Since the 21st century,the Huang–Huai–Hai River Basin(HHHRB)in China has experienced increased frequency and severity of drought–flood abrupt alternation(DFAA)events during early summer,characterized by droughts in...Since the 21st century,the Huang–Huai–Hai River Basin(HHHRB)in China has experienced increased frequency and severity of drought–flood abrupt alternation(DFAA)events during early summer,characterized by droughts in June followed by floods in July.The 2024 event was the most severe since 1981.This study demonstrates that such compound extreme events are closely linked to anomalous subseasonal evolution of large-scale atmospheric circulation.During the drought phase,the East Asian subtropical westerly jet(EAJ)shifts southward,and the western Pacific subtropical high(WPSH)exhibits anomalous strengthening with its western ridge line displaced southward.The flood phase is characterized by acceleration of the EAJ,westward extension of the WPSH,and enhanced southwestern moisture transport from the western Pacific.Beyond these typical features,the 2024 early summer circulation exhibited unique characteristics:Anomalous northeastward intensification of the WPSH facilitated merged moisture influx from both the Indian Ocean and the western Pacific along the southeast pathway into the HHHRB in July,resulting in the highest net moisture inflow at the southern boundary of the HHHRB since 1981.The synergistic effects of multiple factors primarily explain the exceptionally intense DFAA event in 2024.展开更多
Based on the daily rainfall data from China Meteorological Administration,the tropical cyclone（TC） best track data from Japan Meteorological Agency,and the NCEP-NCAR reanalysis data from NOAA,regional mean daily pre...Based on the daily rainfall data from China Meteorological Administration,the tropical cyclone（TC） best track data from Japan Meteorological Agency,and the NCEP-NCAR reanalysis data from NOAA,regional mean daily precipitation extreme（RDPE） events over southeastern China（specifically,the Fujian-Jiangxi region（FJR）） and the associated circulation anomalies are investigated.For the summers of 1979-2011,a total of 105 RDPE events are identified,among which 35 are TC-infiuenced（TCIn-RDPE） and 70 are TCfree events（TCFr-RDPE）.Distinct differences between these two types of RDPEs are found in both their statistical features and the related circulation patterns.TCFr-RDPEs usually occur in June,while TCInRDPEs mainly take place during July-August.When TCFr-RDPEs happen,a center of the anomalous cyclonic circulation is observed over the FJR,with an anomalous anticyclonic circulation to the south of this region.The warm/moist air flows from the South China Sea（SCS） and western Pacific meet with colder air from the north,forming a narrow convergent belt of water vapor over the FJR.Simultaneously,positive diabatic forcing anomalies are observed over the FJR,whereas negative anomalies appear over both its south and north sides,facilitating the formation and maintenance of the cyclonic circulation anomaly,as well as the upward motion of the atmosphere,over the FJR.When TCIn-RDPEs occur,southeastern China is dominated by a TC-related stronger anomalous cyclonic circulation.An anomalous anticyclonic circulation in the mid and high latitudes north of the FJR exists in the mid and upper troposphere,opposite to the situation during TCFr-RDPE events.Abundant warm/wet air is carried into the FJR from both the Indian Ocean and the SCS,leading to a large amount of latent heat release over the FJR and inducing strong ascending motion there.Furthermore,large differences are also found in the manifestation of Rossby wave energy propagation between these two types of RDPE events.The results of this study are helpful to deepen our understanding of the mechanisms behind these two types of RDPE events.展开更多
A robust anomalous anticyclonic circulation (AAC) was observed over Northeast Asia and the Japan Sea in boreal win-ter 1997/98 and over the Japan Sea in spring 1998. The formation mechanism is investigated. On the bac...A robust anomalous anticyclonic circulation (AAC) was observed over Northeast Asia and the Japan Sea in boreal win-ter 1997/98 and over the Japan Sea in spring 1998. The formation mechanism is investigated. On the background of the vertically sheared winter monsoonal flow, anomalous rainfall in the tropical Indo-Western Pacific warm pool excited a wave train towards East Asia in the upper troposphere during boreal winter of 1997/98. The AAC over Northeast Asia and the Japan Sea is part of the wave train of equivalent barotropic structure. The AAC over the Japan Sea persisted from winter to spring and even intensified in spring 1998. The diagnostic calculations show that the vorticity and temperature fluxes by synoptic eddies are an important mechanism for the AAC over the Japan Sea in spring 1998.展开更多
Using the summer (June to August) monthly mean data of the National Centers for Environmental Predictions (NCEP) - National Center for Atmospheric Research (NCAR) reanalysis from 1980 to 1997, atmospheric heat sources...Using the summer (June to August) monthly mean data of the National Centers for Environmental Predictions (NCEP) - National Center for Atmospheric Research (NCAR) reanalysis from 1980 to 1997, atmospheric heat sources and moisture sinks are calculated. Anomalous circulation and the vertically integrated heat source with the vertical integrated moisture sink and outgoing longwave radiation (OLR) flux are examined based upon monthly composites for 16 great wet-spells and 8 great dry-spells over the middle-lower reaches of the Yangtze River. The wind anomaly exhibits prominent differences between the great wet-spell and the great dry-spell over the Yangtze River Valley. For the great wet-spell, the anomalous southerly from the Bay of Bengal and the South China Sea and the anomalous northerly over North China enhanced low-level convergence toward a narrow latitudinal belt area (the middle-lower reaches of the Yangtze River). The southerly anomaly is connected with an anticyclonic anomalous circulation system centered at 22 degreesN, 140 degreesE and the northerly anomaly is associated with a cyclonic anomalous circulation system centered at the Japan Sea. In the upper level, the anomalous northwesterly between an anticyclonic anomalous system with the center at 23 degreesN, 105 degreesE and a cyclonic anomalous system with the center at Korea diverged over the middle-lower reaches of the Yangtze River. On the contrary, for the great dry-spell, the anomalous northerly over South China and the anomalous southerly over North China diverged from the Yangtze River Valley in the low level. The former formed in the western part of a cyclonic anomalous system centered at 23 degreesN, 135 degreesE. The latter was located in the western ridge of an anticyclonic anomalous system in the northwestern Pacific. The upper troposphere showed easterly anomaly that converged over the middle-lower reaches of the Yangtze River. A cyclonic anomalous system in South China and an anticyclonic system centered in the