Decadal Changes in Interannual Dependence of the Bay of Bengal Summer Monsoon Onset on ENSO Modulated by the Pacific Decadal Oscillation

Interannual variations of the Bay of Bengal summer monsoon (BOBSM) onset in association with El Ni-o-Southern Oscillation (ENSO) are reexamined using NCEP1, JRA-55 and ERA20C atmospheric and Hadley sea surface temperature (SST) reanalysis datasets over the period 1900-2017. Decadal changes exist in the dependence of the BOBSM onset on ENSO, varying with the Pacific Decadal Oscillation (PDO). A higher correlation between the BOBSM onset and ENSO arises during the warm PDO epochs, with distinct late (early) onsets following El Ni-o (La Ni-a) events. In contrast, less significant correlations occur during the cold PDO epochs. The mechanism for the PDO modulating the ENSO-BOBSM onset relationship is through the variations in SST anomaly (SSTA) patterns. During the warm PDO epochs, the superimpositions of the PDO-related and ENSO-related SSTAs lead to the SSTA distribution of an El Ni-o (La Ni-a) event exhibiting significant positive (negative) SSTAs over the tropical central-eastern Pacific and Indian Ocean along with negative (positive) SSTAs, especially over the tropical western Pacific (TWP), forming a strong zonal interoceanic SSTA gradient between the TWP and tropical Indian Ocean. Significant anomalous lower tropospheric easterlies (westerlies) together with upper-tropospheric westerlies (easterlies) are thus induced over the BOB, favoring an abnormally late (early) BOBSM onset. During the cold PDO epochs, however, the superimpositions of PDO-related SSTAs with El Ni-o-related (La Ni-a-related) SSTAs lead to insignificant SSTAs over the TWP and a weak zonal SSTA gradient, without distinct circulation anomalies over the BOB favoring early or late BOBSM onsets.

Harmonious Inter-decadal Changes of July–August Upper Tropospheric Temperature Across the North Atlantic, Eurasian Continent,and North Pacific

The authors have developed an integral view of the inter-decadal variability of July-August （JA） tropospheric temperature across the entire subtropical Northern Hemisphere. Using reanalysis data and complementary balloon-borne measurements, the authors identify one major mode of variability for the period 1958 2001 which exhibits a significant cooling center over East Asia and warming centers over the North Atlantic and North Pacific. The cooling （warming） signals barotropically penetrate through the troposphere, with the strongest anomalies at 200-300 hPa. The amplitude of the cooling over East Asia is stronger than that of the warming over the North Atlantic （North Pacific） by a factor of 2 （3）. This dominant mode exhibits a declining tendency for the entire period examined, particularly before 1980. After the mid-1980s, the tendency has leveled off. Variations of the harmonious change of JA upper tropospheric temperature represented by the principal component of Empirical Orthogonal Function analysis exhibit significant negative （positive） correlations with SST anomalies in the eastern tropical Pacific and the western tropical Indian Ocean （mid-latitude North Pacific）. Possible mechanisms are discussed.

A decomposition study of moisture transport divergence for inter-decadal change in East Asia summer rainfall during 1958-2001

In this paper, we report on the results of an investigation into inter-decadal changes in moisture transport and divergence in East Asia for the two periods 1980-2001 and 1958 1979. The aim is to explore the mechanism of summer rainfall change in the region after abrupt changes. The relevant changes are calculated using ERA-40 daily reanalysis datasets. The results show that both stationary and transient eddy moisture transports to the Chinese mainland have declined since the abrupt change in atmospheric general circulation in the late 1970s, leading to more rainfall in South China and less in the North. The anomalous rainfall pattern coincides well with anomalous large-scale moisture divergence in the troposphere, of which stationary-wave or monsoon transport is dominant, in comparison with the contribution of the transient eddies. F^rthermore, their divergences are found to be in opposite phases. In addition, meridional divergence is more important than its zonal counterpart, with an opposite phase in East Asia. Abnormal zonal moisture convergences appear in northwestern and northeastern parts of China, and are related to the excess rainfalls in these regions. An increase in transient eddy activity is one of the major mechanisms for excess rainfall in northern Xinjiang. Consequently, the anomalous rainfall pattern in East Asia results from a decline of the East Asian monsoon after the abrupt change, while the rainfall increase in northwestern China involves anomalies of both stationary waves and transient eddies on boreal westerly over the mid- and high latitudes.

An interdecadal change of tropical cyclone activity in the South China Sea in the early 1990s

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 43TCs from 1994-2008, but only 17 during 1979-1993. For July in particular, 13TCs 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 (850-500hPa) 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.

