Under global warming,understanding the long-term variation in different types of heatwaves is vital for China’s preparedness against escalating heat stress.This study investigates dry and wet heatwave shifts in easte...Under global warming,understanding the long-term variation in different types of heatwaves is vital for China’s preparedness against escalating heat stress.This study investigates dry and wet heatwave shifts in eastern China over recent decades.Spatial trend analysis displays pronounced warming in inland midlatitudes and the Yangtze River Valley,with increased humidity in coastal regions.EOF results indicate intensifying dry heatwaves in northern China,while the Yangtze River Valley sees more frequent dry heatwaves.On the other hand,Indochina and regions north of 25°N also experience intensified wet heatwaves,corresponding to regional humidity increases.Composite analysis is conducted based on different situations:strong,frequent dry or wet heatwaves.Strong dry heatwaves are influenced by anticyclonic circulations over northern China,accompanied by warming SST anomalies around the coastal midlatitudes of the western North Pacific(WNP).Frequent dry heatwaves are related to strong subsidence along with a strengthened subtropical high over the WNP.Strong and frequent wet heatwaves show an intensified Okhotsk high at higher latitudes in the lower troposphere,and a negative circumglobal teleconnection wave train pattern in the upper troposphere.Decaying El Niño SST patterns are observed in two kinds of wet heatwave and frequent dry heatwave years.Risk analysis indicates that El Niño events heighten the likelihood of these heatwaves in regions most at risk.As global warming continues,adapting and implementing mitigation strategies toward extreme heatwaves becomes crucial,especially for the aforementioned regions under significant heat stress.展开更多
The diurnal surface temperature range （DTR） has become significantly smaller over the Tibetan Plateau （TP） but larger in southeastern China, despite the daily mean surface temperature having increased steadily in ...The diurnal surface temperature range （DTR） has become significantly smaller over the Tibetan Plateau （TP） but larger in southeastern China, despite the daily mean surface temperature having increased steadily in both areas during recent decades. Based on ERA-Interim reanalysis data covering 1979-2012, this study shows that the weakened DTR over TP is caused by stronger warming of daily minimum surface temperature （Tmin） and a weak cooling of the daily maximum surface temper- ature （Tmax）; meanwhile, the enhanced DTR over southeastern China is mainly associated with a relatively stronger/weaker warming of Tmax/Tmin. A further quantitative analysis of DTR changes through a process-based decomposition method-- the Coupled Surface-Atmosphere Climate Feedback Response Analysis Method （CFRAM）--indicates that changes in radia- tive processes are mainly responsible for the decreased DTR over the TR In particular, the increased low-level cloud cover tends to induce the radiative cooling/warming during daytime/nighttime, and the increased water vapor helps to decrease the DTR through the stronger radiative wanning during nighttime than daytime. Contributions from the changes in all radiative processes （over -2℃） are compensated for by those from the stronger decreased surface sensible heat flux during daytime than during nighttime （approximately 2.5℃）, but are co-contributed by the changes in atmospheric dynamics （approximately -0.4℃） and the stronger increased latent heat flux during daytime （approximately -0.8℃）. In contrast, the increased DTR over southeastern China is mainly contributed by the changes in cloud, water vapor and atmospheric dynamics. The changes in surface heat fluxes have resulted in a decrease in DTR over southeastern China.展开更多
This study focuses on the decadal variability of tropical cyclones(TC) over the Western North Pacific(WNP)and how these changes are related to the Madden-Julian Oscillation(MJO).It was done with the help of the Real-t...This study focuses on the decadal variability of tropical cyclones(TC) over the Western North Pacific(WNP)and how these changes are related to the Madden-Julian Oscillation(MJO).It was done with the help of the Real-time Multivariate MJO index from the Australian Government Bureau of Meteorology of the Centre for Australian Weather and Climate Research,TC data from the Joint Typhoon Warming Center best track datasets,and daily and monthly datasets from the NCEP/NCAR reanalysis center.The results show that the TC frequency in the WNP exhibited a statistically significant decrease during 1998-2010 compared to during 1979-1997.The decrease in TC frequency in the WNP mainly occurred during MJO active phases(i.e.,phases 4,5,6,and 7).Further investigation of the climate background and the propagation differences of the MJO between 1979-1997 and 1998-2010 was performed.The La Nina-like tropical sea surface temperature cooling caused stronger Walker circulation and thus induced unfavorable atmosphere conditions for WNP TC genesis including a low-level easterly anomaly,a negative relative vorticity anomaly,an increase in sea-level pressure,and stronger vertical wind shear.Moreover,shortening of the MJO cycle,decline in the duration of the active phases in the WNP,and easterly anomaly and shrinkage of the convection area during MJO active phases may also partly explain the decadal variation of TC.展开更多
Decadal changes in the subseasonal evolution and the phase-locked climatological intraseasonal fluctuation of summertime rainfall over the Korean Peninsula before and after the mid-1990s are investigated.The activity ...Decadal changes in the subseasonal evolution and the phase-locked climatological intraseasonal fluctuation of summertime rainfall over the Korean Peninsula before and after the mid-1990s are investigated.The activity and the migration speed of the monsoon rain band over the East Asian region are altered in the recent decade,resulting in the drier conditions in late spring and the earlier onset of Changma.In early August when a climatological monsoon break was clear in the earlier decade,the precipitation has increased dramatically with a meridional coherency.The response to the enhanced convection over the South China Sea and southeastern China provides a favorable condition for more precipitation in early August through the changes in moisture transport and tropical cyclone passage.展开更多
We investigated the decadal changes in the different types of summer mean precipitation over China across the mid-1990 s based on observational datasets.The spatial variations in the observed decadal changes were esti...We investigated the decadal changes in the different types of summer mean precipitation over China across the mid-1990 s based on observational datasets.The spatial variations in the observed decadal changes were estimated by comparing the present day(PD)time period of 1994–2011 with an earlier period of 1964–1981.The summer total precipitation increased in southern China and decreased in northern China from the early period to the PD.The increases of precipitation in southern China were due to increases in the frequency of heavy and moderate rainfall,whereas the decreases over northern China were mainly due to decreases in the frequency of moderate and light rainfall.Based on a set of numerical experiments using an atmospheric general circulation model coupled with a multilevel mixed-layer ocean model,we found that the increase of precipitation frequency forced by greenhouse gases is the main reason of increasing precipitation over southern and northeastern China,while the decrease of frequency caused by anthropogenic aerosol(AA)induces the decreasing precipitation over northern China.The water vapor flux convergence and water vapor flux strengthen in southern China and northeastern China by anthropogenic greenhouse gases.This distribution is also conducive to precipitation in most of southern China and northeastern China.Under the control of weakened southwesterly winds and 850-h Pa divergence,precipitation decreases over northern and southwestern China by AA.展开更多
