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Air pollution of fine particulate matter (PM2.5) is severe in winter and causes vast premature deaths in north China. Many previous studies have concerned winter‐mean haze conditions and identified some useful climate factors. However, obvious differences in the number of haze days also existed between December and January (HDJ‐D) that lacked of understandings. The HDJ‐D maximum was 6.6 days, which occurred between December 2005 and January 2006, accounting for more than 45% of the monthly mean number of haze days in these 2 months. Differences of large‐scale atmospheric circulations were distributed like a Rossby wave pattern in mid‐troposphere, with centres in the west of Europe (+), the north to the Caspian Sea (−) and north China (+). Associated meteorological conditions in north China resulted in more adverse ventilation conditions and stronger chemical productions, and thus larger number of haze days in January than in December. In addition, significant relationships between HDJ‐D and three preceding climate factors during 1979–2018 were also revealed, including August–September–October sea ice over the Laptev Sea and the north of the Kara Sea (correlation coefficient = −0.53), November snow depth in eastern Europe and western Siberia (correlation coefficient = −0.44) and sea surface temperature over the Mid‐Atlantic Ridge (correlation coefficient = −0.48). Abundant analyses, basing on the observation data, large‐ensemble climate simulations and atmospheric chemical simulations, all reproduced the influences of each climate factor on large‐scale atmospheric circulations and local meteorological conditions associated with HDJ‐D changes. [ABSTRACT FROM AUTHOR]

A comparison assessment of model capabilities in simulating precipitation extremes across China was first implemented by using 30 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and using 36 CMIP6 models. The results indicate that the multi‐model median ensembles (MME) of both the CMIP5 and CMIP6 models can reasonably reproduce the climate means for the period from 1986 to 2005, and the biases are lower in most CMIP6 models compared to the CMIP5 models, especially over southern China. To provide further comparisons, 14 CMIP6 models are selected and compared with their predecessors in CMIP5. The results show that the CMIP6 models generally exhibit superior skill in simulating the extreme precipitation indices over China. The model spreads for most of the extreme indices in the CMIP6 version are also smaller. Additionally, the MMEs of the two CMIPs outperform individual models. However, some CMIP6 models also exhibit weaker skill levels in simulating some particular indices compared with those in CMIP5, which merits further investigation. The results from seven reanalyses further show large uncertainties for these indices; therefore, care should be taken in comparison with reanalyses. For future changes in precipitation extremes, total wet day precipitation (PRCPTOT), maximum 5‐day precipitation (RX5day) and very heavy precipitation days (R20mm) are projected to clearly increase across China over the coming century under the shared socioeconomic pathway (SSP) 2‐4.5 and SSP5‐8.5 scenarios. However, the dry condition index of CDD exhibits a decreasing tendency in the future, which implies that the dry conditions induced by precipitation anomalies will be mitigated. However, large uncertainties are still observed for future changes, which are primarily sourced from inter‐model and scenario variabilities, especially for the projected changes at the end of the 21st century. [ABSTRACT FROM AUTHOR]

This study investigates the interannual variability of winter precipitation over the Three River Source (TRS) region in China based on precipitation observations collected at stations in the TRS region and reanalysis datasets for the period of 1980–2015. The results suggest that the TRS winter precipitation has distinct interannual variability with discordant trends in different months, that is, an increasing trend is found in November and February, a decreasing trend is found in other winter months, and interannual variations in precipitation are different in different months. The mechanisms for the interannual variation in monthly precipitation over the TRS region are significantly different in different winter months. The interannual variability of TRS precipitation in November is modulated by an anomalous westerly water vapour transport (WVT) branch. This anomalous WVT branch is related to a North Atlantic‐Europe‐Tibetan Plateau (NA‐E‐TP) wave‐train that originates in the North Atlantic due to the ocean–atmosphere interaction. In December, a circum‐global teleconnection (CGT) wave‐train can induce anomalous westerly WVT in the TRS region, resulting in increased precipitation there. This CGT wave‐train is triggered by warm SST anomalies in the central‐eastern tropical Pacific associated with the El Niño‐Southern Oscillation (ENSO). The interannual variability of TRS precipitation in January and February is affected by southwesterly WVT anomalies over the TRS region, which are associated with a southeastward propagating wave‐train over Eurasia caused by the North Atlantic Oscillation (NAO). In March, the interannual variability of TRS precipitation is modulated by the leading mode of the Eurasian circulation that resembles the Scandinavian (EU1) pattern, which can cause anomalous southwesterly WVT in the TRS region. In winter, the characteristics of sea surface temperature anomalies, weather systems, and atmospheric circulations associated with interannual variations of monthly precipitation in the TRS region are different for individual winter months. [ABSTRACT FROM AUTHOR]

