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Afro-Asian summer monsoon precipitation (AfroASMP) is the life blood of billions of people living in many developing countries covering West Africa and Asia. Its complex variabilities are always accompanied by natural disasters like floods, landslides and droughts. Reliable AfroASMP prediction several months in advance is valuable for not only decision-makers but also regional socioeconomic sustainability. To address the current predicament of the AfroASMP seasonal prediction, this study provides an effective machine-learning model (Y-model). Y-model uses the monsoon related big climate data for searching the potential predictors, encompassing atmospheric internal factors and external forcings. Only the predictors associated with significant anomalies in summer horizonal winds at 850 hPa over the monsoon domain are retained. These selected predictors are then reorganized into a large ensemble based upon different thresholds of four fundamental principles. These principles include the focused sample sizes, the relationships between predictors and predictand, the independence among predictors, and the extremities of predictors in the forecast year. Real-time predictions can be generated based on the ensemble mean of skillful members during an independent hindcast period. Y-model skillfully predicts four monsoon precipitation indices of AfroASMP during 2011–2022 at lead 4–12 months, correlation skills range from 0.58 to 0.90 and root mean square error skills are reduced by 11–53% compared to CFS v2 model at lead 1 month. This study offers an effective method for preprocessing predictors in seasonal climate prediction. [ABSTRACT FROM AUTHOR]

Reconstruction and observational studies imply a potential linkage of moisture and precipitation change in arid central Asia and monsoonal East Asia, in which the evolution of moisture and precipitation in central Asia is out of phase with that in northern China but in phase with that in southern China. In order to ascertain whether there is a robust linkage between the changes in climate in Asian arid regions and monsoon regions and to elucidate the underlying dynamic mechanisms, we analyzed the Last Millennium Reanalysis dataset and outputs from the Community Earth System Model Last Millennium Ensemble (CESM-LME). The results indicate a significant decadal linkage between precipitation changes in central Asia's arid region and the Asian monsoon region during the last millennium, which is primarily driven by the Interdecadal Pacific Oscillation (IPO). In spring, the positive IPO could enhance westerlies over the Mediterranean Sea and to its east, which could transport more water vapor and cause increased precipitation over central Asia. In summer, the positive IPO is accompanied by a weakened Asian monsoon and southward Asian subtropical westerly jet, which can lead to increased (decreased) summer precipitation over southern China (over northern China and South Asia). The IPO plays a dominant role in connecting the decadal variations in precipitation between arid central Asia and monsoonal Asia by modulating the precipitation of their respective major rainy seasons. Model results suggest that this decadal linkage stems entirely from the internal variability present in the CESM-LME control and all single-forcing simulations. Changes in external forcing factors do not alter this inherent linkage caused by the IPO. Moreover, based on analyses of the aridity index and soil moisture content, this relationship of precipitation variation also causes a similar decadal linkage of moisture changes in central Asia and monsoonal Asia. The differences in the multi-centennial-scale moisture and precipitation variations in the Asian arid region and the monsoon region between the Medieval Climate Anomaly and Little Ice Age are also likely caused by IPO-like sea surface temperature anomalies. [ABSTRACT FROM AUTHOR]

To investigate the evolution of precipitation over Asian continent in the Holocene and the associated mechanisms, we used a set of simulations of the transient climate evolution over the past 21,000 years (TraCE‐21ka), multimodel results from the Paleoclimate Modeling Intercomparison Project Phase 4 (PMIP4), and proxy records in Asia. The TraCE‐21ka results showed a tripole pattern in suborbital‐scale precipitation trends over the Asian continent during the Holocene, with a trend of increase over southern parts of the monsoon regions and arid Central Asia (ACA), and a trend of decline over northern parts of the monsoon regions and their areas of transition with ACA. This tripole pattern was corroborated by proxy records from multiple regions and multimodel results from the PMIP4. Further analysis based on single‐forcing simulations of TraCE‐21ka indicated that influences from different external forcings were different on the Asian precipitation in the main rainy seasons in the Holocene and that their combined effects shaped the tripole pattern. In summer, orbital forcing, by reducing solar radiation in mid‐to‐high latitudes and weakening the land‐sea thermal contrast, has been the dominant factor in the long‐term evolution of precipitation in the monsoon region and West Asia. In winter and spring, changes in meltwater flux played dominant roles in intensifying local water cycle and horizontal moisture advection, which drove the trend of increase in precipitation in ACA. Additionally, changes in greenhouse gas concentration and continental ice sheet forcings both also contribute to the increase in precipitation in ACA. Plain Language Summary: The change in precipitation in Asia plays an important role in shaping the pattern of the natural environment and affecting the development of human civilization. However, our understanding of the precipitation evolution in some regions of Asia in the Holocene remains of controversy, and the factors that govern the long‐term changes in Asian precipitation are not fully understood. Simulations and proxy data used in this study revealed a tripole pattern in the trend of precipitation in Asia during the Holocene. The pattern features a trend of increase in precipitation over southern parts of the monsoon regions and arid Central Asia (ACA), and a trend of decline in precipitation over northern parts of the monsoon regions and their areas of transition with ACA. Further analyses showed that solar insolation changes induced by the Earth's orbital movements have dominated the long‐term evolution of precipitation in the Asian monsoon region, and that the increasing greenhouse gas concentration, the changes in topography and extent of the retreating continental ice sheet over the Northwestern Europe and North America, and especially the decreasing amount of freshwater into oceans due to the retreating ice sheets have contributed to the increase in precipitation in ACA. Key Points: Both simulations and proxy records reveal a tripole pattern in suborbital‐scale precipitation trends over Asia during the HoloceneTripole pattern in precipitation trend reflects the combined effects of various external forcings during the HoloceneOrbital forcing dominates precipitation trend in the monsoon region, while meltwater flux dominates precipitation trend in the arid region [ABSTRACT FROM AUTHOR]