Japan Sea enhanced the easterly. Large atmospheric heat source anomalies of opposite signs existed over the western Pacific - the South China Sea, with negative in the great wet-spell and positive in the great dry-spell. The analysis of heat source also revealed positive anomalous heat sources during the great wet-spell and negative anomalous heat sources during the great dry-spell over the Yangtze River valley. The changes of the moisture sink and OLR were correspondingly altered, implying the change of heat source anomaly is due to the latent heat releasing of convective activity. Over the southeastern Tibetan Plateau- the Bay of Bengal, the analysis of heat source shows positive anomalous heat sources during the great wet-spell and negative anomalous heat sources during the great dry-spell because of latent heating change. The change of divergent wind coexisted with the change of heat source. In the great wet-spell, southerly divergent wind anomaly in the low level and northerly divergent wind anomaly in high-level are seen over South China. These divergent wind anomalies are helpful to the low-level convergence anomaly and high-level divergence anomaly over the Yangtze River valley. The low-level northerly divergent wind anomaly and high-level southerly divergent wind anomaly over South China reduced the low-level convergence and high-level divergence over the Yangtze River valley during the great dry-spell.展开更多
With the extreme drought(flood)event in southern China from July to August in 2022(1999)as the research object,based on the comprehensive diagnosis and composite analysis on the anomalous drought and flood years from ...With the extreme drought(flood)event in southern China from July to August in 2022(1999)as the research object,based on the comprehensive diagnosis and composite analysis on the anomalous drought and flood years from July to August in 1961-2022,it is found that there are significant differences in the characteristics of the vertically integrated moisture flux(VIMF)anomaly circulation pattern and the VIMF convergence(VIMFC)anomaly in southern China in drought and flood years,and the VIMFC,a physical quantity,can be regarded as an indicative physical factor for the"strong signal"of drought and flood in southern China.Specifically,in drought years,the VIMF anomaly in southern China is an anticyclonic circulation pattern and the divergence characteristics of the VIMFC are prominent,while those are opposite in flood years.Based on the SST anomaly in the typical draught year of 2022 in southern China and the SST deviation distribution characteristics of abnormal draught and flood years from 1961 to 2022,five SST high impact areas(i.e.,the North Pacific Ocean,Northwest Pacific Ocean,Southwest Pacific Ocean,Indian Ocean,and East Pacific Ocean)are selected via the correlation analysis of VIMFC and the global SST in the preceding months(May and June)and in the study period(July and August)in 1961-2022,and their contributions to drought and flood in southern China are quantified.Our study reveals not only the persistent anomalous variation of SST in the Pacific and the Indian Ocean but also its impact on the pattern of moisture transport.Furthermore,it can be discovered from the positive and negative phase fitting of SST that the SST composite flow field in high impact areas can exhibit two types of anomalous moisture transport structures that are opposite to each other,namely an anticyclonic(cyclonic)circulation pattern anomaly in southern China and the coastal areas of east China.These two types of opposite anomalous moisture transport structures can not only drive the formation of drought(flood)in southern China but also exert its influence on the persistent development of the extreme weather.展开更多
The midwinter suppression(MWS) of the North Pacific storm track(NPST) has been an active research topic for decades. Based on the daily-mean NCEP/NCAR reanalysis from 1948 to 2018, this study investigates the MWS-rela...The midwinter suppression(MWS) of the North Pacific storm track(NPST) has been an active research topic for decades. Based on the daily-mean NCEP/NCAR reanalysis from 1948 to 2018, this study investigates the MWS-related atmospheric circulation characteristics in the Northern Hemisphere by regression analysis with respect to a new MWS index, which may shed more light on this difficult issue. The occurrence frequency of the MWS of the upper-tropospheric NPST is more than 0.8 after the mid-1980 s. The MWS is accompanied by significantly positive sea-level pressure anomalies in Eurasia and negative anomalies over the North Pacific, which correspond to a strengthened East Asian winter monsoon. The intensified East Asian trough and atmospheric blocking in the North Pacific as well as the significantly negative low-level air temperature anomalies, lying upstream of the MNPST, are expected to be distinctly associated with the MWS. However, the relationship between the MWS and low-level atmospheric baroclinicity is somewhat puzzling.From the diagnostics of the eddy energy budget, it is identified that the inefficiency of the barotropic energy conversion related to the barotropic governor mechanism does not favor the occurrence of the MWS. In contrast, weakened baroclinic energy conversion, buoyancy conversion, and generation of eddy available potential energy by diabatic heating are conducive to the occurrence of the MWS. In addition, Ural blocking in the upstream region of the MNPST may be another candidate mechanism associated with the MWS.展开更多
In this study,the impacts of the tropical Pacific–Indian Ocean associated mode(PIOAM)on Madden–Julian Oscillation(MJO)activity were investigated using reanalysis data.In the positive(negative)phase of the PIOAM,the ...In this study,the impacts of the tropical Pacific–Indian Ocean associated mode(PIOAM)on Madden–Julian Oscillation(MJO)activity were investigated using reanalysis data.In the positive(negative)phase of the PIOAM,the amplitudes of MJO zonal wind and outgoing longwave radiation are significantly weakened(enhanced)over the Indian Ocean,while they are enhanced(weakened)over the central and eastern Pacific.The eastward propagation of the MJO can extend to the central Pacific in the positive phase of the PIOAM,whereas it is mainly confined to west of 160°E in the negative phase.The PIOAM impacts MJO activity by modifying the atmospheric circulation and moisture budget.Anomalous ascending(descending)motion and positive(negative)moisture anomalies occur over the western Indian Ocean and central-eastern Pacific(Maritime Continent and western Pacific)during the positive phase of the PIOAM.The anomalous circulation is almost the opposite in the negative phases of the PIOAM.This anomalous circulation and moisture can modulate the activity of the MJO.The stronger moistening over the Indian Ocean induced by zonal and vertical moisture advection leads to the stronger MJO activity over the Indian Ocean in the negative phase of the PIOAM.During the positive phase of the PIOAM,the MJO propagates farther east over the central Pacific owing to the stronger moistening there,which is mainly attributable to the meridional and vertical moisture advection,especially low-frequency background state moisture advection by the MJO’s meridional and vertical velocities.展开更多