Given the threats that climate change poses to solid water reservoirs on the Tibetan plateau (TP), there is significant interest in understanding spatial patterns of climate change and their causes. Weather station observations have been extensively examined, but are scarce, resulting in an incomplete understanding of climate change across the TP, particularly in the west. Using recent (2001–2015) satellite‐based data sets (2 m air temperature, land surface temperature, albedo and snow cover), this study reveals that mean annual 2 m air temperature in the southwestern TP has decreased by 0.15°C/decade in contrast to overall warming (+0.18°C/decade) on the rest of the TP. Up to 45% (74%) of the variance in the annual (spring) 2 m air temperature can be explained by simultaneous change in snow‐induced albedo in the southwestern TP. The free atmosphere column over this region and Northwest India is cooling, providing a favourable environment for the decrease in 2 m air temperature observed. Moreover, the anomalous water vapour transport into the southwestern TP is advantageous for increased snowfall and the associated decrease in 2 m air temperature. The implications of this anomalous cooling under global warming have yet to be fully considered, in particular for the futures of glaciers and snowpack over the Himalayan Mountains in the southwestern TP. [ABSTRACT FROM AUTHOR]

The emerging precursor in December Arctic oscillation (AO) for the following January temperature over East Asia (TEA) has been noticed by recent researches. This study evaluates the precursor in December AO for the following January TEA using 35 climate models from the Coupled Model Intercomparison Project Phase 5. Results indicate that, 24 (multi‐model ensemble mean; MME‐GOOD) out of total 35 models reproduce the statistically significant positive correlation between the December AO and the January TEA identified in the observation, but the remaining 11 models (MME‐BAD) fail. Further, the investigation focuses on these models' simulations for the December AO‐associated atmospheric circulation in January. Eighteen (MME‐GOOD‐18) out of the above 24 models reproduce the persistence of December AO‐related atmospheric circulation, including the seesaw structure of the sea level pressure anomalies, the sweeping southerly 850‐hPa wind anomalies over the East Asia–Northwest Pacific region, the weakening of the East Asian trough, and the weakened meridional shear of the East Asian jet stream in January following an enhanced December AO. It is found that the propagation of stationary planetary waves plays a crucial role in connecting the December AO and the January TEA. Therefore, the models' ability to simulate the stratosphere–troposphere interaction is essential for the successful simulations on the persistence of December AO. Additionally, the future projections indicate that the precursor in December AO for the following January TEA is still robust during the 21st century. [ABSTRACT FROM AUTHOR]

This study investigated the interannual variability of the frequency of extreme high‐temperature events (FEHE) during summer over eastern China and associated mechanisms. Two leading modes of interannual variability of FEHE over eastern China during summer are obtained by Empirical Orthogonal Function (EOF) analysis. The first EOF (EOF1) mode of FEHE shows a diploe pattern characterized by positive (negative) anomalies in northeastern (southern) China. The second EOF (EOF2) mode of FEHE shows a monopole pattern characterized by positive anomalies in the Yangtze River valley (YRV) region. The potential mechanisms of the two leading modes are different. The EOF1 of FEHE is modulated by the EOF1 of 500‐hPa geopotential height (Z500) over Eurasia, which exhibits a Eurasian wave train associated with the air–sea interaction over the North Atlantic. Besides, anomalous SSTs over the western tropical Pacific and associated anomalous convective activity over the Maritime continent may also exert an impact on the EOF1 of FEHE via stimulating a meridional wave train, the so‐called "Pacific‐Japan/East Asia‐Pacific" teleconnection. The EOF2 of FEHE is mainly associated with the NAO mode. [ABSTRACT FROM AUTHOR]