The relationships between the El Niño‐Southern Oscillation (ENSO), Asian‐Pacific region summer precipitation and its corresponding atmospheric circulations exhibit interdecadal changes. From the early 1990s to the early 2000s, these relationships were significant. Therefore, we divided this time frame into three periods for analysis: 1979–1991 (P1), 1992–2005 (P2), and 2006–2019 (P3). The possible mechanism underlying these relationships is as follows: the southern Indian Ocean sea surface temperature (SST) over the east of Madagascar exhibits interdecadal variation. During P2, the SST was relatively cold, which induced an anomalously high mascarene high (MH). The strengthened MH enhanced the Somali jet and Indian monsoon westerlies, which intensified the strong center of the Indian Ocean Walker circulation. Therefore, owing to the strength and location changes of the Indian Walker circulation and the Walker circulation, the connection between the ENSO and the western Pacific vertical motions strengthened, resulting in the close relationship of the ENSO with the atmospheric circulation over the Asian‐Pacific region. Hence, ENSO can influence a north‐south tripole pattern of precipitation over the Asian‐Pacific monsoon region through local vertical activities and meridional Hadley circulation over western Pacific. Numerical experiments using an atmospheric general circulation model, with prescribed three times southern Indian Ocean SST anomalies of 1995–2005 relative to 1979–2019, also lend support to the southern Indian Ocean SST's contribution to modulating the relationship between ENSO and summer precipitation over the Asian‐Pacific monsoon region. Key Points: The linkages of the El Niño‐Southern Oscillation (ENSO), Asian‐Pacific summer precipitation and its corresponding atmospheric circulations exhibit interdecadal changesThe southern Indian Ocean sea surface temperature (SST) is the key role to modulate the relationship between ENSO and Asian‐Pacific climateThe extratropical SST of the southern Indian Ocean can also modulate the relationship between ENSO and Asian‐Pacific climate [ABSTRACT FROM AUTHOR]

Understanding the variations of the westerly jets (WJs) during the Last Interglacial (~ 127 ka; LIG) aids in deciphering the reconstructed hydrological conditions and dust activities at that time. Here, we investigated variations of the WJs over the Northern Hemisphere during the LIG based on the Paleoclimate Modelling Intercomparison Project 4. We demonstrated that WJs exhibited divergent responses across regions and seasons during the LIG. In summer, WJs over the African–Asian–Pacific regions moved poleward by ~ 5° and weakened by 19% during the LIG, whereas there was only a slight change (+ 1°/− 2%) over the North American–Atlantic regions. In contrast, the winter WJs over the North American–Atlantic regions shifted poleward by ~ 2° and weakened by 7%, but with no obvious responses (− 0.2°/− 0.8%) over the African–Asian–Pacific regions. The WJs variations were linked to changes in meridional temperature gradients and maximum eddy growth rates. Furthermore, the poleward shift and weakening of the WJs over the East Asian–Pacific regions in summer resulted from the anomalous anticyclone triggered by the deep warm center over East Siberia and the anomalous cyclone linked with diabatic cooling over the western tropical Pacific. For the poleward-displaced and depressed WJs over the North American–Atlantic regions in winter, the propagation of Rossby waves from the eastern Pacific played a more important role, compared with the effect of tropical heating. Our work highlights divergent responses of the WJs to orbital forcing during the LIG, which helps to decipher the reconstructed westerlies-related processes and hence to obtain a better understanding of the climate change during this orbitally induced warm period. [ABSTRACT FROM AUTHOR]

The East Asian summer monsoon (EASM) is one of the major synoptic systems that affect the summer climate in China. Decadal prediction of the EASM is of great significance, yet few attempts have been made by far. This study represents a preliminary attempt that uses the decadal increment method to predict the decadal variability of the EASM. The 3-year increment of the decadal EASM (DI_EASMI) is firstly predicted by the leading 4 years tropical ocean and the leading 5 years sea ice over Barents Sea. These two leading predictors with quasi-10-year oscillation can explain most variance of the DI_EASMI, especially in recent decades. The predicted DI_EASMI is combined with the observed EASM at 3 years ago to get the final prediction result. The results of cross-validation and independent hindcast show that the decadal increment method can well predict decadal variability of the EASM during the recent century. The real-time prediction results show that the chance for the occurrence of strong decadal EASM would be rare in 2021. The method developed in the present study provides a new approach for decadal prediction of the EASM. [ABSTRACT FROM AUTHOR]

In the early summer (late June–early July) of 2021, a prolonged and extremely high temperature event was observed in the Northwest United States and Southwest Canada. In this study, a connection was found between the subseasonal variations of the East Asian monsoon system and the anomalous heat dome over western North America (WNA). Around 2–3 pentads prior to the period of maximum heat, a northward‐propagating rain belt and abnormally intensified jet stream in East Asia excited a Rossby wave train, causing the high‐pressure anomaly over WNA that was responsible for the occurrence of this heat wave in 2021. Multivariate singular value decomposition analysis results using historical data and a series of numerical experiments further supported the finding of this East Asia–western North America teleconnection. The results provide precursor signals for better monitoring and predicting heat waves over WNA. Plain Language Summary: A persistent and extremely high temperature event was observed in several states in the Northwest United States and Southwest Canada in summer 2021. This severe heat wave resulted in the hottest early‐summer season in western North America (WNA) since 1979 and huge social and economic losses. Although the heat dome (high‐pressure anomaly) over WNA was the factor responsible for this mega heat wave, the cause of this high‐pressure anomaly was unclear. The present study reveals the importance of an atmospheric wave train emanating from East Asia toward North America in inducing the heat dome over WNA. The generation of the wave train is attributable to a northward‐propagating rain belt and abnormally enhanced jet stream over East Asia from 10 to 15 days before the peak of this heat wave. Thus, monitoring the East Asian summer monsoon and associated wave train activity may help in predicting heat waves in WNA, and therefore in knowing when to put measures in place to mitigate against their effects. Key Points: An extreme heat wave occurred in western North America (WNA) in the early summer of 2021, causing huge socioeconomic losses in several big citiesThe subseasonal activity of East Asian summer monsoon contributed to the occurrence of the heat wave via excitation of a Rossby wave trainThe East Asia‐North America teleconnection can provide precursor signals for forecasting heat waves in WNA [ABSTRACT FROM AUTHOR]