NCEP/NCAR data are utilized to analyze an extreme flood year(1998) and an extreme dry year(2006) in the Sichuan-Chongqing region(SCR) and the results are as follows. The positive divergence of South Asia High(SAH) is ...NCEP/NCAR data are utilized to analyze an extreme flood year(1998) and an extreme dry year(2006) in the Sichuan-Chongqing region(SCR) and the results are as follows. The positive divergence of South Asia High(SAH) is stronger in the flood year; the position of the ridge line of SAH is southward compared with the annual average; Western Pacific Subtropical High(WPSH) extends westward and its ridge line is southward. In the drought year, the positive divergence of SAH is weaker, its ridge line is northward, and the position of WPSH is also northward. As shown in the dynamics, in drought(flood) years, negative(positive) vorticity advection in the upper atmosphere can cause the atmosphere to ascend(descend), and anomalous circulation of SAH displays divergence(convergence), and anomalous circulation of the lower atmosphere shows convergence(divergence). Thermal structure of the atmosphere shows that there is warm(cold) temperature advection in the lower atmosphere, and the vertical distribution of diabetic heating causes SAH's local circulation to display convergence(divergence) and affects vertical motion of the lower atmosphere circulation eventually. To some extent, the two extreme years in the SCR is closely related to the vertical motion of atmosphere circulation and the variation of such vertical motion is caused by differences of interactions between SAH and lower atmosphere circulations.展开更多
In the first half of winter 2020/21,China has experienced an extremely cold period across both northern and southern regions,with record-breaking low temperatures set in many stations of China.Meanwhile,a moderate La ...In the first half of winter 2020/21,China has experienced an extremely cold period across both northern and southern regions,with record-breaking low temperatures set in many stations of China.Meanwhile,a moderate La Niña event which exceeded both oceanic and atmospheric thresholds began in August 2020 and in a few months developed into its mature phase,just prior to the 2020/21 winter.In this report,the mid−high-latitude large-scale atmospheric circulation anomalies in the Northern Hemisphere,which were forced by the negative phase of Arctic Oscillation,a strengthened Siberian High,an intensified Ural High and a deepened East Asian Trough,are considered to be the direct reason for the frequent cold surges in winter 2020/21.At the same time,the synergistic effect of the warm Arctic and the cold tropical Pacific(La Niña)provided an indispensable background,at a hemispheric scale,to intensify the atmospheric circulation anomalies in middle-to-high latitudes.In the end,a most recent La Niña prediction is provided and the on-coming evolution of climate is discussed for the remaining part of the 2020/21 winter for the purpose of future decision-making and early warning.展开更多
The relationship between sea surface temperature anomaly (SSTA) in the domain from the northwest of the Pacific to China seas (NWP-CS) and climate (precipitation and surface temperature) anomaly in winters over east ...The relationship between sea surface temperature anomaly (SSTA) in the domain from the northwest of the Pacific to China seas (NWP-CS) and climate (precipitation and surface temperature) anomaly in winters over east of Chinese Mainland (ECM) are investigated with composite analysis. The results suggest that (1) SSTA in NWP-CS usually appears as 'seesaw' pattern, i.e., sign of SSTA in the northwest of the Philippines is positive (negative) while in the southeast it is negative (positive), defined as SSTA 'seesaw' positive (negative) pattern. When SSTA 'seesaw' positive (negative) pattern appears, the surface temperature in ECM is higher (lower) than the normal winters but the precipitation anomaly is not distinct ; (2) there are two anomalous anticyclones (cyclones) locating in the northwest Pacific and east of the Philippines at 850 hPa level and an anomalous anti-Hadley (Hadley) circulation, which descends (ascends) south of 18oN, ascends (descends) north of 18oN; (3) heat flux anomaly from ocean into atmosphere is weaker (stronger) in the northwest of the Philippines but stronger (weaker) in the southeast of the Philippines than the normal winters, because the weaker (stronger) northerly prevails in the northwest of the Philippines and stronger (weaker) northeaster in the southeast of the Philippines induced by anomalous anticyclones (cyclones).展开更多
After compositing three representative ENSO indices,El Nio events have been divided into an eastern pattern(EP) and a central pattern(CP).By using EOF,correlation and composite analysis,the relationship and possible m...After compositing three representative ENSO indices,El Nio events have been divided into an eastern pattern(EP) and a central pattern(CP).By using EOF,correlation and composite analysis,the relationship and possible mechanisms between Indian Ocean Dipole(IOD) and two types of El Nio were investigated.IOD events,originating from Indo-Pacific scale air-sea interaction,are composed of two modes,which are associated with EP and CP El Ni o respectively.The IOD mode related to EP El Nio events(named as IOD1) is strongest at the depth of 50 to 150 m along the equatorial Indian Ocean.Besides,it shows a quasi-symmetric distribution,stronger in the south of the Equator.The IOD mode associated with CP El Nio(named as IOD2) has strongest signal in tropical southern Indian Ocean surface.In terms of mechanisms,before EP El Nio peaks,anomalous Walker circulation produces strong anomalous easterlies in equatorial Indian Ocean,resulting in upwelling in the east,decreasing sea temperature there;a couple of anomalous anticyclones(stronger in the south) form off the Equator where warm water accumulates,and thus the IOD1 occurs.When CP El Nio develops,anomalous Walker circulation is weaker and shifts its center to the west,therefore anomalous easterlies in equatorial Indian Ocean is less strong.Besides,the anticyclone south of Sumatra strengthens,and the southerlies east of it bring cold water from higher latitudes and northerlies west of it bring warm water from lower latitudes to the 15° to 25°S zone.Meanwhile,there exists strong divergence in the east and convergence in the west part of tropical southern Indian Ocean,making sea temperature fall and rise separately.Therefore,IOD2 lies farther south.展开更多
