicating that coherent mesoscale eddy plays an important role in eddy diffusivity.
基金supported by the National Natural Science Foundation of China[grant numbers U2442207,42122034,42075043,and 42330609]the Youth Innovation Promotion Association[grant number 2021427]+2 种基金the West Light Foundation[grant number xbzgzdsys-202409]of the Chinese Academy of Sciencesthe Key Talent Projects in Gansu Provincethe Central Guidance Fund for Local Science and Technology Development Projects in Gansu Province[grant number 24ZYQA031].
文摘Previous studies have indicated a global reversal of near-surface wind speeds from a declining trend to an increasing trend around 2010;however,it remains unclear whether upper-air wind speeds exhibit a similar reversal.This study evaluates reanalysis products using surface and radiosonde observations to analyze upper-air wind speed variations in the Northern Hemisphere,focusing on their seasonal and latitudinal differences.Results demonstrate that JRA-55 effectively captures wind speed variations in the Northern Hemisphere.Notably,upper-air wind speeds over land experienced a reversal in winter 2010 with significant latitudinal differences.The trend reversal of upper wind speed between the midlatitudes and subtropics presents a dipole pattern.From 1990 to 2010,upper-air wind speeds in the midlatitudes(40°-70°N)significantly declined,while the subtropical zone(20°-40°N)displayed an opposite trend.However,during 2010-2020,wind speeds in the midlatitudes shifted to a significant positive trend,whereas the subtropics experienced a significant negative trend.The variations in Northern Hemisphere winter wind speeds can be attributed to changes in low-level baroclinicity driven by tropical diabatic heating and midlatitude transient eddy feedback.Enhanced diabatic heating and weakened eddy feedback during 1990-2010 contributed to reduced wind speeds in the midlatitudes and increased speeds in the subtropics,while reduced diabatic heating and strengthened eddy feedback during 2010-2020 resulted in increased wind speeds in the midlatitudes and decreased speeds in the subtropics.The reversal of upper-air wind speeds could affect surface wind speeds by downward momentum transfer,which could contribute to the reversal of surface wind speeds.
文摘In this paper, the boundary value problem (BVP) of 3 D transient eddy current field in the end region in the case that the generator is affected by impact load is specified. Besides, ways to implement discrete methods in both time domain and space domain during the solution of the problem are investigated. The Crank Nicolson scheme is utilized to attain the iterative format of time differential, after taking factors that can ensure both computation precision and stability into consideration. In this paper, the magnetic distribution in the end region of a turbogenerator in the case that the generator is affected by impact load is specified. As a result, it provides foundation for further study of electromagnetic force and electromagnetic vibration in the end region of the turbogenerator.
文摘Based on diagnostic analysis of reanalysis data for 58-year,the distribution characteristics of decadal variability in diabatic heating,transient eddy heating and transient eddy vorticity forcing related to the sea surface temperature(SST)anomalies over the North Pacific,as well as their relationship with anomalous atmospheric circulation have been investigated in this paper.A linear baroclinic model(LBM)was used to investigate atmospheric responses to idealized and realistic heat and vorticity forcing anomalies,and then to compare relative roles of different kinds of forcing in terms of geopotential height responses.The results illustrate that the responses of atmospheric height fields to the mid-latitude heating can be either baroclinic or barotropic.The response structure is sensitive to the relative horizontal location of heating with respect to the background jet flow,as well as to the vertical profile of heating.The response to the idealized deep heating over the eastern North Pacific,mimicking the observed heating anomaly,is baroclinic.The atmospheric response to the mid-latitude vorticity forcing is always barotropic,resulting in a geopotential low that is in phase with the forcing.The atmospheric responses to the realistic heat and vorticity forcing show the similar results,suggesting that diabatic heating,transient eddy heating and transient eddy vorticity forcing can all cause atmospheric anomalies and that the vorticity forcing plays a relatively more important role in maintaining the equivalent-barotropic structure of geopotential height anomalies.