The reversal of surface air temperature anomalies (SATA) in winter brings a great challenge for short-term climate prediction, and the mechanisms are not well understood. This study found that the reversal of SATA between December and January over China could be demonstrated by the second leading mode of multivariate empirical orthogonal function analysis on the December–January SATA. It further reveals that the central Pacific El Niño–Southern Oscillation (CP ENSO) has contributed more influence on such a reversal of SATA since 1997. CP ENSO shows positive but weak correlations with SATA over China in both December and January during the pre-1996 period, whereas it shows significant negative and positive correlations with the SATA in December and January, respectively, during the post-1997 period. The CP ENSO–related circulations suggest that the change of the Siberian high has played an essential role in the reversal of SATA since 1997. The pattern of sea surface temperature anomalies associated with the CP ENSO leads to a westward-replaced Walker circulation that alters the local meridional circulation and, further, has impacted the Siberian high and SATA over China since 1997. Moreover, the seasonal northward march of the convergence zone from December to January causes a northward-replaced west branch of the Walker circulation in January compared with that in December. The west branch of the Walker circulation in December and January directly modulates local Hadley and Ferrel circulations and then causes contrasting Siberian high anomalies by inducing opposite vertical motion anomalies over Siberia. The reversal of SATA between December and January, therefore, has been more frequently observed over China since 1997. The abovementioned mechanisms are validated by the analysis at pentad time scales and confirmed by numerical simulations. [ABSTRACT FROM AUTHOR]

The East Asian winter monsoon (EAWM) is greatly influenced by many factors that can be classified as anthropogenic forcing and natural forcing. Here we explore the contribution of anthropogenic influence to the change in the EAWM over the past decades. Under all forcings observed during 1960–2013 (All-Hist run), the atmospheric general circulation model is able to reproduce the climatology and variability of the EAWM-related surface air temperature and 500 hPa geopotential height and shows a statistically significant decreasing EAWM intensity with a trend coefficient of ∼-0.04 yr -1 , which is close to the observed trend. By contrast, the simulation, which is driven by the same forcing as the All-Hist run but with the anthropogenic contribution to them removed, shows no decreasing trend in the EAWM intensity. By comparing the simulations under two different forcing scenarios, we further reveal that the responses of the EAWM to the anthropogenic forcing include a rise of 0.6 ∘ in surface air temperature over East Asia as well as weakening of the East Asian trough, which may result from the poleward expansion and intensification of the East Asian jet forced by the change in temperature gradient in the troposphere. Additionally, compared with the simulation without anthropogenic forcing, the frequency of strong (weak) EAWM occurrence is reduced (increased) by 45 % (from 0 to 10/7). These results indicate that the weakening of the EAWM during 1960–2013 may be mainly attributed to the anthropogenic influence. [ABSTRACT FROM AUTHOR]

Observations show that the East Asian winter monsoon (EAWM) experienced an interdecadal weakening in the mid-1980s. This is evident for all members of the EAWM system (i.e., East Asian trough, upper-tropospheric jet stream, and lower-tropospheric monsoon circulation). Here, we investigate the relative contributions of natural (volcanic aerosols and solar variability) and anthropogenic [greenhouse gases (GHGs) and anthropogenic aerosols] forcings to this interdecadal weakening using multiple coupled models within phase 5 of the Coupled Model Intercomparison Project (CMIP5). The results indicate that in the midtroposphere, the increased GHG concentrations play an important role in weakening the East Asian trough (EAT) by increasing the sea surface temperatures (SSTs) over the North Pacific. In the upper troposphere, natural external forcings contribute to the observed weakening of the meridional shear of the East Asian jet stream (EAJS) by regulating the meridional temperature gradient (MTG) over the East Asian region. In the lower troposphere, both anthropogenic and natural forcings can weaken the Siberian high during this period. Overall, based on the present analysis of the CMIP5 output, GHGs and natural forcings play key roles in shaping the observed interdecadal weakening of the EAWM during the mid-1980s. Additionally, contributions from internal variability cannot be neglected and require further investigation. [ABSTRACT FROM AUTHOR]

In this study, we investigate the influence of low-frequency solar forcing on the East Asian winter monsoon (EAWM) by analyzing a four-member ensemble of 600-year simulations performed with HadCM3 (Hadley Centre Coupled Model, version 3). We find that the EAWM is strengthened when total solar irradiance (TSI) increases on the multidecadal time scale. The model results indicate that positive TSI anomalies can result in the weakening of Atlantic meridional overturning circulation, causing negative sea surface temperature (SST) anomalies in the North Atlantic. Especially for the subtropical North Atlantic, the negative SST anomalies can excite an anomalous Rossby wave train that moves from the subtropical North Atlantic to the Greenland Sea and finally to Siberia. In this process, the positive sea-ice feedback over the Greenland Sea further enhances the Rossby wave. The wave train can reach the Siberian region, and strengthen the Siberian high. As a result, low-level East Asian winter circulation is strengthened and the surface air temperature in East Asia decreases. Overall, when solar forcing is stronger on the multidecadal time scale, the EAWM is typically stronger than normal. Finally, a similar linkage can be observed between the EAWM and solar forcing during the period 1850-1970. [ABSTRACT FROM AUTHOR]

The Year-to-year variability of the western Pacific subtropical high (WPSH) is primarily controlled by atmosphere-ocean interaction (AOI) between the WPSH and the Indo-Pacific warm pool dipole SST anomalies (AOI mode) and the anomalous SST forcing from the equatorial central Pacific (the CP forcing mode). In this study, we show that the impacts of the WPSH variability on Asian summer monsoon rainfall have changed after the late 1990s. Before the late 1990s (the PRE epoch), the WPSH primarily affects East Asian summer monsoon (EASM) and had little influence on Indian summer monsoon (ISM), whereas after the late 1990s (the POST epoch), the WPSH has strengthened its linkage to the ISM while weakened its relationship with the EASM. This epochal change is associated with a change in the leading circulation mode in the Asia-WP region. During the PRE (POST) epoch the WPSH variation is mainly controlled by the AOI (CP forcing) that mainly affects EASM (ISM). The epochal change of the leading mode may be attributed to the change of the ENSO properties in late 1990s: the CP types of El Nino become a leading ENSO mode in the POST epoch. This work provides a new perspective for understanding decadal changes of the ENSO-monsoon relationship through subtropical dynamics. [ABSTRACT FROM AUTHOR]