Based on three reanalysis datasets—ERA-Interim,NCAR–NCEP and JRA-55—the classification of25 commonly used indexes of the East Asian summer monsoon(EASM)was investigated.The physical nature of two categories of mons...Based on three reanalysis datasets—ERA-Interim,NCAR–NCEP and JRA-55—the classification of25 commonly used indexes of the East Asian summer monsoon(EASM)was investigated.The physical nature of two categories of monsoon index,together with their circulation pattern,climate anomalies,and driving factors,were investigated.Results suggest that the selected 25 monsoon indexes can be classified into two typical categories(CategoryⅠandⅡ),which are dominated by interannual and decadal variabilities of the EASM,respectively.The anomalous circulation patterns and summer rainfall patterns related to the two categories of index also exhibit evident differences.CategoryⅠis closely linked to the low-latitude circulation system and the anomalous circulation pattern is a typical East Asia–Pacific teleconnection pattern.The summer rainfall anomaly exhibits a typical tripole pattern.However,CategoryⅡmainly reflects the impacts of the middle–high latitude circulation system on the summer monsoon and is closely linked to a typical Eurasian teleconnection pattern,which corresponds to a dipole of summer rainfall anomalies.Further analysis suggests that the underlying thermal driving factors of the two categories of monsoon are distinct.The main driving factors of CategoryⅠare the tropical sea surface temperature anomalies(SSTAs),especially ENSO-related SSTAs in the preceding winter and summer SSTAs in the tropical Indian Ocean.The winter signal of Category II summer monsoon anomalous activity mainly originates from the polar region and the middle and high latitudes of the Eurasian continent.CategoryⅡmonsoon activity is also associated with summer SSTAs in the equatorial central Pacific.展开更多
Experimental predictions with a hybrid coupled ocean-atmosphere model(L9R15 AGCM-ZC ocean model)were performed for the 1986/87 El Nino event and the 1988/89 La Nina event with and without the Tibetan Plateau respectiv...Experimental predictions with a hybrid coupled ocean-atmosphere model(L9R15 AGCM-ZC ocean model)were performed for the 1986/87 El Nino event and the 1988/89 La Nina event with and without the Tibetan Plateau respectively(called TP FORC and NTP FORC hereinafter). Comparison shows that,to some extent,the existence of the Tibetan Plateau orography weakens or restrains(strengthens or facilitates)the formation of the anomalous circulation of Asian monsoon during El Nino(La Nina)period.Opposite results are found in the uncoupled AGCM simulation.展开更多
The spatial-temporal features of the extremely severe drought and the anomalous atmospheric circulation in summer 2006 are analyzed based on the NCEP/NCAR reanalysis data,the characteristic circulation indices given b...The spatial-temporal features of the extremely severe drought and the anomalous atmospheric circulation in summer 2006 are analyzed based on the NCEP/NCAR reanalysis data,the characteristic circulation indices given by the National Climate Center of China,and the daily precipitation data of 20 stations in the east of Southwest China（ESC） from 1959 to 2006.The results show that the rainless period started from early June and ended in early September 2006 with a total of more than 80 days,and the rainfall was especially scarce from around 25 July to 5 September 2006.Precipitation for each month was less than normal,and analysis of the precipitation indices shows that the summer precipitation in 2006 was the least since 1959.The extremely severe drought in the ESC in summer 2006 was closely related to the persistent anomalies of the atmospheric circulation in the same period,i.e.,anomalies of mid-high latitude atmospheric circulation,western Pacific subtropical high（WPSH）,westerlies,South Asian high,lower-level flow,water vapor transport,vertical motion,and so on.Droughts usually occur when the WPSH lies anomalously northward and westward,or anomalously weak and eastward.The extreme drought in summer 2006 was caused by the former.When the WPSH turned stronger and shifted to the north and west of its normal position,and the South Asian high was also strong and lay eastward,downdrafts prevailed over the ESC and suppressed the water vapor transfer toward this area.At the same time,the disposition of the westerlies and the mid-high latitude circulation disfavored the southward invasion of cold air,which jointly resulted in the extremely severe drought in the ESC in summer 2006.The weak heating over the Tibetan Plateau and vigorous convective activities over the Philippine area were likely responsible for the strong WPSH and its northwestward shift in summer 2006.展开更多
Based on the daily NCEP/DOE reanalysis II data,dates of the boreal spring Stratospheric Final Warming(SFW) events during 1979–2010 are defined as the time when the zonal-mean zonal wind at the central latitudes(65...Based on the daily NCEP/DOE reanalysis II data,dates of the boreal spring Stratospheric Final Warming(SFW) events during 1979–2010 are defined as the time when the zonal-mean zonal wind at the central latitudes(65°–75°N) of the westerly polar jet drops below zero and never recovers until the subsequent autumn.It is found that the SFW events occur successively from the mid to the lower stratosphere and averagely from the mid to late April with a temporal lag of about 13 days from 10 to 50 hPa.Over the past 32 years,the earliest SFW occurs in mid March whereas the latest SFW happens in late May,showing a clear interannual variability of the time of SFW.Accompanying the SFW onset,the stratospheric circulation transits from a winter dynamical regime to a summertime state,and the maximum negative tendency of zonal wind and the strongest convergence of planetary-wave are observed.Composite results show that the early/late SFW events in boreal spring correspond to a quicker/slower transition of the stratospheric circulation,with the zonal-mean zonal wind reducing about 20/5 m s-1 at 30 hPa within 10 days around the onset date.Meanwhile,the planetary wave activities are relatively strong/weak associating with an out-of-/in-phase circumpolar circulation anomaly before and after the SFW events in the stratosphere.All these results indicate that,the earlier breakdown of the stratospheric polar vortex(SPV),as for the winter stratospheric sudden warming(SSW) events is driven mainly by wave forcing;and in contrast,the later breakdown of the SPV exhibits more characteristics of its seasonal evolution.Nevertheless,after the breakdown of SPV,the polar temperature anomalies always exhibit an out-of-phase relationship between the stratosphere and the troposphere for both the early and late SFW events,which implies an intimate stratosphere–troposphere dynamical coupling in spring.In addition,there exists a remarkable interdecadal change of the onset time of SFW in the mid 1990s.On average,the SFW onset time before the mid 1990s is 11 days earlier than that afterwards,corresponding to the increased/decreased planetary wave activities in late winter-early spring before/after the 1990s.展开更多
基金supported by the National Natural Science Foundation of China[grant numbers U2142207 and U2342205]the National Key R&D Program of China[grant number 2024YFC3013100]China Meteorological Administration(CMA)Youth Innovation Team[CMA2024QN06].