In 2013, we re-inventoried all lianas （≥1 cm diameter measured at 1.3 m from the rooting point） in four I-ha permanent plots distributed one each in four sites of inland tropical dry evergreen forest on the Coroman...In 2013, we re-inventoried all lianas （≥1 cm diameter measured at 1.3 m from the rooting point） in four I-ha permanent plots distributed one each in four sites of inland tropical dry evergreen forest on the Coromandel Coast （Pudukottai district） of peninsular India, established in 2003. Among the four sites, Shanmuganathapuram （SP） and Araiyapatti （AP） were much disturbed and the other two sites （Karisakkadu--KR and Maramadakki--MM） were moderately disturbed. We inventoried a total of 3425 lianas representing 37 species of 33 genera and 22 families. Over a decade （2003-2013） liana species richness increased at two sites （MM and SP） and no changes occurred at the other two sites. Liana abundance increased by 210, 211,164 and 162 individuals at sites AP, KR, MM and SP, respectively, and basal area increased （from 1.09 to 1.76 m2 at AP, 0.67 to 0.86 m2 at KR, 1.68 to 2.06 mz at MM, and from 0.44 to 1.06 m2 at SP）. Over a 10-year period, three species （Abrus precatorius, Canavalia virosa, and Cocculus hirsutus） were lost and five species （Gloriosa superba, Ampelocissus tomentosa, Capparis sepiaria, Aganosma cymosa and Tiliacora acuminata） were newly added. Total aboveground biomass increased by 18.5, 0.74, 3.6 and 9.5 Mg ha-1, respectively, at sites AP, KR, MM and SP.展开更多
During the boreal winter,abundant persistent heavy rainfall(PHR)amount and significant rainfall variability at subseasonal timescale are generally observed over the southern sector of East China,where the large-scale ...During the boreal winter,abundant persistent heavy rainfall(PHR)amount and significant rainfall variability at subseasonal timescale are generally observed over the southern sector of East China,where the large-scale circulation and moisture transport are tightly connected with the equatorial Madden-Julian Oscillation(MJO).As the MJO convections occur over the equatorial Indian Ocean(MJO phases 1-4),the low-level moisture convergence is enhanced over southern China(SC,108°-120°E,21°-26°N)with the divergence to the north.Thus,a positive anomaly of PHR amount appears in SC but a negative anomaly of PHR amount is seen in the Yangtze River valley(YR,113°-122°E,28°-30°N).In contrast,the divergence(convergence)of moisture flux anomalies in the SC(YR)associated with the western equatorial Pacific MJO convections(phases 5-8)limits(benefits)the occurrence of PHR in the SC(YR).The wintertime PHR over southern China is found to undergo a long-term change over the past three decades(1979-2011)with a decreasing(an increasing)trend of PHR amount in the SC(YR).The change in PHR amount occurs consistently with the decadal change in MJO activity.In the earlier decade(1979-1994,E1),the active Indian Ocean(western Pacific)MJO events appeared more frequently while they became less frequent in the recent decade(1995-2011,E2).Accordingly,the Indian Ocean(western Pacific)MJO-related moisture convergence(divergence)anomalies in the SC tend to be weakened(enhanced),contributing to the decrease in PHR amount over the SC in the recent decade.展开更多
Tropical cyclone （TC） genesis in the South China Sea （SCS） during 1979-2008 underwent a decadal variation around 1993. A total of 55 TCs formed in the SCS from May to September during 1994- 2008, about twice that ...Tropical cyclone （TC） genesis in the South China Sea （SCS） during 1979-2008 underwent a decadal variation around 1993. A total of 55 TCs formed in the SCS from May to September during 1994- 2008, about twice that during 1979-1993 （27）. During the TC peak season （July-September, JAS）, there were 43 TCs fi＇om 1994-2008, but only 17 during 1979-1993. For July in particular, 13 TCs formed from 1994-2008, but there were none during 1979-1993. The change in TC number is associated with changes of key environmental conditions in atmosphere and ocean. Compared to 1979-1993, the subtropical high was significantly weaker and was displaced more eastward during 1994-2008. In the former period, a stronger subtropical high induced downward flow, inhibiting TC formation. In the latter period, vertical wind shear and outgoing longwave radiation all weakened. Mid-level （850-500 hPa） humidity, and relative vorticity were higher. Sea surface temperature and upper layer heat content were also higher in the area. All these factors favor TC genesis during the latter period. The decadal change of TC genesis led to more landfalling TCs in Southern China during the period 1994-2008, which contributed to an abrupt increase in regional rainfall.展开更多
It is well known that on the interannual timescale,the westward extension of the western North Pacific subtropical high(WNPSH)results in enhanced rainfall over the Yangtze River basin(YRB)in summer,and vice versa.This...It is well known that on the interannual timescale,the westward extension of the western North Pacific subtropical high(WNPSH)results in enhanced rainfall over the Yangtze River basin(YRB)in summer,and vice versa.This study identifies that this correspondence experiences a decadal change in the late 1970s.That is,the WNPSH significantly affects YRB precipitation(YRBP)after the late 1970s(P2)but not before the late 1970s(P1).It is found that enhanced interannual variability of the WNPSH favors its effect on YRB rainfall in P2.On the other hand,after removing the strong WNPSH cases in P2 and making the WNPSH variability equivalent to that in P1,the WNPSH can still significantly affect YRB rainfall,suggesting that the WNPSH variability is not the only factor that affects the WNPSH-YRBP relationship.Further results indicate that the change in basic state of thermal conditions in the tropical WNP provides a favorable background for the enhanced WNPSH-YRBP relationship.In P2,the lower-tropospheric atmosphere in the tropical WNP gets warmer and wetter,and thus the meridional gradient of climatological equivalent potential temperature over the YRB is enhanced.As a result,the WNPSH-related circulation anomalies can more effectively induce YRB rainfall anomalies through affecting the meridional gradient of equivalent potential temperature over the YRB.展开更多
The winter temperature changes in East China during the past 100 years are investigated by using the Twentieth Century Version 2(20th-v2)Reanalysis.Four typical warm(P1,1911–30;P4,1991–2010)and cold(P2,1938–57;P3,1...The winter temperature changes in East China during the past 100 years are investigated by using the Twentieth Century Version 2(20th-v2)Reanalysis.Four typical warm(P1,1911–30;P4,1991–2010)and cold(P2,1938–57;P3,1961–80)periods are identified for the East China winter temperature index.Comparison of160-station observational data,NCAR sea level pressure(SLP)data,and NCEP/NCAR Reanalysis shows that the20th-v2 Reanalysis can successfully depict the major features of the warming from P3 to P4,which is part of the global warming phenomenon.The cooling from P1 to P2is a regional phenomenon under global warming.However,both changes are consistent with the phase change of the Arctic Oscillation(AO),while the second change is also accompanied by the phase change of Antarctic Oscillation(AAO)from negative to positive.Original sources of the interdecadal shifts of the AO and winter temperature in East China require further research.展开更多