In this study, the relationship between the East Asian winter monsoon (EAWM) and the Arctic oscillation (AO) and El Niño southern oscillation (ENSO) are reanalysed using long‐term reanalysis data. The AO and ENSO influence the EAWM by means of a significant inter‐decadal oscillation with a periodicity of approximately 60 years. When the linkage between the AO and EAWM is strengthened (weakened), the ENSO has a weakened (strengthened) relationship with the EAWM. The AO and ENSO alternately affect the EAWM for approximately 30 years. Furthermore, an analysis is completed regarding the possible mechanism of the inter‐decadal relationship between the EAWM and ENSO, as well as the AO. Such an inter‐decadal relationship could be due to the phase transition of the Pacific multi‐decadal oscillation (PMO). When the PMO is positive, sea surface temperatures (SSTs) in the Bering Strait and Alaska Current region are warmer than average, which results in a decrease in the East Asian trough and an increase in the Aleutian low and northeastern Pacific vertical wave activity flux. In this case, the vertical wave activity flux propagates upward and is reflected in the polar vortex, which significantly strengthens the relationship between the Asia‐Pacific climate and AO. Simultaneously, during the positive phase of the PMO, the increase in the SST in the eastern South Pacific is stronger than in the west, which leads to a weakening of the east–west temperature gradient and southern oscillation. Thus, the relationship between the ENSO and Asia‐Pacific region is significantly weakened and vice versa. The results of this study deepen the understanding of the Asia‐Pacific climate and may improve the inter‐decadal prediction of the Asia‐Pacific regional climate in the future. [ABSTRACT FROM AUTHOR]

The severe drought over northeast Asia in summer 2014 and the contribution to it by sea surface temperature (SST) anomalies in the tropical Indo-Pacific region were investigated from the month-to-month perspective. The severe drought was accompanied by weak lower-level summer monsoon flow and featured an obvious northward movement during summer. The mid-latitude Asian summer (MAS) pattern and East Asia/Pacific teleconnection (EAP) pattern, induced by the Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) rainfall anomalies respectively, were two main bridges between the SST anomalies in the tropical Indo-Pacific region and the severe drought. Warming in the Arabian Sea induced reduced rainfall over northeast India and then triggered a negative MAS pattern favoring the severe drought in June 2014. In July 2014, warming in the tropical western North Pacific led to a strong WNPSM and increased rainfall over the Philippine Sea, triggering a positive EAP pattern. The equatorial eastern Pacific and local warming resulted in increased rainfall over the off-equatorial western Pacific and triggered an EAP-like pattern. The EAP pattern and EAP-like pattern contributed to the severe drought in July 2014. A negative Indian Ocean dipole induced an anomalous meridional circulation, and warming in the equatorial eastern Pacific induced an anomalous zonal circulation, in August 2014. The two anomalous cells led to a weak ISM and WNPSM, triggering the negative MAS and EAP patterns responsible for the severe drought. Two possible reasons for the northward movement of the drought were also proposed. [ABSTRACT FROM AUTHOR]

We investigate the interannual variability of the South Asian summer monsoon (SASM) circulation, which has experienced a significant interdecadal change since 2000. This change is primarily influenced by sea surface temperatures (SSTs) in the tropical Pacific and North Atlantic oceans. During the pre-2000 period examined in this study (1979-99), the SASM is negatively correlated with eastern Pacific SSTs (the canonical ENSO mode) and positively correlated with the negative phase of the North Atlantic SST tripole (NAT). During the post-2000 period (2000-14), the SASM is negatively correlated with central Pacific SSTs and positively correlated with the positive phase of the NAT pattern. The associated Pacific SSTs change from the eastern to central region, leading to the rising (subsiding) branch of the Walker circulation moving westwards to the Maritime Continent in the latter period, which can impact the interannual variability of the SASM through modulating the wind field in the troposphere. In addition to Pacific SSTs, the NAT SSTs can propagate energy from the North Atlantic to the South Asian High (SAH) region through the wave activity flux, and then further impact the SASM via the SAH. Because the SASM is intimately related with precipitation over the Asian region, we briefly discuss the features of the precipitation patterns associated with the SASM during the two periods. The westward shifting Walker circulation leads to the shrinking and weakened anomalous westerlies of the SASM in the lower level, inducing the Maritime Continent rainfall location to move westwards and more moisture to arrive in southern China from the Pacific Ocean in the latter period. [ABSTRACT FROM AUTHOR]

Haze is a serious issue in China today, presenting a substantial pollution problem to the public. To enhance understanding of haze mechanisms in recent years, this study examined the relationship between the subtropical western Pacific sea surface temperature ( SST) and the haze pollution over North-Central North China Plain. The empirical orthogonal function result showed an opposite trend of boreal winter (December- January-February) haze days ( WHD) between the south and north of the Huaihe River. The negative sea surface temperature anomaly ( SSTA) over the subtropical western Pacific could intensify the WHD over North-Central North China Plain. As an effective external forcing, the preceding boreal autumn (Sepember-October-November) negative SSTA could weaken the East Asian winter monsoon ( EAWM) circulation, leading to conditions such as a weaker Siberian High, reduced wind speed, higher humidity and an abnormal southerly wind. These conditions provide a favourable environment for haze with static stability and potential for hygroscopic growth. A series of SST sensitivity experiments were carried out using version 3.1 of the Community Atmosphere Model. When negative SSTA with different strengths were added into the model, all the key factors exhibited favourable features for haze, such as smaller EAWM index, stronger inversion potential, weaker vertical velocity, lower boundary layer height and higher relative humidity. [ABSTRACT FROM AUTHOR]