基金supported by the National Key Basic Research Program of China (Grant No.2009CB421401)the Special Program of China Meteorological Administration (Grant No. GYHY200906017)contributing to the Norwegian Research Councilproject East Asian DecCen (Grant No. 193690)
文摘The most severe snowstorm and freezing-rain event in the past 50 years hit central and southern China in January 2008. One of the main reasons for the anomalous climate event was the occurrence of atmospheric circulation anomalies over middle and high latitudes, particularly the persistent blocking that occurred over the Ural Mountains. Along with atmospheric anomalies, a strong La Nia event in the Pacific and warm sea surface temperature anomalies （SSTAs） in the North Atlantic were the most significant in the lower boundary. Since a brief analysis suggests that La Nia exerts no significant impact on the Urals, the key point of focus in this study is on the role of the warmer SSTAs in the North Atlantic. Based on an observational composite, North Atlantic SSTAs pattern when the height anomaly over the Urals is strongly positive is found similar to that in January 2008, but no significant SSTAs occurred elsewhere, such as the Pacific. Using an atmospheric general circulation model, ECHAM5, the impact of North Atlantic SSTAs on the extratropical atmosphere circulation in the event was investigated. The results show that the warm SSTAs strengthened the blocking high over the Urals, through anomalous transient eddies. The consistency between the study model and the observational composite indicates that the warm SSTAs in the North Atlantic were indeed an important factor in the formation of the snowstorm disaster of January 2008.
基金supported by the National Key Basic Research Program of China(Grants No.2010CB428602 and No. 2009CB421401)the Innovative Key Project of the Chinese Academy of Sciences(Grant No.KZCX2-YW-BR-14)the National Natural Science Foundation of China(Grant No.40775053)
文摘During recent decades, the tropical Indo-Pacific Ocean has become increasingly warmer. Meanwhile, both the northern and southern hemispheric polar vortices （NPV and SPV） have exhibited a deepening trend in boreal winter. Although previous studies have revealed that the tropical Indian Ocean warming （IOW） favors an intensifying NPV and a weakening SPV, how the tropical Pacific Ocean warming （POW） influences the NPV and SPV remains unclear. In this study, a comparative analysis has been conducted through ensemble atmospheric general circulation model （AGCM） experiments. The results show that, for the Northern Hemisphere, the two warmings exerted opposite impacts in boreal winter, in that the IOW intensified the NPV while the POW weakened the NPV. For the Southern Hemisphere, both the IOW and POW warmed the southern polar atmosphere and weakened the SPV. A diagnostic analysis based on the vorticity budget revealed that such an interhemispheric difference in influences from the IOW and POW in boreal winter was associated with different roles of transient eddy momentum flux convergence between the hemispheres. Furthermore, this difference may have been linked to different strengths of stationary wave activity between the hemispheres in boreal winter.
基金the CAS Key project KZCX2-203 the Research Program for the Excellent State Key Laboratory! (Grant No.49823002)the Study on F
文摘The 1998 summer-time floods at the Yangtze River basin of China, the severest in last 50 years or so, directly resulted from the abnormal extension of Meiyu (rainy season), which was related to a weak East Asian summer monsoon and persistent anomalies of extratropical circulation. The long persistence of blocking over the Ural Mountains is a conspicuous feature. The physical processes responsible for the prolonged maintenance of this key system are investigated in terms of internal forcing (transient eddy upon basic flow) and external forcing (tropical heating forcing) via diagnosis and numerical experiments in the paper. Using the adjoint method, the location and structure of optimal perturbations favorable for the development and maintenance of Ural blocking are identified, which shows an apparent coincidence with the observed storm track at the eastern Atlantic to Europe sector. The diagnosis of E-vector and the response of baroclinic stationary wave to transient forcing both suggest further that the enhanced transient eddy activity favors the occurrence and maintenance of positive anomalies. The upper-level jet and heat sources (sinks) during that period are calculated, and the results indicate that the anomaly of upper jet and tropical heating is evident. The ensemble forecasting experiments by a GCM, IAP T42L9 show that the anomalous heating over the tropics, especially over the central-western Pacific and Atlantic, favors the formation of positive anomalies of height at the Ural region. Finally, a self-sustain mechanism of positive anomalies through two-way interaction between planetary stationary wave and transient eddy under the stimulation of anomalous tropical heating is proposed.