Using the CAM3.0 model, we investigated the respective effects of aerosol concentration increasing and decadal variation of global sea surface temperature(SST) around year 1976/77 on the East Asian precipitation in bo...Using the CAM3.0 model, we investigated the respective effects of aerosol concentration increasing and decadal variation of global sea surface temperature(SST) around year 1976/77 on the East Asian precipitation in boreal summer. By doubling the concentration of the sulfate aerosol and black carbon aerosol separately and synchronously in East Asia(100-150 °E, 20-50 °N), the climate effects of these aerosols are specifically investigated. The results show that both the decadal SST changing and aerosol concentration increasing could lead to rainfall decreasing in the center of East Asia, but increasing in the regions along southeast coast areas of China. However, the different patterns of rainfall over ocean and lower wind field over Asian continent between aerosol experiments and SST experiments in CAM3.0 indicate the presence of different mechanisms. In the increased aerosol concentration experiments, scattering effect is the main climate effect for both sulfate and black carbon aerosols in the Eastern Asian summer. Especially in the increased sulfate aerosol concentration experiment, the climate scattering effect of aerosol leads to the most significant temperature decreasing, sinking convection anomalies and decreased rainfall in the troposphere over the central part of East Asia. However, in an increased black carbon aerosol concentration experiment, weakened sinking convection anomalies exist at the southerly position. This weakened sinking and its compensating rising convection anomalies in the south lead to the heavy rainfall over southeast coast areas of China. When concentrations of both sulfate and black carbon aerosols increase synchronously, the anomalous rainfall distribution is somewhat like that in the increased black carbon concentration aerosol experiment but with less intensity.展开更多
In the past nearly two decades,the Argo Program has created an unprecedented global observing array with continuous in situ salinity observations,providing opportunities to extend our knowledge on the variability and ...In the past nearly two decades,the Argo Program has created an unprecedented global observing array with continuous in situ salinity observations,providing opportunities to extend our knowledge on the variability and effects of ocean salinity.In this study,we utilize the Argo data during 2004–2017,together with the satellite observations and a newly released version of ECCO ocean reanalysis,to explore the decadal salinity variability in the Southeast Indian Ocean(SEIO)and its impacts on the regional sea level changes.Both the observations and ECCO reanalysis show that during the Argo era,sea level in the SEIO and the tropical western Pacific experienced a rapid rise in 2005–2013 and a subsequent decline in 2013–2017.Such a decadal phase reversal in sea level could be explained,to a large extent,by the steric sea level variability in the upper 300 m.Argo data further show that,in the SEIO,both the temperature and salinity changes have significant positive contributions to the decadal sea level variations.This is different from much of the Indo-Pacific region,where the halosteric component often has minor or negative contributions to the regional sea level pattern on decadal timescale.The salinity budget analyses based on the ECCO reanalysis indicate that the decadal salinity change in the upper 300 m of SEIO is mainly caused by the horizontal ocean advection.More detailed decomposition reveals that in the SEIO,there exists a strong meridional salinity front between the tropical low-salinity and subtropical high salinity waters.The meridional component of decadal circulation changes will induce strong cross-front salinity exchange and thus the significant regional salinity variations.展开更多
In this study,physical mechanism of the impacts of the tropical Atlantic sea surface temperature(SST)on decadal change of the summer North Atlantic Oscillation(SNAO)was explored using an atmospheric general circulatio...In this study,physical mechanism of the impacts of the tropical Atlantic sea surface temperature(SST)on decadal change of the summer North Atlantic Oscillation(SNAO)was explored using an atmospheric general circulation model(AGCM)developed at the International Centre for Theoretical Physics(ICTP).The simulation results indicate that the decadal warming of the SST over the tropical Atlantic after the late 1970s could have significantly enhanced the convection over the region.This enhanced convection would have strengthened the local meridional circulation over the Eastern Atlantic-North Africa-Western Europe region,exciting a meridional teleconnection.This teleconnection might have brought the signal of the tropical Atlantic SST to the Extratropics,consequently activating the variability of the eastern part of the SNAO southern center,which led to an eastward shift of the SNAO southern center around the late 1970s.Such physical processes are highly consistent with the previous observations.展开更多
Previous studies have suggested a poleward shift of the zonally averaged jet stream due to rapid warming over continents.However,the regional characteristics of the change in the jet stream are not yet understood.Here...Previous studies have suggested a poleward shift of the zonally averaged jet stream due to rapid warming over continents.However,the regional characteristics of the change in the jet stream are not yet understood.Here,we present evidence suggesting that the East Asian westerly jet did not shift poleward in past decades（1980-2004 relative to 1958-1979）,both in winter and summer.Rather,the jet axis has moved southward in summer,but its meridional position is steady in winter.The main change of the jet stream in winter is the enhancement of its intensity.These changes in both summer and winter are consistent with the corresponding changes in the large meridional tropospheric temperature-gradient zone.Based on these results,we suggest that the changes of the jet stream over East Asia are unique and are different from the zonal mean jet stream over the Northern Hemisphere and over the North Atlantic region.展开更多
Owing to limited observations,it remains unknown whether the impact of El Niño-Southern Oscillation(ENSO)on the Indian Ocean-Northwest Pacific(IO-NWP)climate showed decadal changes in the early 20th century.Using...Owing to limited observations,it remains unknown whether the impact of El Niño-Southern Oscillation(ENSO)on the Indian Ocean-Northwest Pacific(IO-NWP)climate showed decadal changes in the early 20th century.Using multi-source reanalysis and hindcast datasets from the ECMWF and NOAA extending back to 1901,this study investigates interdecadal variations of the impact of ENSO on the IO-NWP climate from 1901 to 2009.It is found that the influence of ENSO on the IO-NWP climate shows“strong-weak-strong”interdecadal change during 1901-2009.This is characterized by much weaker Indian Ocean sea surface temperature(SST)warming and a weaker NWP subtropical anticyclone(NWPSA)in the following summer of El Ni?o during 1946-1967,compared with those in the other two periods(1901-1945 and 1968-2009).Analyses of the datasets indicate that the interdecadal variation is mainly associated with the change in ENSO amplitude.In contrast to the period of 1946-1967,a greater SST variance occurred in the central-eastern equatorial Pacific during 1901-1945 and 1968-2009.A stronger El Ni?o tends to generate more significant anticyclonic anomalies over the southeast Indian Ocean through teleconnection.The northwesterly anomalies to the south of the anticyclone weaken the southeast trade winds and warm the south Indian Ocean SST via wind-evaporation-SST feedback,and the positive south Indian Ocean SST anomalies trigger westward-propagating oceanic Rossby waves to induce stronger warming of the southwest Indian Ocean,leading to a significant asymmetric wind pattern across the equator in spring.The profound northeastward winds on the north side weaken the southwest monsoon,leading to a“second warming”over the north Indian Ocean in summer,which anchors the eastward-propagating warm Kelvin waves and results in a stronger NWPSA by inducing surface divergence and suppressing deep convection.展开更多