文摘Since the 21st century,the Huang–Huai–Hai River Basin(HHHRB)in China has experienced increased frequency and severity of drought–flood abrupt alternation(DFAA)events during early summer,characterized by droughts in June followed by floods in July.The 2024 event was the most severe since 1981.This study demonstrates that such compound extreme events are closely linked to anomalous subseasonal evolution of large-scale atmospheric circulation.During the drought phase,the East Asian subtropical westerly jet(EAJ)shifts southward,and the western Pacific subtropical high(WPSH)exhibits anomalous strengthening with its western ridge line displaced southward.The flood phase is characterized by acceleration of the EAJ,westward extension of the WPSH,and enhanced southwestern moisture transport from the western Pacific.Beyond these typical features,the 2024 early summer circulation exhibited unique characteristics:Anomalous northeastward intensification of the WPSH facilitated merged moisture influx from both the Indian Ocean and the western Pacific along the southeast pathway into the HHHRB in July,resulting in the highest net moisture inflow at the southern boundary of the HHHRB since 1981.The synergistic effects of multiple factors primarily explain the exceptionally intense DFAA event in 2024.
基金Supported by the China Meteorological Administration Special Public Welfare Research Fund Province(GYHY201406024)National Natural Science Foundation of China(41330425)+1 种基金Creative Program of Science&Technology of Jiangsu(KYZZ-0239and CXZZ12_0485)Creative Program of the State Key Laboratory of Severe Weather(2015LASW-A03)
文摘Based on the daily rainfall data from China Meteorological Administration,the tropical cyclone（TC） best track data from Japan Meteorological Agency,and the NCEP-NCAR reanalysis data from NOAA,regional mean daily precipitation extreme（RDPE） events over southeastern China（specifically,the Fujian-Jiangxi region（FJR）） and the associated circulation anomalies are investigated.For the summers of 1979-2011,a total of 105 RDPE events are identified,among which 35 are TC-infiuenced（TCIn-RDPE） and 70 are TCfree events（TCFr-RDPE）.Distinct differences between these two types of RDPEs are found in both their statistical features and the related circulation patterns.TCFr-RDPEs usually occur in June,while TCInRDPEs mainly take place during July-August.When TCFr-RDPEs happen,a center of the anomalous cyclonic circulation is observed over the FJR,with an anomalous anticyclonic circulation to the south of this region.The warm/moist air flows from the South China Sea（SCS） and western Pacific meet with colder air from the north,forming a narrow convergent belt of water vapor over the FJR.Simultaneously,positive diabatic forcing anomalies are observed over the FJR,whereas negative anomalies appear over both its south and north sides,facilitating the formation and maintenance of the cyclonic circulation anomaly,as well as the upward motion of the atmosphere,over the FJR.When TCIn-RDPEs occur,southeastern China is dominated by a TC-related stronger anomalous cyclonic circulation.An anomalous anticyclonic circulation in the mid and high latitudes north of the FJR exists in the mid and upper troposphere,opposite to the situation during TCFr-RDPE events.Abundant warm/wet air is carried into the FJR from both the Indian Ocean and the SCS,leading to a large amount of latent heat release over the FJR and inducing strong ascending motion there.Furthermore,large differences are also found in the manifestation of Rossby wave energy propagation between these two types of RDPE events.The results of this study are helpful to deepen our understanding of the mechanisms behind these two types of RDPE events.
基金supported by the Ministry of Science and Technology of China(National Basic Research Program of China(Grant No.2012CB955602))the National Key Program for Developing Basic Science(Grant No.2010CB428904)the Natural Science Foundation of China(Grant Nos.40830106,40921004,41176006)
文摘A robust anomalous anticyclonic circulation (AAC) was observed over Northeast Asia and the Japan Sea in boreal win-ter 1997/98 and over the Japan Sea in spring 1998. The formation mechanism is investigated. On the background of the vertically sheared winter monsoonal flow, anomalous rainfall in the tropical Indo-Western Pacific warm pool excited a wave train towards East Asia in the upper troposphere during boreal winter of 1997/98. The AAC over Northeast Asia and the Japan Sea is part of the wave train of equivalent barotropic structure. The AAC over the Japan Sea persisted from winter to spring and even intensified in spring 1998. The diagnostic calculations show that the vorticity and temperature fluxes by synoptic eddies are an important mechanism for the AAC over the Japan Sea in spring 1998.
基金Supported by National Key Programme for Developing Basic Sciences G1998040900 Part 1 and IAPInnovation Foundation 8-1308.
文摘Using the summer (June to August) monthly mean data of the National Centers for Environmental Predictions (NCEP) - National Center for Atmospheric Research (NCAR) reanalysis from 1980 to 1997, atmospheric heat sources and moisture sinks are calculated. Anomalous circulation and the vertically integrated heat source with the vertical integrated moisture sink and outgoing longwave radiation (OLR) flux are examined based upon monthly composites for 16 great wet-spells and 8 great dry-spells over the middle-lower reaches of the Yangtze River. The wind anomaly exhibits prominent differences between the great wet-spell and the great dry-spell over the Yangtze River Valley. For the great wet-spell, the anomalous southerly from the Bay of Bengal and the South China Sea and the anomalous northerly over North China enhanced low-level convergence toward a narrow latitudinal belt area (the middle-lower reaches of the Yangtze River). The southerly anomaly is connected with an anticyclonic anomalous circulation system centered at 22 degreesN, 140 degreesE and the northerly anomaly is associated with a cyclonic anomalous circulation system centered at the Japan Sea. In the upper level, the anomalous northwesterly between an anticyclonic anomalous system with the center at 23 degreesN, 105 degreesE and a cyclonic anomalous system with the center at Korea diverged over the middle-lower reaches of the Yangtze River. On the contrary, for the great dry-spell, the anomalous northerly over South China and the anomalous southerly over North China diverged from the Yangtze River Valley in the low level. The former formed in the western part of a cyclonic anomalous system centered at 23 degreesN, 135 degreesE. The latter was located in the western ridge of an anticyclonic anomalous system in the northwestern Pacific. The upper troposphere showed easterly anomaly that converged over the middle-lower reaches of the Yangtze River. A cyclonic anomalous system in South China and an anticyclonic system centered in the Japan Sea enhanced the easterly. Large atmospheric heat source anomalies of opposite signs existed over the western Pacific - the South China Sea, with negative in the great wet-spell and positive in the great dry-spell. The analysis of heat source also revealed positive anomalous heat sources during the great wet-spell and negative anomalous heat sources during the great dry-spell over the Yangtze River valley. The changes of the moisture sink and OLR were correspondingly altered, implying the change of heat source anomaly is due to the latent heat releasing of convective activity. Over the southeastern Tibetan Plateau- the Bay of Bengal, the analysis of heat source shows positive anomalous heat sources during the great wet-spell and negative anomalous heat sources during the great dry-spell because of latent heating change. The change of divergent wind coexisted with the change of heat source. In the great wet-spell, southerly divergent wind anomaly in the low level and northerly divergent wind anomaly in high-level are seen over South China. These divergent wind anomalies are helpful to the low-level convergence anomaly and high-level divergence anomaly over the Yangtze River valley. The low-level northerly divergent wind anomaly and high-level southerly divergent wind anomaly over South China reduced the low-level convergence and high-level divergence over the Yangtze River valley during the great dry-spell.