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 40775048 and 41075058)the Special Scientific Research Fund of Public Welfare Profession of China (Grant No. GYHY201106016)
文摘In this paper, we report on the results of an investigation into inter-decadal changes in moisture transport and divergence in East Asia for the two periods 1980-2001 and 1958 1979. The aim is to explore the mechanism of summer rainfall change in the region after abrupt changes. The relevant changes are calculated using ERA-40 daily reanalysis datasets. The results show that both stationary and transient eddy moisture transports to the Chinese mainland have declined since the abrupt change in atmospheric general circulation in the late 1970s, leading to more rainfall in South China and less in the North. The anomalous rainfall pattern coincides well with anomalous large-scale moisture divergence in the troposphere, of which stationary-wave or monsoon transport is dominant, in comparison with the contribution of the transient eddies. F^rthermore, their divergences are found to be in opposite phases. In addition, meridional divergence is more important than its zonal counterpart, with an opposite phase in East Asia. Abnormal zonal moisture convergences appear in northwestern and northeastern parts of China, and are related to the excess rainfalls in these regions. An increase in transient eddy activity is one of the major mechanisms for excess rainfall in northern Xinjiang. Consequently, the anomalous rainfall pattern in East Asia results from a decline of the East Asian monsoon after the abrupt change, while the rainfall increase in northwestern China involves anomalies of both stationary waves and transient eddies on boreal westerly over the mid- and high latitudes.
基金The National Key Research and Development Program of China under contract Nos 2016YFC0402708,2017YFC1502501,2018YFC1508002 and 2016YFA0602102the China Special Fund for Meteorological Research in the Public Interest under contract No.GYHY201506011the National Natural Science Foundation of China under contract Nos 41975134 and 40975040。
文摘Based on the climatological reanalysis data of the European Center for Medium-Range Weather Forecasts and the Arctic sea ice data of the National Snow and Ice Data Center, the relationship between the Arctic sea ice area(SIA)and the interannual variation of atmospheric meridional heat transport(AMHT) was analyzed. The results show that the atmospheric meridional heat transported by transient eddy(TAMHT) dominates the June AMHT in midhigh latitudes of the Northern Hemisphere, while the western Baffin Bay(B) and the eastern Greenland(G) are two gates for TAMHT entering the Arctic. TAMHT in the western Baffin Bay(B-TAMHT) and eastern Greenland(G-TAMHT) has a concurrent variation of reverse phase, which is closely related to the summer Arctic SIA.Possible mechanism is that the three Arctic atmospheric circulation patterns(AD, AO and NAO) in June can cause the concurrent variation of TAMHT in the B and G regions. This concurrent variation helps to maintain AD anomaly in summer through wave action and changes the polar air temperature, thus affecting the summer Arctic SIA. Calling the heat entering the Arctic as warm transport and the heat leaving Arctic as cold transport, then the results are classified into three situations based on B-TAMHT and G-TAMHT: warm B corresponding to cold G(WC), cold B corresponding to warm G(CW), cold B corresponding to cold G(CC), while warm B corresponding to warm G is virtually non-existent. During the WC situation, the SIA in the Pacific Arctic sediments and Kara Sea decreases;during the CW situation, the SIA in the Laptev Sea and Kara Sea decreases;during the CC situation, the SIA in the Kara Sea, Laptev Sea and southern Beaufort Sea increases.