In this study, a 600-yr integration performed with the Bergen Climate Model (BCM), version 2.0, was used to investigate the impact of strong tropical volcanic eruptions (SVEs) on the East Asian winter monsoon (EAWM). It is found that SVEs have an important influence on the East Asian winter climate. The volcanic forcing can cause changes in surface heat fluxes and tropospheric circulation, particularly over the tropics and high-latitude regions. As a result, Arctic Oscillation enters into its positive phase in the first two winters after SVEs. The associated circulation weakens the Siberian high and reduces the cold air transport to East Asia, which is not conducive to the enhancement of the EAWM during this period. At the same time, the North Pacific Ocean gradually cools and shapes La Niña-like sea surface temperature (SST) anomalies in the third winter after SVEs. The Walker circulation is strengthened over the Pacific, and two anomalous lower-tropospheric cyclones are located over the South China Sea and southeast of Japan, respectively. Therefore, related northeasterly wind anomalies appear along the East Asian coast, indicating a strengthened EAWM during this period. Meanwhile, the enhanced Siberian high and East Asian trough further contribute to the enhancement of EAWM in the third winter. It is therefore concluded that the SVEs-induced climate changes over the tropical Pacific and north polar regions play an important role in regulating the EAWM in the posteruption winters. [ABSTRACT FROM AUTHOR]

In this study, two seasonal scale prediction systems were established based on the Community Climate System Model Version 3 (CCSM3) distributed by the National Center for Atmospheric Research (NCAR) of the United States. The initialization schemes are different between the two systems: one is based on sea surface temperature and the other is based on ocean assimilation data of the Climate Forecast System Reanalysis (CFSR). Two sets of hindcasts aiming at summer season (JJA) were generated to verify the predictive ability of the two systems for 1983-2011. The results indicate that both prediction systems perform generally well in predicting sea surface temperature anomalies (SSTA), geopotential height fields, and the typical Asian summer monsoon indices. Their prediction skills of the systems for 2-m air temperature and precipitation are generally confined to the low and middle latitudes. The system based on CFSR ocean assimilation data obtained more accurate results for SSTA, geopotential height fields, and surface weather elements compared with the other system. [ABSTRACT FROM AUTHOR]

ABSTRACT Twenty-one climate models from Coupled Model Intercomparison Project Phase 3 ( CMIP3) and thirty-one models from the project's Phase 5 ( CMIP5) were used to evaluate model reproducibility in assessing interannual variability of summer precipitation in Pan-Asian monsoon region. The results show that both the multi-model ensemble means of the best eight models and of the thirty-one CMIP5 models are more skilful than those of the CMIP3 models in simulating the climatological pattern and the dominant mode of summer precipitation in Pan-Asian monsoon region. CMIP5 models show improved skill in representing the main characteristic of the first mode of summer precipitation in Pan-Asian monsoon region, which is a meridional tripole pattern from north to south occurring east of the 80°E region. That is, owing to the improved El Niño-Southern Oscillation ( ENSO) pattern and the relationship between Antarctic oscillation in the southern Pacific Ocean ( AAOSP) and ENSO, the first dominant mode of summer precipitation in Pan-Asian monsoon region are captured by CMIP5 models, which indicates that these models are more skilful in simulating the air-sea interaction of the Southern Hemisphere. [ABSTRACT FROM AUTHOR]

The interdecadal variation of the western Pacific subtropical high (WPSH) in summer (June-July-August) during 1979-2009 relative to 1948-1978 was investigated based upon NCEP-NCAR reanalysis data. Since the WPSH appeared to be much more stable and stronger in the lower troposphere compared to the middle troposphere and had great influences on the East Asian summer climate with the lower-level southerlies or southwesterlies at its western and southern edge, the interdecadal changes of the WPSH at low levels were examined in this study. Over the area of interest (~15°-30°N, 120°-150°E), the 850-hPa anticyclone circulation was weakened, the sea level pressure decreased, the relative vorticity at middle and lower levels (1,000-600 hPa) increased, and the maximum meridional wind at each latitude (between 17.5° and 32.5°N) moved eastward. All the above decadal variations suggested that the WPSH recessed eastward during 1979-2009 relative to 1948-1978, which can be displayed in the 850-hPa eddy geopotential height field. In association with the eastward recession of the WPSH, the water vapor transport from the Indian summer monsoon played a more important role in the summer precipitation over the middle and lower reaches of the Yangtze River valley during 1979-2009 relative to 1948-1978. [ABSTRACT FROM AUTHOR]

ABSTRACT We present observational evidence and model simulation demonstrating a significant change in the early 1990s in the relationships of the autumn Eurasian snow depth ( AESD) and the autumn Arctic sea ice cover ( ASIC) with the East Asian winter monsoon ( EAWM). A weakening of the AESD-EAWM relationship occurs over a period when the ASIC-EAWM relationship strengthens. The possible physical process can be described as follows: before the early 1990s, the positive AESD anomaly tends to persist into winter. In contrast, after the early 1990s, the negative ASIC anomaly tends to excite a stationary Rossby wave across the North America-Atlantic sector in autumn. This wave train propagates eastward in winter and favours the winter Eurasian snow depth ( WESD) increase. The WESD anomaly is associated with the strengthening and expansions of the Siberian high west and north across the pole and the zonally elongated cold anomalies from Europe to the Far East. It has a stable connection with the EAWM throughout the period observed (1979-2006) and may serve as a bridge linking AESD or ASIC with the EAWM. [ABSTRACT FROM AUTHOR]

Observational analysis and purposely designed coupled atmosphere-ocean (AOGCM) and atmosphere-only (AGCM) model simulations are used together to investigate a new mechanism describing how spring Arctic sea ice impacts the East Asian summer monsoon (EASM). Consistent with previous studies, analysis of observational data from 1979 to 2009 show that spring Arctic sea ice is significantly linked to the EASM on inter-annual timescales. Results of a multivariate Empirical Orthogonal Function analysis reveal that sea surface temperature (SST) changes in the North Pacific play a mediating role for the inter-seasonal connection between spring Arctic sea ice and the EASM. Large-scale atmospheric circulation and precipitation changes are consistent with the SST changes. The mechanism found in the observational data is confirmed by the numerical experiments and can be described as follows: spring Arctic sea ice anomalies cause atmospheric circulation anomalies, which, in turn, cause SST anomalies in the North Pacific. The SST anomalies can persist into summer and then impact the summer monsoon circulation and precipitation over East Asia. The mediating role of SST changes is highlighted by the result that only the AOGCM, but not the AGCM, reproduces the observed sea ice-EASM linkage. [ABSTRACT FROM AUTHOR]