基金The Second Tibetan Plateau Scientific Expedition and Research(STEP)Program(2019QZKK0105)the Science and Technology Development Fund of the Chinese Academy of Meteorological Sciences(2022KJ022)+2 种基金Special Fund for the Basic Scientific Research Expenses of the Chinese Academy of Meteorological Sciences(2021Z013)the Science and Technology Development Fund of the Chinese Academy of Meteorological Sciences(2022KJ021)Major Projects of the Natural Science Foundation of China(91337000)。
文摘With the extreme drought(flood)event in southern China from July to August in 2022(1999)as the research object,based on the comprehensive diagnosis and composite analysis on the anomalous drought and flood years from July to August in 1961-2022,it is found that there are significant differences in the characteristics of the vertically integrated moisture flux(VIMF)anomaly circulation pattern and the VIMF convergence(VIMFC)anomaly in southern China in drought and flood years,and the VIMFC,a physical quantity,can be regarded as an indicative physical factor for the"strong signal"of drought and flood in southern China.Specifically,in drought years,the VIMF anomaly in southern China is an anticyclonic circulation pattern and the divergence characteristics of the VIMFC are prominent,while those are opposite in flood years.Based on the SST anomaly in the typical draught year of 2022 in southern China and the SST deviation distribution characteristics of abnormal draught and flood years from 1961 to 2022,five SST high impact areas(i.e.,the North Pacific Ocean,Northwest Pacific Ocean,Southwest Pacific Ocean,Indian Ocean,and East Pacific Ocean)are selected via the correlation analysis of VIMFC and the global SST in the preceding months(May and June)and in the study period(July and August)in 1961-2022,and their contributions to drought and flood in southern China are quantified.Our study reveals not only the persistent anomalous variation of SST in the Pacific and the Indian Ocean but also its impact on the pattern of moisture transport.Furthermore,it can be discovered from the positive and negative phase fitting of SST that the SST composite flow field in high impact areas can exhibit two types of anomalous moisture transport structures that are opposite to each other,namely an anticyclonic(cyclonic)circulation pattern anomaly in southern China and the coastal areas of east China.These two types of opposite anomalous moisture transport structures can not only drive the formation of drought(flood)in southern China but also exert its influence on the persistent development of the extreme weather.
基金supported by the National Key Research and Development Program of China(Grant No.2018YFC1505901)the National Natural Science Foundation of China(Grant Nos.41490642,4160501,and 41520104008)。
文摘The midwinter suppression(MWS) of the North Pacific storm track(NPST) has been an active research topic for decades. Based on the daily-mean NCEP/NCAR reanalysis from 1948 to 2018, this study investigates the MWS-related atmospheric circulation characteristics in the Northern Hemisphere by regression analysis with respect to a new MWS index, which may shed more light on this difficult issue. The occurrence frequency of the MWS of the upper-tropospheric NPST is more than 0.8 after the mid-1980 s. The MWS is accompanied by significantly positive sea-level pressure anomalies in Eurasia and negative anomalies over the North Pacific, which correspond to a strengthened East Asian winter monsoon. The intensified East Asian trough and atmospheric blocking in the North Pacific as well as the significantly negative low-level air temperature anomalies, lying upstream of the MNPST, are expected to be distinctly associated with the MWS. However, the relationship between the MWS and low-level atmospheric baroclinicity is somewhat puzzling.From the diagnostics of the eddy energy budget, it is identified that the inefficiency of the barotropic energy conversion related to the barotropic governor mechanism does not favor the occurrence of the MWS. In contrast, weakened baroclinic energy conversion, buoyancy conversion, and generation of eddy available potential energy by diabatic heating are conducive to the occurrence of the MWS. In addition, Ural blocking in the upstream region of the MNPST may be another candidate mechanism associated with the MWS.
基金We thank the anonymous reviewers for their careful comments and suggestions.This work was supported by the National Key Research and Development Program of China(Grant No.2018YFC1505901)the National Natural Science Foundation of China(Grant Nos.41605051,41520104008,41475070 and 41575062).
文摘In this study,the impacts of the tropical Pacific–Indian Ocean associated mode(PIOAM)on Madden–Julian Oscillation(MJO)activity were investigated using reanalysis data.In the positive(negative)phase of the PIOAM,the amplitudes of MJO zonal wind and outgoing longwave radiation are significantly weakened(enhanced)over the Indian Ocean,while they are enhanced(weakened)over the central and eastern Pacific.The eastward propagation of the MJO can extend to the central Pacific in the positive phase of the PIOAM,whereas it is mainly confined to west of 160°E in the negative phase.The PIOAM impacts MJO activity by modifying the atmospheric circulation and moisture budget.Anomalous ascending(descending)motion and positive(negative)moisture anomalies occur over the western Indian Ocean and central-eastern Pacific(Maritime Continent and western Pacific)during the positive phase of the PIOAM.The anomalous circulation is almost the opposite in the negative phases of the PIOAM.This anomalous circulation and moisture can modulate the activity of the MJO.The stronger moistening over the Indian Ocean induced by zonal and vertical moisture advection leads to the stronger MJO activity over the Indian Ocean in the negative phase of the PIOAM.During the positive phase of the PIOAM,the MJO propagates farther east over the central Pacific owing to the stronger moistening there,which is mainly attributable to the meridional and vertical moisture advection,especially low-frequency background state moisture advection by the MJO’s meridional and vertical velocities.