The increase in southern China summer rainfall around 1993 was accompanied by an increase in tropical cyclones that formed in the South China Sea. This study documents the connection of these two features. Our analysi...The increase in southern China summer rainfall around 1993 was accompanied by an increase in tropical cyclones that formed in the South China Sea. This study documents the connection of these two features. Our analysis shows that the contribution of tropical cyclones that formed in the South China Sea to southern China summer rainfall experienced a significant increase around 1993, in particular, along the coast and in the heavy rain category. The number of tropical cyclones that formed in the western North Pacific and entered the South China Sea decreased, and their contribution to summer rainfall was reduced in eastern part of southern China （but statistically insignificant）. The increase in tropical cyclone-induced rainfall contributed up to -30＆ of the total rainfall increase along the coastal regions. The increase of tropical cyclones in the South China Sea appears to be related to an increase in local sea surface temperature.展开更多
This study examined the variability in frequency of tropical night occurrence （i.e., minimum air tem- perature 25℃） in Beijing, using a homogenized daily temperature dataset during the period 1960–2008. Our result...This study examined the variability in frequency of tropical night occurrence （i.e., minimum air tem- perature 25℃） in Beijing, using a homogenized daily temperature dataset during the period 1960–2008. Our results show that tropical nights occur most frequently in late July and early August, which is consis- tent with relatively high air humidity associated with the rainy season in Beijing. In addition, year-to-year variation of tropical night occurrence indicates that the tropical nights have appeared much more frequently since 1994, which can be illustrated by the yearly days of tropical nights averaged over two periods： 9.2 days of tropical nights per year during 1994–2008 versus 3.15 days during 1960–1993. These features of tropical night variations suggest a distinction between tropical nights and extreme heat in Beijing. We further investigated the large-scale circulations associated with the year-to-year variation of tropical night occurrence in July and August, when tropical nights appear most frequently and occupy 95% of the annual sum. After comparing the results in the two reanalysis datasets （NCEP/NCAR and ERA-40） and considering the possible effects of decadal change in the frequency of tropical nights that occurred around 1993/94, we conclude that on the interannual time scale, the cyclonic anomaly with a barotropic structure centered over Beijing is responsible for less frequent tropical nights, and the anticyclonic anomaly is responsible for more frequent occurrence of tropical nights over Beijing.展开更多
This paper documents a decadal strengthened co-variability of the Antarctic Oscillation （AAO） and ENSO in austral spring after the mid-1990s. During the period 1979-93, the ENSO （AAO） spatial signatures are restri...This paper documents a decadal strengthened co-variability of the Antarctic Oscillation （AAO） and ENSO in austral spring after the mid-1990s. During the period 1979-93, the ENSO （AAO） spatial signatures are restricted to the tropicsmidlatitudes （Antarctic-midlatitudes） of the Southern Hemisphere （SH）, with a weak connection between the two oscillations. Comparatively, after the mid-1990s, the E1 Nifio-related atmospheric anomalies project on a negative AAO pattern with a barotropic structure in the mid-high latitudes of the SH. The expansion of E1 Nifio-related air temperature anomalies have a heightened impact on the meridional thermal structure of the SH, contributing to a weakened circumpolar westerly and strengthened subtropical jet. Meanwhile, the ENSO-related southern three-cell circulations expand poleward and then strongly couple the Antarctic and the tropics. Numerical simulation results suggest that the intensified connection between ENSO and SST in the South Pacific since the mid-1990s is responsible for the strengthened AAO-ENSO relationship.展开更多
Using a homogenized daily maximum temperature(T_(max))dataset across China,this study characterized the spatiotemporal variation of the onset date of extreme hot days in a year(i.e.,FirstEHD)during 1960-2018.Inhomogen...Using a homogenized daily maximum temperature(T_(max))dataset across China,this study characterized the spatiotemporal variation of the onset date of extreme hot days in a year(i.e.,FirstEHD)during 1960-2018.Inhomogeneous trends of FirstEHD over China during 1960-2018 can be found,with the advanced trend of FirstEHD over most parts in China,while a number of stations in North-Central China(NC)show the delayed trend of FirstEHD.Moreover,there exist interdecadal changes of FirstEHD trend,with a remarkable difference in the trend magnitude before and after the 1990s over South China(SC),and the sign of trend can even reverse from negative to positive after the 1990s in Xinjiang(XJ)and Yangtze River Basin(YR),and from positive to negative in NC.The overall trends of FirstEHD over NC,YR,and XJ during 1960-2018 are dominated by the trends before the 1990s,while they are dominated by the sharp advance after the 1990s over SC.It is further found that the trend of FirstEHD can generally be explained by the long-term trend in T_(max) over most parts of China,but the contribution from T_(max) variabilities is also non-negligible and can even account for more than 75% of the overall trend over NC.The possible factors responsible for the decadal changes in FirstEHD trends are also discussed.展开更多
This study utilizes ML classifiers to estimate canopy density based on three decades of data (1990-2021). The Support Vector Machine (SVM) classifier outperformed other classifiers, such as Random Tree and Maximum Lik...This study utilizes ML classifiers to estimate canopy density based on three decades of data (1990-2021). The Support Vector Machine (SVM) classifier outperformed other classifiers, such as Random Tree and Maximum Likelihood. Satellite data from Landsat and Sentinel 2 was classified using a developed python model, providing an economical and time-saving approach. The accuracy of the classification was evaluated through a confusion matrix and area computation. The findings indicate a negative trend in the overall decadal change, with significant tree loss attributed to jhum cultivation, mining, and quarry activities. However, positive changes were observed in recent years due to the ban on illegal mining. The study highlights the dynamic nature of tree cover and emphasizes the need for biennial assessments using at least five time-series data. Micro-level analysis in Shallang, West Khasi hills, revealed a concerning trend of shortening jhum cycles. Automation in canopy change analysis is crucial for effective forest monitoring, providing timely information for law enforcement proposals and involving forest managers, stakeholders, and watchdog organizations.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.42120104001,42192563 and 42005010)the Hong Kong RGC General Research Fund 11300920.