Recent strong inter-decadal change of Meiyu in 121-year variations

The strongest change in Meiyu periods in the mid-lower Yangtze Basin （MLY） since 1885 occurred in the late 1970s： a stage of weak Meiyu from 1958 to 1978 abruptly transformed into a stage of plentiful Meiyu from 1979 to 1999. The average Meiyu amount of the latter 21 years increased by 66% compared with that of the former 21 years, accompanied by a significant increase in the occurrence of summer floods in the MLY. This change was closely related with the frequent phenomenon of postponed Meiyu ending dates （MED） and later onset dates of high summer （ODHS） in the MLY. To a considerable degree, this reflects an abrupt change of the summer climate in East China. Further analysis showed that the preceding factors contributing to inter-annual changes in Meiyu in the two 21-year stages delimited above were also very different from each other. The causes of change were associated with the following： China’s industrialization has greatly accelerated since the 1970s, accompanied by an increase in atmospheric pollution and a reduction of the solar radiation reaching the ground. The sand area of North China has also expanded due to overgrazing. The enhanced greenhouse effect is manifested in warm winters （especially in February）. Meanwhile, the January precipitation of the MLY has for the most part increased, and El Ni？o events have occurred more frequently since the late 1970s. A correlative scatter diagram consisting of these five factors mentioned above clearly shows that the two stages with opposite Meiyu characteristics are grouped in two contrasting locations with very different environmental （land-atmosphere） conditions. It is quite possible that we are now entering a new stage of lesser Meiyu, beginning in 2000.

On the Contrasting Decadal Changes of Diurnal Surface Temperature Range between the Tibetan Plateau and Southeastern China during the 1980s–2000s

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.

Decadal (2003–2013) changes in liana diversity,abundance and aboveground biomass in four inland tropical dry evergreen forest sites of peninsular India

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.

DECADAL CHANGES IN WESTERN NORTH PACIFIC TROPICAL CYCLONES ASSOCIATED WITH MADDEN-JULIAN OSCILLATION

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.

Decadal Changes in Climatological Intraseasonal Fluctuation of Subseasonal Evolution of Summer Precipitation over the Korean Peninsula in the mid-1990s

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.

Decadal Change in the Influence of the Western North Pacific Subtropical High on Summer Rainfall over the Yangtze River Basin in the Late 1970s

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.

Impacts of the upper-ocean salinity variations on the decadal sea level change in the Southeast Indian Ocean during the Argo era

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.

Physical Mechanism of the Impacts of the Tropical Atlantic Sea Surface Temperature on the Decadal Change of the Summer North Atlantic Oscillation

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.

A Review of Decadal/Interdecadal Climate Variation Studies in China

Decadal/interdecadal climate variability is an important element in the CLIVAR (Climate Variability and Predictability) and has received much attention in the world. Many studies in relation to interdecadal variation have also been completed by Chinese scientists in recent years. In this paper, an introduction in outline for interdecadal climate variation research in China is presented. The content includes the features of interdecadal climate variability in China, global warming and interdecadal temperature variability, the NAO (the North Atlantic Oscillation)/NPO (the North Pacific Oscillation) and interdecadal climate variation in China, the interdecadal variation of the East Asian monsoon, the interdecadal mode of SSTA (Sea Surface Temperature Anomaly) in the North Pacific and its climate impact, and abrupt change feature of the climate.

Decadal/Interdecadal Variations of the Ocean Temperature and its Impacts on Climate

Decadal/interdecadal climate variability is an important research focus of the CLIVAR Program and has been paid more attention. Over recent years, a lot of studies in relation to interdecadal climate variations have been also completed by Chinese scientists. This paper presents an overview of some advances in the study of decadal/interdecadal variations of the ocean temperature and its climate impacts, which includes interdccadal climate variability in China, the interdecadal modes of sea surface temperature （SST） anomalies in the North Pacific, and in particular, the impacts of interdecadal SST variations on the Asian monsoon rainfall. As summarized in this paper, some results have been achieved by using climate diagnostic studies of historical climatic datasets. Two fundamental interdecadal SST variability modes （7- 10-years mode and 25 35-years mode） have been identified over the North Pacific associated with different anomalous patterns of atmospheric circulation. The southern Indian Ocean dipole （SIOD） shows a major feature of interdecadal variation, with a positive （negative） phase favoring a weakened （enhanced） Asian summer monsoon in the following summer. It is also found that the China monsoon rainfall exhibits interdecadal variations with more wet （dry） monsoon years in the Yangtze River （South China and North China） before 1976, but vice versa after 1976. The weakened relationship between the Indian summer rainfall and ENSO is a feature of interdecadal variations, suggesting an important role of the interdecadal variation of the SIOD in the climate over the south Asia and southeast Asia. In addition, evidence indicates that the climate shift in the 1960s may be related to the anomalies of the North Atlantic Oscillation （NAO） and North Pacific Oscillation （NPO）. Overall, the present research has improved our understanding of the decadal/interdecadal variations of SST and their impacts on the Asian monsoon rainfall. However, the research also highlights a number of problems for future research, in particular the mechanisms responsible for the monsoon long4erm predictability, which is a great challenge in climate research.