基金jointly supported by the Ministry of Science and Technology of China,through the National Basic Research Program of China(Grant No.2012CB955602)the National Natural Science Foundation of China(Grant Nos.41575077,41490643 and 41275094)+1 种基金a project funded by the PAPD(Priority Academic Program Development of Jiangsu Higher Education Institutions)supported by the Innovation Project for Graduate Student of Jiangsu Province(Grant No.KYLX15-0860)
文摘This study concerns atmospheric responses to the North Pacific subtropical front （NPSTF） in boreal spring over the period 1982-2014. Statistical results show that a strong NPSTF in spring can significantly enhance the East Asian jet stream （EAJS）. Both transient eddy activity and the atmospheric heat source play important roles in this process. The enhanced atmospheric temperature gradient due to a strong NPSTF increases atmospheric baroclinicity, resulting in an intensification of transient eddy and convection activities. On the one hand, the enhanced transient eddy activities can excite an anomalous cyclonic circulation with a quasi-baraotropical structure in the troposphere to the north of the NPSTF. Accordingly, the related westerly wind anomalies around 30°N can intensify the component of the EAJS over the Northeast Pacific. On the other hand, an enhanced atmospheric heat source over the NPSTF, which is related to increased rainfall, acts to excite an anomalous cyclonic circulation system in the troposphere to the northwest of the NPSTF, which can explain the enhanced component of the EAJS over the Northwest Pacific. The two mechanisms may combine to enhance the EAJS.
基金supported by the National Basic Research Program of China (973 Program) under Grant 2009CB421406the National Key Technologies R & D Programof China under Grant 2009BAC51B02
文摘In this paper,the dynamical evolutions of two types of Arctic Oscillation (AO),the stratospheric (S) and tropospheric (T) types,have been investigated on an intermediate time scale in terms of transient eddy feedback forcing and three-dimensional Rossby wave propagation.S-Type (T-type) events are characterized by an anomalous stratospheric polar vortex that is in phase (out of phase) with its tropospheric counterpart.Approximately onethird of AO events,both positive and negative,are T-type events.For the positive phase of a T-type event,the formation and maintenance of stratospheric positive anomalies over the polar cap are associated with an upward propagation of Rossby wave packets originating from the near-tropopause altitude over northeastern Asia.However,such upward propagating features are not found for S-type events.In the troposphere,transient eddy feedback forcing is primarily responsible for the meridional seesaw structure of both the S-and T-type events,with an additional contribution from Rossby wave propagation.
基金the National Natural Science Foundation of China(Grant No.41490642).
文摘The Kuroshio extension(KE)exhibits interdecadal variability,oscillating from a stable state to an unstable state.In this paper,ERA-Interim reanalysis data are used to discuss the possible reasons for the asymmetric response of the atmosphere to symmetric sea surface temperature anomaly(SSTA)during periods of differential KE states.The analysis has the following results:the SSTA presents a nearly symmetrical distribution with opposite signs during the KE stable and unstable periods.During the KE stable period,the storm track is located north of 40°N and is significantly enhanced in the northeast Pacific Ocean.The atmospheric response is similar to the West Pacific/North Pacific Oscillation teleconnection(WP/NPO like pattern)and presents a barotropic structure.The inversion results of the potential vorticity equation show that the feedback of transient eddy vorticity manifests a WP/NPO like pattern and presents a barotropic structure,which is the main reason for bringing about the response of the WP/NPO like pattern.The magnitude of the feedbacks of both diabatic heating and transient eddy heating is small,which can offset one another.During the KE unstable period,the main body of the storm track is located to the south of 40°N,and there is no significant response signal in the atmosphere,except near the west coast of North America.Compared with the KE stable period,the asymmetry of response of the transient eddy vorticity is the main reason for the asymmetric response of the atmosphere.
基金supported by the National Natural Science Foundation of China(Grant No.41975083)。
文摘Since the 2000 s,extratropical extremes have been more frequent,which are closely related to anomalies of planetary-scale and synoptic-scale systems.This study focuses on a key synoptic system,the extratropical cyclonic vortex(ECV)over land,to investigate its relations with extreme precipitation.It was found that ECVs have been more active post-2000,which has induced more extreme precipitation,and such variation is projected to persist along with increasing temperature within 1.5℃of global warming.An enhanced quasi-stationary vortex(QSV)primarily contributes to the ECV,rather than inactive synoptic-scale transient eddies(STEs).Inactive STEs respond to a decline in baroclinicity due to the tendency of the homogeneous temperature gradient.However,such conditions are helpful to widening the westerly jet belt,favoring strong dynamic processes of quasi-resonant amplification and interaction of STEs with the quasi-stationary wave,and the result favors an increasing frequency and persistence of QSVs,contributing to extreme precipitation.