文摘Under global warming,understanding the long-term variation in different types of heatwaves is vital for China’s preparedness against escalating heat stress.This study investigates dry and wet heatwave shifts in eastern China over recent decades.Spatial trend analysis displays pronounced warming in inland midlatitudes and the Yangtze River Valley,with increased humidity in coastal regions.EOF results indicate intensifying dry heatwaves in northern China,while the Yangtze River Valley sees more frequent dry heatwaves.On the other hand,Indochina and regions north of 25°N also experience intensified wet heatwaves,corresponding to regional humidity increases.Composite analysis is conducted based on different situations:strong,frequent dry or wet heatwaves.Strong dry heatwaves are influenced by anticyclonic circulations over northern China,accompanied by warming SST anomalies around the coastal midlatitudes of the western North Pacific(WNP).Frequent dry heatwaves are related to strong subsidence along with a strengthened subtropical high over the WNP.Strong and frequent wet heatwaves show an intensified Okhotsk high at higher latitudes in the lower troposphere,and a negative circumglobal teleconnection wave train pattern in the upper troposphere.Decaying El Niño SST patterns are observed in two kinds of wet heatwave and frequent dry heatwave years.Risk analysis indicates that El Niño events heighten the likelihood of these heatwaves in regions most at risk.As global warming continues,adapting and implementing mitigation strategies toward extreme heatwaves becomes crucial,especially for the aforementioned regions under significant heat stress.
基金jointly supported by the China Meteorological Administration Special Public Welfare Research Fund(Grant No.GYHY201406001)the National Natural Science Foundation of China(Grant Nos.91437105,41575041 and 41430533)Special Foundation for National Commonweal Institutes of China(Grant No.IUMKY201614)
文摘The diurnal surface temperature range （DTR） has become significantly smaller over the Tibetan Plateau （TP） but larger in southeastern China, despite the daily mean surface temperature having increased steadily in both areas during recent decades. Based on ERA-Interim reanalysis data covering 1979-2012, this study shows that the weakened DTR over TP is caused by stronger warming of daily minimum surface temperature （Tmin） and a weak cooling of the daily maximum surface temper- ature （Tmax）; meanwhile, the enhanced DTR over southeastern China is mainly associated with a relatively stronger/weaker warming of Tmax/Tmin. A further quantitative analysis of DTR changes through a process-based decomposition method-- the Coupled Surface-Atmosphere Climate Feedback Response Analysis Method （CFRAM）--indicates that changes in radia- tive processes are mainly responsible for the decreased DTR over the TR In particular, the increased low-level cloud cover tends to induce the radiative cooling/warming during daytime/nighttime, and the increased water vapor helps to decrease the DTR through the stronger radiative wanning during nighttime than daytime. Contributions from the changes in all radiative processes （over -2℃） are compensated for by those from the stronger decreased surface sensible heat flux during daytime than during nighttime （approximately 2.5℃）, but are co-contributed by the changes in atmospheric dynamics （approximately -0.4℃） and the stronger increased latent heat flux during daytime （approximately -0.8℃）. In contrast, the increased DTR over southeastern China is mainly contributed by the changes in cloud, water vapor and atmospheric dynamics. The changes in surface heat fluxes have resulted in a decrease in DTR over southeastern China.
基金National Natural Science Foundation of China(41305050,41275011,41330527)National Key Program for Global Change Research of China(2014CB953903)Fundamental Research Funds for the Central Universities(2013NT02)
文摘This study focuses on the decadal variability of tropical cyclones(TC) over the Western North Pacific(WNP)and how these changes are related to the Madden-Julian Oscillation(MJO).It was done with the help of the Real-time Multivariate MJO index from the Australian Government Bureau of Meteorology of the Centre for Australian Weather and Climate Research,TC data from the Joint Typhoon Warming Center best track datasets,and daily and monthly datasets from the NCEP/NCAR reanalysis center.The results show that the TC frequency in the WNP exhibited a statistically significant decrease during 1998-2010 compared to during 1979-1997.The decrease in TC frequency in the WNP mainly occurred during MJO active phases(i.e.,phases 4,5,6,and 7).Further investigation of the climate background and the propagation differences of the MJO between 1979-1997 and 1998-2010 was performed.The La Nina-like tropical sea surface temperature cooling caused stronger Walker circulation and thus induced unfavorable atmosphere conditions for WNP TC genesis including a low-level easterly anomaly,a negative relative vorticity anomaly,an increase in sea-level pressure,and stronger vertical wind shear.Moreover,shortening of the MJO cycle,decline in the duration of the active phases in the WNP,and easterly anomaly and shrinkage of the convection area during MJO active phases may also partly explain the decadal variation of TC.
基金the Korea Meteorological Administration(KMA) Research and Development Program under Grant CATER 2006- 4205 and by BK21 program.Partial support for M.Kimoto was given by the Innovative Program of Climate Change for the 21st Century of the Japanese Ministry of Education, Culture,Sports,Science and Technology.
文摘Decadal changes in the subseasonal evolution and the phase-locked climatological intraseasonal fluctuation of summertime rainfall over the Korean Peninsula before and after the mid-1990s are investigated.The activity and the migration speed of the monsoon rain band over the East Asian region are altered in the recent decade,resulting in the drier conditions in late spring and the earlier onset of Changma.In early August when a climatological monsoon break was clear in the earlier decade,the precipitation has increased dramatically with a meridional coherency.The response to the enhanced convection over the South China Sea and southeastern China provides a favorable condition for more precipitation in early August through the changes in moisture transport and tropical cyclone passage.
基金Supported by the National Natural Science Foundation of China(41905091)Support Plan of the National Science and Technology(2015BAC03B04)+1 种基金Fund Project of the National Meteorological Center Forecaster(Y201904)Buwen DONG is supported by the UK National Centre for Atmospheric Science-Climate(NCAS-Climate)at the University of Reading。
文摘We investigated the decadal changes in the different types of summer mean precipitation over China across the mid-1990 s based on observational datasets.The spatial variations in the observed decadal changes were estimated by comparing the present day(PD)time period of 1994–2011 with an earlier period of 1964–1981.The summer total precipitation increased in southern China and decreased in northern China from the early period to the PD.The increases of precipitation in southern China were due to increases in the frequency of heavy and moderate rainfall,whereas the decreases over northern China were mainly due to decreases in the frequency of moderate and light rainfall.Based on a set of numerical experiments using an atmospheric general circulation model coupled with a multilevel mixed-layer ocean model,we found that the increase of precipitation frequency forced by greenhouse gases is the main reason of increasing precipitation over southern and northeastern China,while the decrease of frequency caused by anthropogenic aerosol(AA)induces the decreasing precipitation over northern China.The water vapor flux convergence and water vapor flux strengthen in southern China and northeastern China by anthropogenic greenhouse gases.This distribution is also conducive to precipitation in most of southern China and northeastern China.Under the control of weakened southwesterly winds and 850-h Pa divergence,precipitation decreases over northern and southwestern China by AA.