基金National Key Basic Research Development Program Project of China(2012CB417202)Key Project of National Natural Science Foundation(91337215)National Natural Science Foundation of China(41275051)
文摘NCEP/NCAR data are utilized to analyze an extreme flood year(1998) and an extreme dry year(2006) in the Sichuan-Chongqing region(SCR) and the results are as follows. The positive divergence of South Asia High(SAH) is stronger in the flood year; the position of the ridge line of SAH is southward compared with the annual average; Western Pacific Subtropical High(WPSH) extends westward and its ridge line is southward. In the drought year, the positive divergence of SAH is weaker, its ridge line is northward, and the position of WPSH is also northward. As shown in the dynamics, in drought(flood) years, negative(positive) vorticity advection in the upper atmosphere can cause the atmosphere to ascend(descend), and anomalous circulation of SAH displays divergence(convergence), and anomalous circulation of the lower atmosphere shows convergence(divergence). Thermal structure of the atmosphere shows that there is warm(cold) temperature advection in the lower atmosphere, and the vertical distribution of diabetic heating causes SAH's local circulation to display convergence(divergence) and affects vertical motion of the lower atmosphere circulation eventually. To some extent, the two extreme years in the SCR is closely related to the vertical motion of atmosphere circulation and the variation of such vertical motion is caused by differences of interactions between SAH and lower atmosphere circulations.
基金supported by the national key R&D Program of China(Grant No 2018YFC1505603)the Key Research Program of Frontier Sciences,CAS(Grant No.ZDBS-LY-DQC010)the National Natural Science Foundation of China(Grant Nos.41876012,41861144015).
文摘In the first half of winter 2020/21,China has experienced an extremely cold period across both northern and southern regions,with record-breaking low temperatures set in many stations of China.Meanwhile,a moderate La Niña event which exceeded both oceanic and atmospheric thresholds began in August 2020 and in a few months developed into its mature phase,just prior to the 2020/21 winter.In this report,the mid−high-latitude large-scale atmospheric circulation anomalies in the Northern Hemisphere,which were forced by the negative phase of Arctic Oscillation,a strengthened Siberian High,an intensified Ural High and a deepened East Asian Trough,are considered to be the direct reason for the frequent cold surges in winter 2020/21.At the same time,the synergistic effect of the warm Arctic and the cold tropical Pacific(La Niña)provided an indispensable background,at a hemispheric scale,to intensify the atmospheric circulation anomalies in middle-to-high latitudes.In the end,a most recent La Niña prediction is provided and the on-coming evolution of climate is discussed for the remaining part of the 2020/21 winter for the purpose of future decision-making and early warning.
基金the project for the Knowledge Innovation Program of South China Sea Institute of Oceanology,Chinese Academy of Sciences under contract No.50601-30.the National Natural Science Foundation of China(NSFC)under contract Nos 40l75018,40136010
文摘The relationship between sea surface temperature anomaly (SSTA) in the domain from the northwest of the Pacific to China seas (NWP-CS) and climate (precipitation and surface temperature) anomaly in winters over east of Chinese Mainland (ECM) are investigated with composite analysis. The results suggest that (1) SSTA in NWP-CS usually appears as 'seesaw' pattern, i.e., sign of SSTA in the northwest of the Philippines is positive (negative) while in the southeast it is negative (positive), defined as SSTA 'seesaw' positive (negative) pattern. When SSTA 'seesaw' positive (negative) pattern appears, the surface temperature in ECM is higher (lower) than the normal winters but the precipitation anomaly is not distinct ; (2) there are two anomalous anticyclones (cyclones) locating in the northwest Pacific and east of the Philippines at 850 hPa level and an anomalous anti-Hadley (Hadley) circulation, which descends (ascends) south of 18oN, ascends (descends) north of 18oN; (3) heat flux anomaly from ocean into atmosphere is weaker (stronger) in the northwest of the Philippines but stronger (weaker) in the southeast of the Philippines than the normal winters, because the weaker (stronger) northerly prevails in the northwest of the Philippines and stronger (weaker) northeaster in the southeast of the Philippines induced by anomalous anticyclones (cyclones).
基金National Key Basic Research Program of China(973 Program,2012CB417403)
文摘After compositing three representative ENSO indices,El Nio events have been divided into an eastern pattern(EP) and a central pattern(CP).By using EOF,correlation and composite analysis,the relationship and possible mechanisms between Indian Ocean Dipole(IOD) and two types of El Nio were investigated.IOD events,originating from Indo-Pacific scale air-sea interaction,are composed of two modes,which are associated with EP and CP El Ni o respectively.The IOD mode related to EP El Nio events(named as IOD1) is strongest at the depth of 50 to 150 m along the equatorial Indian Ocean.Besides,it shows a quasi-symmetric distribution,stronger in the south of the Equator.The IOD mode associated with CP El Nio(named as IOD2) has strongest signal in tropical southern Indian Ocean surface.In terms of mechanisms,before EP El Nio peaks,anomalous Walker circulation produces strong anomalous easterlies in equatorial Indian Ocean,resulting in upwelling in the east,decreasing sea temperature there;a couple of anomalous anticyclones(stronger in the south) form off the Equator where warm water accumulates,and thus the IOD1 occurs.When CP El Nio develops,anomalous Walker circulation is weaker and shifts its center to the west,therefore anomalous easterlies in equatorial Indian Ocean is less strong.Besides,the anticyclone south of Sumatra strengthens,and the southerlies east of it bring cold water from higher latitudes and northerlies west of it bring warm water from lower latitudes to the 15° to 25°S zone.Meanwhile,there exists strong divergence in the east and convergence in the west part of tropical southern Indian Ocean,making sea temperature fall and rise separately.Therefore,IOD2 lies farther south.