We studied long-term variation of winter snowfall in Northeast China (NEC) for 1951-2010. Results show that NEC snowfall increased about 20% during 1986-2010 relative to 1951-1985. Further investigation suggests that the snowfall increase is closely associated with weakening of the East Asian winter monsoon (EAWM). The physical processes were portrayed by this research. Weakening of EAWM led to weakened cold air flow from the north, thus resulted in the warming of the surface ocean along the Northeast Asia coast and more water vapor evaporated from the ocean surface to the atmosphere and further transported to NEC. Also, because of EAWM weakening, more water vapor from south, east and west of NEC was transported to NEC, increasing water vapor content and hence snowfall there. From an atmospheric circulation viewpoint, EAWM weakening strengthened convergence at low levels and divergence at high levels, thereby favoring increased vertical convection and snowfall. [ABSTRACT FROM AUTHOR]

In this study, the interdecadal changes in the zonal symmetry of both Arctic Oscillation (AO) and Antarctic Oscillation (AAO) were analyzed. To describe the zonal asymmetry, a local index of AO and AAO was defined using the normalized sea level pressure (SLP) differences between 40° and 65° (latitudes) in both hemispheres. The zonal covariability of local AO and AAO can well represent the zonal symmetry of AO and AAO. Results show that the zonal asymmetry of both AO and AAO significantly changed in the late 1970s. AO was less asymmetric in the zonal direction in the boreal winter season during the latter period, while in the boreal summer it became more asymmetric after 1979. The zonal symmetry of AAO in both austral summer and winter has also significantly decreased since the late 1970s. These changes may imply interdecadal transition in the atmospheric circulation at middle and high latitudes, which is of vital importance to understanding climate variability and predictability across the globe, including the African-Asian-Australian monsoon regions. [ABSTRACT FROM AUTHOR]

In this study, the relationship between year-to-year variations in the Bering Sea ice cover (BSIC) and the East Asian winter monsoon (EAWM) for the period 1969-2001 was documented. The time series of total ice cover in the eastern Bering Sea correlated with the EAWM index at −0.49, indicating that they are two tightly related components. Our results show that the BSIC was closely associated with the simultaneous local and large-scale atmosphere over the Asian-northern Pacific region. Heavy BSIC corresponded to weaker EAWM circulations and light BSIC corresponded to stronger EAWM circulations. Thus, the BSIC should be considered as one of the possible factors affecting the EAWM variation. [ABSTRACT FROM AUTHOR]

This paper addresses the interdecadal variation of the West African summer monsoon (WASM) along with its background of atmospheric circulation and possible physical mechanism over the past 32 years (1979-2010). It is indicated that the WASM starts to strengthen from 1998 as the rainfall begins to increase over western West Africa on the whole, which shows a new interdecadal variation. In this interdecadal variation, the strengthened ascending motion corresponding to enhanced divergence (convergence) movement on the upper (lower) troposphere is prone to develop the local circulation of the monsoon. Moreover, the strengthened southwestern (eastern) wind on the lower (upper) level leads to more moisture from the Atlantic and the Gulf of Guinea transported to the West African continent. In addition, the summer subtropical high over the north Atlantic and western West Africa is strong and northward, and the tropical east wind is also strong. Statistically, the weaker (stronger) the spring North Atlantic Oscillation (NAO) is, the stronger (weaker) the tropical easterly is, and then the WASM is also stronger. But the effect of the NAO on the decadal variation of the WASM is not so significant from the north Atlantic anomaly sensitivity simulation with a single model. This is also an indication that the relationship between the WASM and NAO is complicated in an interdecadal time scale and is needed further study. In terms of sea surface temperature (SST) variation, the tendency is toward warming in the subtropical north Pacific, the south Pacific and north Atlantic. Numerical simulation experiments and data analysis show that the SST variation in the north Pacific plays an important role in the latest interdecadal strengthening of the WASM during the past 32 years, while the influences of the south Pacific and the north Atlantic SST anomalies are not so significant to the associated atmospheric circulation changes. [ABSTRACT FROM AUTHOR]

Here we propose a new concept, the Pan-Asian monsoon, and use empirical orthogonal function (EOF) analysis and linear regression approach to define it and to analyze the monsoon-related rainfall variability. The Pan-Asian monsoon is referred to as the monsoon occurred over the great region (60°E-140°E, 10°S-35°N), consisting of the Indian monsoon, Southeast Asian monsoon, East Asian monsoon, and Western North Pacific monsoon. The Pan-Asian monsoon region is the principal region of the summer rainfall over the Asian-Pacific monsoon region and is also water vapor channel connecting several Asian-Pacific sub-monsoon systems. The first EOF mode of the Pan-Asian monsoon precipitation (PAMP_F) shows a meridional tripole pattern with more (less) rainfall zonal belt over the Bay of Bengal (BOB), the Indo-China Peninsula, South China, the South China Sea (SCS), Philippines and the Philippine Sea, and less (more) rainfall on both sides. The first rainfall mode is associated with the weakened Somali cross-equatorial flows, enhanced southerly over the eastern coast of Australia, and strengthened westerly over the tropical Pacific. The first EOF rainfall mode shows a close relationship with the simultaneous El Niño-Southern Oscillation (ENSO) and Pacific South America (PSA). The preceding spring and simultaneous summer Antarctic Oscillation (AAO) in the western Hemisphere (AAO in Pacific) has a connection with the first summer rainfall mode of the Pan-Asian monsoon. Because the main influence factors are over the Pacific, the first rainfall mode is named as the Pacific mode. The second mode of the Pan-Asian monsoon precipitation (PAMP_S) shows a dipole pattern from northeast to southwest, which is associated with the weakened Somali cross-equatorial flows, enhanced easterlies over the Maritime Continent, and weak easterly over the tropical Pacific. The second rainfall model has a close relationship with the atmospheric convection activity and the sea surface temperature variability over the Maritime Continent and South Indian Ocean. Because the influence factors are mainly over the eastern Hemisphere, the second rainfall mode of the Pan-Asian monsoon is named as the Indian Ocean mode. [ABSTRACT FROM AUTHOR]