基金Pondicherry University for financial support received through UGC University fellowship
文摘In 2013, we re-inventoried all lianas （≥1 cm diameter measured at 1.3 m from the rooting point） in four I-ha permanent plots distributed one each in four sites of inland tropical dry evergreen forest on the Coromandel Coast （Pudukottai district） of peninsular India, established in 2003. Among the four sites, Shanmuganathapuram （SP） and Araiyapatti （AP） were much disturbed and the other two sites （Karisakkadu--KR and Maramadakki--MM） were moderately disturbed. We inventoried a total of 3425 lianas representing 37 species of 33 genera and 22 families. Over a decade （2003-2013） liana species richness increased at two sites （MM and SP） and no changes occurred at the other two sites. Liana abundance increased by 210, 211,164 and 162 individuals at sites AP, KR, MM and SP, respectively, and basal area increased （from 1.09 to 1.76 m2 at AP, 0.67 to 0.86 m2 at KR, 1.68 to 2.06 mz at MM, and from 0.44 to 1.06 m2 at SP）. Over a 10-year period, three species （Abrus precatorius, Canavalia virosa, and Cocculus hirsutus） were lost and five species （Gloriosa superba, Ampelocissus tomentosa, Capparis sepiaria, Aganosma cymosa and Tiliacora acuminata） were newly added. Total aboveground biomass increased by 18.5, 0.74, 3.6 and 9.5 Mg ha-1, respectively, at sites AP, KR, MM and SP.
基金supported by the National Key R&D Program of China [grant number 2018YFC1505804]
文摘During the boreal winter,abundant persistent heavy rainfall(PHR)amount and significant rainfall variability at subseasonal timescale are generally observed over the southern sector of East China,where the large-scale circulation and moisture transport are tightly connected with the equatorial Madden-Julian Oscillation(MJO).As the MJO convections occur over the equatorial Indian Ocean(MJO phases 1-4),the low-level moisture convergence is enhanced over southern China(SC,108°-120°E,21°-26°N)with the divergence to the north.Thus,a positive anomaly of PHR amount appears in SC but a negative anomaly of PHR amount is seen in the Yangtze River valley(YR,113°-122°E,28°-30°N).In contrast,the divergence(convergence)of moisture flux anomalies in the SC(YR)associated with the western equatorial Pacific MJO convections(phases 5-8)limits(benefits)the occurrence of PHR in the SC(YR).The wintertime PHR over southern China is found to undergo a long-term change over the past three decades(1979-2011)with a decreasing(an increasing)trend of PHR amount in the SC(YR).The change in PHR amount occurs consistently with the decadal change in MJO activity.In the earlier decade(1979-1994,E1),the active Indian Ocean(western Pacific)MJO events appeared more frequently while they became less frequent in the recent decade(1995-2011,E2).Accordingly,the Indian Ocean(western Pacific)MJO-related moisture convergence(divergence)anomalies in the SC tend to be weakened(enhanced),contributing to the decrease in PHR amount over the SC in the recent decade.
基金Supported by the National Basic Research Program of China (973 Program)(No.2011CB403500)the Knowledge Innovation Program of Chinese Academy of Sciences (Nos. KZCX2-YW-Q11-02, XDA05090404)+1 种基金the National Basic Research Program of China (973 Program) (No. 2010CB950302)the Qianren and Changjiang Scholar Projects, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)and SOEST-8711 & IPRC-901
文摘Tropical cyclone （TC） genesis in the South China Sea （SCS） during 1979-2008 underwent a decadal variation around 1993. A total of 55 TCs formed in the SCS from May to September during 1994- 2008, about twice that during 1979-1993 （27）. During the TC peak season （July-September, JAS）, there were 43 TCs fi＇om 1994-2008, but only 17 during 1979-1993. For July in particular, 13 TCs formed from 1994-2008, but there were none during 1979-1993. The change in TC number is associated with changes of key environmental conditions in atmosphere and ocean. Compared to 1979-1993, the subtropical high was significantly weaker and was displaced more eastward during 1994-2008. In the former period, a stronger subtropical high induced downward flow, inhibiting TC formation. In the latter period, vertical wind shear and outgoing longwave radiation all weakened. Mid-level （850-500 hPa） humidity, and relative vorticity were higher. Sea surface temperature and upper layer heat content were also higher in the area. All these factors favor TC genesis during the latter period. The decadal change of TC genesis led to more landfalling TCs in Southern China during the period 1994-2008, which contributed to an abrupt increase in regional rainfall.
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.41905055 and 41721004)the Natural Science Foundation of Jiangsu Province(Grant No.BK20190500)the Fundamental Research Funds for the Central Universities(Grant No.B200202145).
文摘It is well known that on the interannual timescale,the westward extension of the western North Pacific subtropical high(WNPSH)results in enhanced rainfall over the Yangtze River basin(YRB)in summer,and vice versa.This study identifies that this correspondence experiences a decadal change in the late 1970s.That is,the WNPSH significantly affects YRB precipitation(YRBP)after the late 1970s(P2)but not before the late 1970s(P1).It is found that enhanced interannual variability of the WNPSH favors its effect on YRB rainfall in P2.On the other hand,after removing the strong WNPSH cases in P2 and making the WNPSH variability equivalent to that in P1,the WNPSH can still significantly affect YRB rainfall,suggesting that the WNPSH variability is not the only factor that affects the WNPSH-YRBP relationship.Further results indicate that the change in basic state of thermal conditions in the tropical WNP provides a favorable background for the enhanced WNPSH-YRBP relationship.In P2,the lower-tropospheric atmosphere in the tropical WNP gets warmer and wetter,and thus the meridional gradient of climatological equivalent potential temperature over the YRB is enhanced.As a result,the WNPSH-related circulation anomalies can more effectively induce YRB rainfall anomalies through affecting the meridional gradient of equivalent potential temperature over the YRB.
基金supported jointly by the strategic technological program of the Chinese Academy of Sciences(Grant No.XDA05090405)the National Basic Research Program of China(Grant Nos.2009CB421406 and 2010CB950304)+1 种基金the Special Fund for the Public Welfare Industry(MeteorologyGrant Nos.GYHY201006022 and GYHY200906018)
文摘The winter temperature changes in East China during the past 100 years are investigated by using the Twentieth Century Version 2(20th-v2)Reanalysis.Four typical warm(P1,1911–30;P4,1991–2010)and cold(P2,1938–57;P3,1961–80)periods are identified for the East China winter temperature index.Comparison of160-station observational data,NCAR sea level pressure(SLP)data,and NCEP/NCAR Reanalysis shows that the20th-v2 Reanalysis can successfully depict the major features of the warming from P3 to P4,which is part of the global warming phenomenon.The cooling from P1 to P2is a regional phenomenon under global warming.However,both changes are consistent with the phase change of the Arctic Oscillation(AO),while the second change is also accompanied by the phase change of Antarctic Oscillation(AAO)from negative to positive.Original sources of the interdecadal shifts of the AO and winter temperature in East China require further research.