基金supported by the National Natural Science Foundation of China [grant number 41625019]
文摘Based on three reanalysis datasets—ERA-Interim,NCAR–NCEP and JRA-55—the classification of25 commonly used indexes of the East Asian summer monsoon(EASM)was investigated.The physical nature of two categories of monsoon index,together with their circulation pattern,climate anomalies,and driving factors,were investigated.Results suggest that the selected 25 monsoon indexes can be classified into two typical categories(CategoryⅠandⅡ),which are dominated by interannual and decadal variabilities of the EASM,respectively.The anomalous circulation patterns and summer rainfall patterns related to the two categories of index also exhibit evident differences.CategoryⅠis closely linked to the low-latitude circulation system and the anomalous circulation pattern is a typical East Asia–Pacific teleconnection pattern.The summer rainfall anomaly exhibits a typical tripole pattern.However,CategoryⅡmainly reflects the impacts of the middle–high latitude circulation system on the summer monsoon and is closely linked to a typical Eurasian teleconnection pattern,which corresponds to a dipole of summer rainfall anomalies.Further analysis suggests that the underlying thermal driving factors of the two categories of monsoon are distinct.The main driving factors of CategoryⅠare the tropical sea surface temperature anomalies(SSTAs),especially ENSO-related SSTAs in the preceding winter and summer SSTAs in the tropical Indian Ocean.The winter signal of Category II summer monsoon anomalous activity mainly originates from the polar region and the middle and high latitudes of the Eurasian continent.CategoryⅡmonsoon activity is also associated with summer SSTAs in the equatorial central Pacific.
文摘Experimental predictions with a hybrid coupled ocean-atmosphere model(L9R15 AGCM-ZC ocean model)were performed for the 1986/87 El Nino event and the 1988/89 La Nina event with and without the Tibetan Plateau respectively(called TP FORC and NTP FORC hereinafter). Comparison shows that,to some extent,the existence of the Tibetan Plateau orography weakens or restrains(strengthens or facilitates)the formation of the anomalous circulation of Asian monsoon during El Nino(La Nina)period.Opposite results are found in the uncoupled AGCM simulation.
基金Supported by the Sci. & Tech. Climbing Project of Chongqing Municipal (CSTC 2008BA0022)National Natural Science Foundation of China (40975058 and 40633018)Special Project for the Provincial Institutes of China Meteorological Administration(CMATG2008S09)
文摘The spatial-temporal features of the extremely severe drought and the anomalous atmospheric circulation in summer 2006 are analyzed based on the NCEP/NCAR reanalysis data,the characteristic circulation indices given by the National Climate Center of China,and the daily precipitation data of 20 stations in the east of Southwest China（ESC） from 1959 to 2006.The results show that the rainless period started from early June and ended in early September 2006 with a total of more than 80 days,and the rainfall was especially scarce from around 25 July to 5 September 2006.Precipitation for each month was less than normal,and analysis of the precipitation indices shows that the summer precipitation in 2006 was the least since 1959.The extremely severe drought in the ESC in summer 2006 was closely related to the persistent anomalies of the atmospheric circulation in the same period,i.e.,anomalies of mid-high latitude atmospheric circulation,western Pacific subtropical high（WPSH）,westerlies,South Asian high,lower-level flow,water vapor transport,vertical motion,and so on.Droughts usually occur when the WPSH lies anomalously northward and westward,or anomalously weak and eastward.The extreme drought in summer 2006 was caused by the former.When the WPSH turned stronger and shifted to the north and west of its normal position,and the South Asian high was also strong and lay eastward,downdrafts prevailed over the ESC and suppressed the water vapor transfer toward this area.At the same time,the disposition of the westerlies and the mid-high latitude circulation disfavored the southward invasion of cold air,which jointly resulted in the extremely severe drought in the ESC in summer 2006.The weak heating over the Tibetan Plateau and vigorous convective activities over the Philippine area were likely responsible for the strong WPSH and its northwestward shift in summer 2006.
基金supported by the National Basic Research Program of China(Grant Nos.2010CB428603 and 2010CB950400)National Natural Science Foundation of China(Grant No.41275094)
文摘Based on the daily NCEP/DOE reanalysis II data,dates of the boreal spring Stratospheric Final Warming(SFW) events during 1979–2010 are defined as the time when the zonal-mean zonal wind at the central latitudes(65°–75°N) of the westerly polar jet drops below zero and never recovers until the subsequent autumn.It is found that the SFW events occur successively from the mid to the lower stratosphere and averagely from the mid to late April with a temporal lag of about 13 days from 10 to 50 hPa.Over the past 32 years,the earliest SFW occurs in mid March whereas the latest SFW happens in late May,showing a clear interannual variability of the time of SFW.Accompanying the SFW onset,the stratospheric circulation transits from a winter dynamical regime to a summertime state,and the maximum negative tendency of zonal wind and the strongest convergence of planetary-wave are observed.Composite results show that the early/late SFW events in boreal spring correspond to a quicker/slower transition of the stratospheric circulation,with the zonal-mean zonal wind reducing about 20/5 m s-1 at 30 hPa within 10 days around the onset date.Meanwhile,the planetary wave activities are relatively strong/weak associating with an out-of-/in-phase circumpolar circulation anomaly before and after the SFW events in the stratosphere.All these results indicate that,the earlier breakdown of the stratospheric polar vortex(SPV),as for the winter stratospheric sudden warming(SSW) events is driven mainly by wave forcing;and in contrast,the later breakdown of the SPV exhibits more characteristics of its seasonal evolution.Nevertheless,after the breakdown of SPV,the polar temperature anomalies always exhibit an out-of-phase relationship between the stratosphere and the troposphere for both the early and late SFW events,which implies an intimate stratosphere–troposphere dynamical coupling in spring.In addition,there exists a remarkable interdecadal change of the onset time of SFW in the mid 1990s.On average,the SFW onset time before the mid 1990s is 11 days earlier than that afterwards,corresponding to the increased/decreased planetary wave activities in late winter-early spring before/after the 1990s.