The interannual variability of East Asian winter monsoon (EAWM) circulation from the Development of a European Multi-Model Ensemble (MME) System for Seasonal to Inter-Annual Prediction (DEMETER) hindcasts was evaluated against observation reanalysis data. We evaluated the DEMETER coupled general circulation models (CGCMs)' retrospective prediction of the typical EAWM and its associated atmospheric circulation. Results show that the EAWM can be reasonably predicted with statistically significant accuracy, yet the major bias of the hindcast models is the underestimation of the related anomalies. The temporal correlation coefficient (TCC) of the MME-produced EAWM index, defined as the first EOF mode of 850-hPa air temperature within the EAWM domain (20°-60°N, 90°-150°E), was 0.595. This coefficient was higher than those of the corresponding individual models (range: 0.39-0.51) for the period 1969-2001; this result indicates the advantage of the super-ensemble approach. This study also showed that the ensemble models can reasonably reproduce the major modes and their interannual variabilities for sea level pressure, geopotential height, surface air temperature, and wind fields in Eurasia. Therefore, the prediction of EAWM interannual variability is feasible using multimodel ensemble systems and that they may also reveal the associated mechanisms of the EAWM interannual variability. [ABSTRACT FROM AUTHOR]

The climatological characteristics and interdecadal variability of the water vapor transport and budget over the Yellow River-Huaihe River valleys (YH1) and the Yangtze River-Huaihe River valleys (YH2) of East China were investigated in this study, using the NCEP/NCAR monthly mean reanalysis datasets from 1979 to 2009. Changes in the water vapor transport pattern occurred during the late 1990s over YH1 (YH2) that corresponded with the recent interdecadal changes in the eastern China summer precipitation pattern. The net moisture influx in the YH1 increased and the net moisture influx in the YH2 decreased during 2000-2009 in comparison to 1979-1999. Detailed features in the moisture flux and transport changes across the four boundaries were explored. The altered water vapor transport over the two domains can be principally attributed to the additive effects of the changes in the confluent southwesterly moisture flow by the Indian summer monsoon and East Asian summer monsoon (related with the eastward recession of the western Pacific subtropical high). The altered water vapor transport over YH1 was also partly caused by the weakened midlatitude westerlies. [ABSTRACT FROM AUTHOR]

Based on the reanalysis data throughout 19482002 as derived from the United States National Centers for Environmental Prediction and National Center for Atmospheric Research, it is revealed that East Asian summer monsoon (EASM) intensity weakens on an interdecadal timescale since the mid-1960s, and twice interdecadal jumps are recorded in the EASM intensity index series in the late 20th century, respectively occurring in the mid-1960s and mid- to late 1970s. Six globally coupled atmosphere-ocean models' outputs under the SRES A2 greenhouse gas and aerosol emission scenario, provided by the IPCC Data Distribution Center and the Hadley Center for Climate Prediction and Research, are then systematically examined. It follows that the above EASM weakening is not closely related to synchronizing anthropogenic global warming, and, therefore, it should be qualitatively natural change process. Over the 21st century, the EASM intensity is likely increased slightly by continually intensified greenhouse effect relative to the late 20th century. [ABSTRACT FROM AUTHOR]

The western North Pacific monsoon in the Northern Hemisphere (NH) and the local Hadley circulation in the Southern Hemisphere (SH) are important components of the vertical meridional circulation over the western Pacific Ocean. Here, we define the SH meridional circulation (SHMC) and NH meridional circulation (NHMC) and investigate their relationship over the western Pacific Ocean. Although they are consistent integrally in the climatological circulation, the NHMC and SHMC do not have a positive change relationship but a significantly negative relationship. In addition, this negative correlation experiences an interdecadal change, with one closely related period during 1989–2003 (Period 2, P2) and two weakened related periods during 1979–1988 (Period 1, P1) and 2004–2017 (Period 3, P3). Through the analysis of the meridional wind field at lower and upper levels, we found that the SHMC moves northward during P2, which can result in a close relationship with the NHMC. The possible mechanism is as follows: The Walker circulation strengthened in P2, which strengthened the ascending motion over the western Pacific Ocean, thus inducing the SHMC to strengthen and move northward, resulting in a significantly stronger relationship with the NHMC. [ABSTRACT FROM AUTHOR]

The investigation into the past behavior of the East Asian summer monsoon (EASM) and Australian summer monsoon (ASM) is potentially helpful for advancing our knowledge of projected future changes. Geological evidence supports an in-phase change of the two monsoon systems over the last millennium, but the dynamic mechanisms and timescale-dependence are not fully understood. Using model outputs from the Paleoclimate Modelling Intercomparison Project Phase III, we investigated EASM and ASM variations, their phase relationship during the last millennium, and their dynamic mechanisms. According to selected “best-performance” models capable of reproducing reconstructed Asian–Australian monsoon changes, EASM and ASM showed significant in-phase changes only on centennial timescales, with intensified EASM/ASM during the Medieval Climate Anomaly (MCA) and weakened monsoons during the Little Ice Age (LIA). Moreover, the synchronous variation was more robust during the LIA relative to the MCA. The strengthened (weakened) EASM during the MCA (LIA) is attributed to an enhanced (reduced) land–sea temperature contrast and a northward (southward) shift of the subtropical westerly jet stream during boreal summer. During the same period, the ASM was reinforced (suppressed) due to the enhanced (reduced) lower-level easterly from the western Pacific and southward (northward) shift of upper-level westerly during austral summer. Meanwhile, the stronger (weaker) EASM/ASM during the MCA (LIA) was associated with expansion (retreat) of the local Intertropical Convergence Zone and an enhanced (reduced) zonal temperature gradient over the equatorial Pacific. Our results imply that the synchronous change in the Asian–Australian monsoon may be caused by inherent solar variations, further strengthening previous findings. [ABSTRACT FROM AUTHOR]