基金National Key Program for Developing Basic Science(2016YFA0600303)National Natural Science Foundation of China(41675064,41621005,41330420,41275068)+2 种基金Jiangsu Province Science Foundation(SBK2015020577)Key Laboratory Project Foundation(KLME1501)Jiangsu Collaborative Innovation Center for Climate Change
文摘Using the CAM3.0 model, we investigated the respective effects of aerosol concentration increasing and decadal variation of global sea surface temperature(SST) around year 1976/77 on the East Asian precipitation in boreal summer. By doubling the concentration of the sulfate aerosol and black carbon aerosol separately and synchronously in East Asia(100-150 °E, 20-50 °N), the climate effects of these aerosols are specifically investigated. The results show that both the decadal SST changing and aerosol concentration increasing could lead to rainfall decreasing in the center of East Asia, but increasing in the regions along southeast coast areas of China. However, the different patterns of rainfall over ocean and lower wind field over Asian continent between aerosol experiments and SST experiments in CAM3.0 indicate the presence of different mechanisms. In the increased aerosol concentration experiments, scattering effect is the main climate effect for both sulfate and black carbon aerosols in the Eastern Asian summer. Especially in the increased sulfate aerosol concentration experiment, the climate scattering effect of aerosol leads to the most significant temperature decreasing, sinking convection anomalies and decreased rainfall in the troposphere over the central part of East Asia. However, in an increased black carbon aerosol concentration experiment, weakened sinking convection anomalies exist at the southerly position. This weakened sinking and its compensating rising convection anomalies in the south lead to the heavy rainfall over southeast coast areas of China. When concentrations of both sulfate and black carbon aerosols increase synchronously, the anomalous rainfall distribution is somewhat like that in the increased black carbon concentration aerosol experiment but with less intensity.
基金The National Key Research and Development Program of China under contract No.2019YFA0606702the SOA Global Change and Air-Sea Interaction Project under contract No.GASI-IPOVAI-01-04the National Natural Science Foundation of China under contract Nos 41776003,91858202 and 41630963。
文摘In the past nearly two decades,the Argo Program has created an unprecedented global observing array with continuous in situ salinity observations,providing opportunities to extend our knowledge on the variability and effects of ocean salinity.In this study,we utilize the Argo data during 2004–2017,together with the satellite observations and a newly released version of ECCO ocean reanalysis,to explore the decadal salinity variability in the Southeast Indian Ocean(SEIO)and its impacts on the regional sea level changes.Both the observations and ECCO reanalysis show that during the Argo era,sea level in the SEIO and the tropical western Pacific experienced a rapid rise in 2005–2013 and a subsequent decline in 2013–2017.Such a decadal phase reversal in sea level could be explained,to a large extent,by the steric sea level variability in the upper 300 m.Argo data further show that,in the SEIO,both the temperature and salinity changes have significant positive contributions to the decadal sea level variations.This is different from much of the Indo-Pacific region,where the halosteric component often has minor or negative contributions to the regional sea level pattern on decadal timescale.The salinity budget analyses based on the ECCO reanalysis indicate that the decadal salinity change in the upper 300 m of SEIO is mainly caused by the horizontal ocean advection.More detailed decomposition reveals that in the SEIO,there exists a strong meridional salinity front between the tropical low-salinity and subtropical high salinity waters.The meridional component of decadal circulation changes will induce strong cross-front salinity exchange and thus the significant regional salinity variations.
基金supported by the National Basic Research Program of China(Grant No.2012CB955401)the National Natural Science Foundation of China(Grant No.40905041)
文摘In this study,physical mechanism of the impacts of the tropical Atlantic sea surface temperature(SST)on decadal change of the summer North Atlantic Oscillation(SNAO)was explored using an atmospheric general circulation model(AGCM)developed at the International Centre for Theoretical Physics(ICTP).The simulation results indicate that the decadal warming of the SST over the tropical Atlantic after the late 1970s could have significantly enhanced the convection over the region.This enhanced convection would have strengthened the local meridional circulation over the Eastern Atlantic-North Africa-Western Europe region,exciting a meridional teleconnection.This teleconnection might have brought the signal of the tropical Atlantic SST to the Extratropics,consequently activating the variability of the eastern part of the SNAO southern center,which led to an eastward shift of the SNAO southern center around the late 1970s.Such physical processes are highly consistent with the previous observations.
基金provided by the National Centers for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR)supported by National Natural Science Foundation of China (Grant No41130963)+1 种基金Research Fund for the Doctoral Program of Higher Education (Grant No.20100091110003)the Fundamental Research Funds for the Central Universities(Grant Nos.1184020702 and 1107020730)
文摘Previous studies have suggested a poleward shift of the zonally averaged jet stream due to rapid warming over continents.However,the regional characteristics of the change in the jet stream are not yet understood.Here,we present evidence suggesting that the East Asian westerly jet did not shift poleward in past decades（1980-2004 relative to 1958-1979）,both in winter and summer.Rather,the jet axis has moved southward in summer,but its meridional position is steady in winter.The main change of the jet stream in winter is the enhancement of its intensity.These changes in both summer and winter are consistent with the corresponding changes in the large meridional tropospheric temperature-gradient zone.Based on these results,we suggest that the changes of the jet stream over East Asia are unique and are different from the zonal mean jet stream over the Northern Hemisphere and over the North Atlantic region.
基金Supported by the National Key Research and Development Program of China(2020YFA0608000)National Natural Science Foundation of China(42088101 and 42030605)。
文摘Owing to limited observations,it remains unknown whether the impact of El Niño-Southern Oscillation(ENSO)on the Indian Ocean-Northwest Pacific(IO-NWP)climate showed decadal changes in the early 20th century.Using multi-source reanalysis and hindcast datasets from the ECMWF and NOAA extending back to 1901,this study investigates interdecadal variations of the impact of ENSO on the IO-NWP climate from 1901 to 2009.It is found that the influence of ENSO on the IO-NWP climate shows“strong-weak-strong”interdecadal change during 1901-2009.This is characterized by much weaker Indian Ocean sea surface temperature(SST)warming and a weaker NWP subtropical anticyclone(NWPSA)in the following summer of El Ni?o during 1946-1967,compared with those in the other two periods(1901-1945 and 1968-2009).Analyses of the datasets indicate that the interdecadal variation is mainly associated with the change in ENSO amplitude.In contrast to the period of 1946-1967,a greater SST variance occurred in the central-eastern equatorial Pacific during 1901-1945 and 1968-2009.A stronger El Ni?o tends to generate more significant anticyclonic anomalies over the southeast Indian Ocean through teleconnection.The northwesterly anomalies to the south of the anticyclone weaken the southeast trade winds and warm the south Indian Ocean SST via wind-evaporation-SST feedback,and the positive south Indian Ocean SST anomalies trigger westward-propagating oceanic Rossby waves to induce stronger warming of the southwest Indian Ocean,leading to a significant asymmetric wind pattern across the equator in spring.The profound northeastward winds on the north side weaken the southwest monsoon,leading to a“second warming”over the north Indian Ocean in summer,which anchors the eastward-propagating warm Kelvin waves and results in a stronger NWPSA by inducing surface divergence and suppressing deep convection.