Inter-decadal variations,causes and future projection of the Asian summer monsoon

The present paper presents a concise summary of our recent studies on the Asian summer monsoon,with highting decadal and inter- decadal scales. The studies on the long- term variations of the Asian summer monsoon and its impacts on the change in the summer precipitation in China are reviewed. Moreover,recent changes in the Asian summer monsoon and summer precipitation in East Asia(including Meiyu precipitation)are discussed. Finally,the future changes of the Asian summer monsoon are also pointed out in this paper.

Southern Ocean SST Variability and Its Relationship with ENSO on Inter-Decadal Time Scales

Empirical orthogonal function (EOF) analysis reveals a co-variability of Sea surface temperatures (SSTs) in the Southern Hemisphere (0°-60°S). In the South Indian and Atlantic Oceans, there is a subtropical dipole pattern slanted in the southwest- north-east direction. In the South Pacific Ocean, a meridional tripole structure emerges, whose middle pole co-varies with the dipoles in the South Indian and Atlantic Oceans and is used in this study to track subtropical Pacific variability. The South Indian and Atlantic Ocean dipoles and the subtropical Pacific variability are phase-locked in austral summer. On the inter-decadal time scales, the dipoles in the South Indian and Atlantic Oceans weaken in amplitude after 1979/1980. No such weakening is found in the subtropical South Pacific Ocean. Interestingly, despite the reduced amplitude, the correlation of the Indian Ocean and Atlantic dipoles with El Nio and Southern Oscillation (ENSO) are enhanced after 1979/1980. The same increase in correlation is found for subtropical South Pacific variability after 1979/1980. These inter-decadal modulations imply that the Southern Hemisphere participates in part of the climate shift in the late 1970s. The correlation between Southern Hemisphere SST and ENSO reduces after 2000.

Analysis of the Changes in China's Grain Production Costs over the Past Decade

China's grain production cost has risen constantly since 2003. This paper starts from the grain production cost changes of unit area and unit quality,to analyze the reasons for China's grain production cost change in the last decade. It points out that the direct cause of changes in grain production cost is the steady growth of material and services costs,the fastest growth of labor costs and fast growth of land costs. The indirect cause of grain production high cost is the low efficiency in numerous and dispersive farmer production management,obviously insufficient agricultural infrastructure construction,waste phenomenon grain in production process and low contribution rate of agricultural technology.On this basis,it is recommended to reduce China's grain production cost and improve grain production conditions.

Modeled Influence of East Asian Black Carbon on Inter-Decadal Shifts in East China Summer Rainfall

Two inter-decadal shifts in East China summer rainfall during the last three decades of the 20th century have been identified.One shift occurred in the late 1970s and featured more rainfall in the Yangtze River valley and prolonged drought in North China.The other shift occurred in the early 1990s and featured increased rainfall in South China.The role of black carbon(BC) aerosol in the first shift event is controversial,and it has not been documented for the second event.In this study,the authors used Geophysical Fluid Dynamics Laboratory's(GFDL's) atmospheric general circulation model known as Atmosphere and Land Model(AM2.1) ,which has been shown to capture East Asian climate variability well,to investigate these issues by conducting sensitive experiments with or without historical BC in East Asia. The results suggest that the model reproduces the first shift well,including intensified rainfall in the Yangtze River and weakened monsoonal circulation.However,the model captures only a fraction of the observed variations for the second shift event.Thus,the role of BC in modulating the two shift events is different,and its impact is relatively less important for the early 1990s event.

Interdecadal Variations of Winter Temperatures in East China During the Past 100 Years and Related Atmospheric Circulation

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.

Long-term changes in wintertime persistent heavy rainfall over southern China contributed by the Madden-Julian Oscillation

本文利用中国地区高分辨率降水数据及热带季节内振荡(MJO)指数,研究1979-2011年冬季(DJF)的MJO活动变化对中国南方持续性强降水(PHR)的影响。当MJO对流活跃于印度洋地区(位相1-4)时,其引发的水汽通量异常造成华南(SC)PHR增加;而当MJO对流活跃于西太平洋(位相5-8)时,长江流域(YR)PHR显示出正异常。印度洋和西太平洋MJO活动显现年代际变化,与1979-1994年(E1)相较,印度洋(西太平洋)MJO对流在近十几年(1995-201 1,E2)明显减少(增加),为SC(YR)的PHR提供了不利(有利)的水汽条件,有利SC(YR)PHR下降(上升)趋势的发生。由该结果推测,MJO活动的年代际变化可作为中国南方PHR长期预报的潜在预报因子。