Investigation of climate change during the Last Interglacial (LIG, ~130–115 ka BP) may deepen our understanding of the response of climate to Earth's orbit. Here we examine climate change over China and associated mechanisms during the LIG using the lig127k experiments from the Paleoclimate Modeling Intercomparison Project Phase 4. We illustrate that annual mean surface air temperature over China decreases by 0.22 °C during the LIG relative to the preindustrial, with a colder climate in winter and spring, whereas a warmer climate in summer and autumn. Annual mean precipitation increases (up to 54%) over northern and southwestern China during the LIG, whereas it decreases (up to 11%) over the other parts. This pattern largely holds for precipitation change in summer during the LIG, but precipitation generally decreases in winter. According to the moisture budget equation, the nonlinear and horizontal dynamic terms contribute largely to the drier (wetter) conditions in summer over the south (north) of eastern China, whereas the horizontal dynamic term and the horizontal thermodynamic term is more important in regulating drier conditions over south and north of eastern China in winter, respectively. Furthermore, East Asian summer monsoon is intensified during the LIG manifested by anomalous southerly at 850 hPa, a westward shift of western North Pacific subtropical high, and poleward migration of East Asian westerly jet. East Asian winter monsoon also gets stronger during the LIG, with anomalous northerly at low-level troposphere, enhanced mid-tropospheric East Asian trough, and an equatorward shift of westerly jet at upper troposphere. Additionally, we note obvious data-model discrepancies in annual temperature change over China during the LIG, with cooling seen in the models but warming suggested by multiproxies. This highlights close collaboration between model and data communities in future to deepen our understanding of climate change during the LIG. • A colder climate in the annual mean during the LIG based on the PMIP4. • A dipole wet-dry anomaly pattern over eastern China during the LIG. • Enhanced East Asian summer and winter monsoon circulation system during the LIG. • Considerable model-data mismatch in climate change over China during the LIG. [ABSTRACT FROM AUTHOR]

Abstract: Paleoenvironmental reconstructions illustrated that Asia was dominated by a zonal climate pattern in the Eocene epoch, with an almost latitudinal arid/semiarid band extending from the eastern coast of China to Central Asia. However, the mechanism behind the formation of the broad zonal arid/semiarid region and the possible occurrence of a monsoonal system remains unclear for that period. Here, using a reconstructed land-sea distribution and topography of 50Ma, we employed a global climate model to simulate the Early Eocene climate in China. A set of experiments with consideration to different topography, orbital parameters, vegetation, sea surface temperature and atmospheric CO2 concentration demonstrated that a large extent of arid/semi-arid region with annual precipitation less than 800mm was distributed latitudinally between 20°N and 40°N paleolatitude in China. This arid region was controlled by desert (BWh) and steppe (BSh) climates (according to the Köppen climate classification systems). On the north of the zonal desert/steppe region, a Mediterranean-like climate occurred in Central Asia. These simulations agree well with the geological reconstructions of a zonal desert/steppe region and indicate that this pattern was primarily caused by subtropical high pressures associated with strong descending air. However, it is likely that the aridity in the zonal desert/steppe was reduced occasionally by intensification of monsoon circulations over East Asia. The change in orbital parameters, causing enhanced insolation in summer, was the most likely factor that could enlarge the land-sea thermal contrast and further intensify the monsoon circulations in the Eocene epoch, though its effect was reversible and impermanent. In addition, simulations show that East Asian monsoon did not play a leading role in Chinese Paleogene climate although it may have affected Eastern China occasionally. [Copyright &y& Elsevier]

The rapid expansion of lake area in the Tibetan Plateau (TP) has attracted a considerable amount of concern in recent decades. Apart from the arid zone with rare precipitation on the western TP, the highest precipitation rates on the TP are cases where monsoon precipitation prevails. The high consistency of decadal shifts as a result of variances in the SAM onset, TP precipitation and lake areas indicates that an advanced SAM onset would play an important role in modulating lake variations. [Extracted from the article]

Observations show that the East Asian winter monsoon (EAWM) experienced an interdecadal weakening in the mid-1980s. This is evident for all members of the EAWM system (i.e., East Asian trough, upper-tropospheric jet stream, and lower-tropospheric monsoon circulation). Here, we investigate the relative contributions of natural (volcanic aerosols and solar variability) and anthropogenic [greenhouse gases (GHGs) and anthropogenic aerosols] forcings to this interdecadal weakening using multiple coupled models within the phase 5 of the Coupled Model Intercomparison Program (CMIP5). The results indicate that in the mid-troposphere, the increased GHG concentrations play an important role in weakening the East Asian trough (EAT) by increasing the sea surface temperatures (SSTs) over the North Pacific. In the upper troposphere, natural external forcings contribute to the observed weakening of the meridional shear of the East Asian jet stream (EAJS) by regulating the meridional temperature gradient (MTG) over the East Asian region. In the lower troposphere, both anthropogenic and natural forcings can weaken the Siberian high during this period. Overall, based on the present analysis of the CMIP5 output, GHGs and natural forcings play key roles in shaping the observed interdecadal weakening of the EAWM during the mid-1980s. Additionally, contributions from internal variability cannot be neglected and require further investigation. [ABSTRACT FROM AUTHOR]

An analysis of dust storms and blowing sand records at 681 stations in China during 1961–2000 and in Mongolia during 1980–2000 reveals that the dust source regions in central Asia and the Tibetan Plateau have monthly and interannual northward shifts from the Tibetan Plateau to Mongolia. The monthly shift is closely related to the location of the 200hPa westerly jet and to the location and intensity of the 500hPa westerly jet over central Asia and the Tibetan Plateau. The causes for interannual shifts vary by region. The westerly jet appears to drive the shift on the Tibetan Plateau; the intensity of the central Asian mid-latitude westerly, the Arctic oscillation (AO) and the East Asian winter monsoon may be the drivers in the Hexi Corridor region to the north of the Tibetan Plateau; and the AO and the location of the westerly jet in the Mongolian region. All these causes are closely related to the AO variations, which may provide a connection between climatic changes in East Asia and the North Atlantic, and the long-distance transport of dust. This connection could explain the shifts of dust source regions at interannual and millennial timescales and the roughly synchronous climate changes recorded in the Chinese loess and deep-sea sediments in the North Atlantic Ocean. [Copyright &y& Elsevier]