基金supported by the National Key Basic Research Program of China (GrantNo. 2009CB421404)the National Natural Science Foundation of China (Grant No. 40730951)+2 种基金the Fundamental Research Funds for the Central Universities (Grant No.11lgjc10)the National Natural Science Foundation of major international collaborative research project (GrantNo. 40810059005)the support of a Direct Grant of the Chinese University of HongKong (Grant No. 2021090)
文摘The increase in southern China summer rainfall around 1993 was accompanied by an increase in tropical cyclones that formed in the South China Sea. This study documents the connection of these two features. Our analysis shows that the contribution of tropical cyclones that formed in the South China Sea to southern China summer rainfall experienced a significant increase around 1993, in particular, along the coast and in the heavy rain category. The number of tropical cyclones that formed in the western North Pacific and entered the South China Sea decreased, and their contribution to summer rainfall was reduced in eastern part of southern China （but statistically insignificant）. The increase in tropical cyclone-induced rainfall contributed up to -30＆ of the total rainfall increase along the coastal regions. The increase of tropical cyclones in the South China Sea appears to be related to an increase in local sea surface temperature.
基金supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST Grant No 2010-0028715)
文摘This study examined the variability in frequency of tropical night occurrence （i.e., minimum air tem- perature 25℃） in Beijing, using a homogenized daily temperature dataset during the period 1960–2008. Our results show that tropical nights occur most frequently in late July and early August, which is consis- tent with relatively high air humidity associated with the rainy season in Beijing. In addition, year-to-year variation of tropical night occurrence indicates that the tropical nights have appeared much more frequently since 1994, which can be illustrated by the yearly days of tropical nights averaged over two periods： 9.2 days of tropical nights per year during 1994–2008 versus 3.15 days during 1960–1993. These features of tropical night variations suggest a distinction between tropical nights and extreme heat in Beijing. We further investigated the large-scale circulations associated with the year-to-year variation of tropical night occurrence in July and August, when tropical nights appear most frequently and occupy 95% of the annual sum. After comparing the results in the two reanalysis datasets （NCEP/NCAR and ERA-40） and considering the possible effects of decadal change in the frequency of tropical nights that occurred around 1993/94, we conclude that on the interannual time scale, the cyclonic anomaly with a barotropic structure centered over Beijing is responsible for less frequent tropical nights, and the anticyclonic anomaly is responsible for more frequent occurrence of tropical nights over Beijing.
基金jointly supported by the National Natural Science Foundation of China(Grant Nos.41421004 and 41210007)the Special Fund for Public Welfare Industry(Meteorology)(Grant No.GYHY201306026)
文摘This paper documents a decadal strengthened co-variability of the Antarctic Oscillation （AAO） and ENSO in austral spring after the mid-1990s. During the period 1979-93, the ENSO （AAO） spatial signatures are restricted to the tropicsmidlatitudes （Antarctic-midlatitudes） of the Southern Hemisphere （SH）, with a weak connection between the two oscillations. Comparatively, after the mid-1990s, the E1 Nifio-related atmospheric anomalies project on a negative AAO pattern with a barotropic structure in the mid-high latitudes of the SH. The expansion of E1 Nifio-related air temperature anomalies have a heightened impact on the meridional thermal structure of the SH, contributing to a weakened circumpolar westerly and strengthened subtropical jet. Meanwhile, the ENSO-related southern three-cell circulations expand poleward and then strongly couple the Antarctic and the tropics. Numerical simulation results suggest that the intensified connection between ENSO and SST in the South Pacific since the mid-1990s is responsible for the strengthened AAO-ENSO relationship.
基金funded by the National Key Research and De-velopment Program of China[Grant number 2017YFA0604304]the National Natural Science Foundation of China[Grant number 41661144032].
文摘Using a homogenized daily maximum temperature(T_(max))dataset across China,this study characterized the spatiotemporal variation of the onset date of extreme hot days in a year(i.e.,FirstEHD)during 1960-2018.Inhomogeneous trends of FirstEHD over China during 1960-2018 can be found,with the advanced trend of FirstEHD over most parts in China,while a number of stations in North-Central China(NC)show the delayed trend of FirstEHD.Moreover,there exist interdecadal changes of FirstEHD trend,with a remarkable difference in the trend magnitude before and after the 1990s over South China(SC),and the sign of trend can even reverse from negative to positive after the 1990s in Xinjiang(XJ)and Yangtze River Basin(YR),and from positive to negative in NC.The overall trends of FirstEHD over NC,YR,and XJ during 1960-2018 are dominated by the trends before the 1990s,while they are dominated by the sharp advance after the 1990s over SC.It is further found that the trend of FirstEHD can generally be explained by the long-term trend in T_(max) over most parts of China,but the contribution from T_(max) variabilities is also non-negligible and can even account for more than 75% of the overall trend over NC.The possible factors responsible for the decadal changes in FirstEHD trends are also discussed.
文摘This study utilizes ML classifiers to estimate canopy density based on three decades of data (1990-2021). The Support Vector Machine (SVM) classifier outperformed other classifiers, such as Random Tree and Maximum Likelihood. Satellite data from Landsat and Sentinel 2 was classified using a developed python model, providing an economical and time-saving approach. The accuracy of the classification was evaluated through a confusion matrix and area computation. The findings indicate a negative trend in the overall decadal change, with significant tree loss attributed to jhum cultivation, mining, and quarry activities. However, positive changes were observed in recent years due to the ban on illegal mining. The study highlights the dynamic nature of tree cover and emphasizes the need for biennial assessments using at least five time-series data. Micro-level analysis in Shallang, West Khasi hills, revealed a concerning trend of shortening jhum cycles. Automation in canopy change analysis is crucial for effective forest monitoring, providing timely information for law enforcement proposals and involving forest managers, stakeholders, and watchdog organizations.