This study explored the interannual relationship between autumn Arctic sea ice concentration (SIC) and the subsequent summer precipitation over East Asia (EASP). Since the late‐1990s, the declining SIC in the Kara–Laptev Seas has been significantly correlated with EASP as well as extremely positive anomalies in northern China and intensely negative anomalies in central‐eastern East Asia. However, there was a weak correlation between autumn SIC and EASP before the late‐1990s. Furthermore, the anomalous precipitation pattern in summer and its connection with autumn SIC variability can be explained by the seasonal persistence of continental processes (snow depth and soil moisture) into the spring. In particular, a decreasing SIC was connected with simultaneously positive and negative precipitation anomalies over northeastern China and the Siberian region, respectively, since the late‐1990s and tends to produce corresponding soil moisture anomalies over the Eurasian continent. Declining SIC also favours increased snow depth anomalies in winter over northeastern East Asia. These anomalous signals of surface processes can persist from winter into the subsequent spring, making the connection between the autumn SIC and EASP possible. The Community Earth System Model Large Ensemble simulations further verified these physical processes. More detailed mechanism for this relationship needs to be stressed in further work by numerical simulations. The results have important implications for extending the seasonal prediction validity of EASP. Moreover, before the late‐1990s, SIC‐related circulation anomalies shifted westward and northward as negative precipitation anomalies developed over west Siberia in autumn. As a result, anomalous dry soil conditions in Siberia persisted into the subsequent spring and then led to wetter‐than‐normal conditions through locally negative soil moisture–precipitation feedback before the late‐1990s. [ABSTRACT FROM AUTHOR]

The impact of North American (NA) snow cover on the Northern Hemisphere climate has drawn increasing attention in recent years. This study reveals a negative relationship between NA snow cover in December and surface air temperature (SAT) over the midlatitude Asia in the following January. The results show that, in December, the extensive snow cover decreases SAT over NA via local diabatic cooling, which further intensifies the Atlantic jet stream by enlarging the meridional gradient of the 1,000‐ to 300‐hPa layer thickness, especially along 40°N. Meanwhile, we found negative sea surface temperature (SST) anomalies over the Western North Atlantic and enhanced transient eddy activity over the Eastern North Atlantic due to anomalous net energy conversion from the mean flow. In January, a zonally oriented dipole of transient eddy anomalies straddles the North Atlantic, with the negative–positive (west–east) anomaly centres attributing to local oceanic cooling and the persistence of the eddy–mean flow interaction, respectively. Such dipole of transient eddy activity in turn favours the downstream development of the Atlantic jet stream. Besides, the changes in surface turbulent heat flux over the North Atlantic trigger a Rossby wave train that propagates across Europe and towards the Far East. Coincident with the strengthened Siberian High and the accelerated East Asian jet stream in January, SAT decreases over the midlatitude Asia. Hence, our analysis indicates that December NA snow cover could potentially be exploited for subseasonal predictability over Asia. [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]

Previous studies have documented that the summer precipitation over South China (SC) has experienced a prominent inter-decadal increase in 1992/1993, and the possible mechanism has been well revealed. The aim of this study is to investigate the changes in extreme precipitation and clustered extreme precipitation events in recent decades using station observations and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. The results indicate that extreme precipitation also experienced a significant inter-decadal increase around 1992/1993. Significant changes can also be found in the associated atmospheric circulations, such as the western North Pacific subtropical high (WNPSH) and the westerly jet stream over East Asia. In addition, the water vapour transport (WVT) related to extreme precipitation differed from mean conditions. For extreme precipitation events, the WVT from the Northwest Pacific and Indian Oceans was much stronger when compared with mean precipitation. When extreme precipitation events were clustered, the increased WVT mainly originated from the South China Sea and Northwest Pacific Ocean. Further analysis indicates that the WVT increased from the Indian Ocean but decreased from the Northwest Pacific after 1992/1993; this inding explains the significant increase in the clustered extreme precipitation events over SC after 1992/1993. In addition, the atmospheric stratification has become more unstable since 1992/1993. [ABSTRACT FROM AUTHOR]

ABSTRACT This study investigated the potential connection of the October-December Arctic Oscillation ( AO-OND) with the following January East Asian temperature extremes and the possible mechanisms. It was found that the extreme cold (warm) events are less (more) frequent in January over East Asia following a positive phase of AO-OND, which might be attributed to the intrinsic persistence of AO-OND in stratosphere and the memory of Eurasian snow cover. It is revealed that the barotropic structure of AO-OND could extend to stratosphere. By the following January, wave activities propagate from stratosphere downwards to upper troposphere, then bent equatorwards to mid-latitudes. Consequently, the January circumpolar jet (polar vortex) gets strengthened (colder) because the equatorward-pointing (upwards) wave activities correspond to poleward meridional eddy momentum (eddy heat) flux. In such a way, the anomalous AO-OND persists into the following January. On the other hand, positive phase of AO-OND causes significant surface warming in most parts of Eurasia, leading to Eurasian snow melt. The October-December snow melt has intrinsic climatic memory and further weakens the January Siberian High through reducing surface albedo. Besides, the surface warming in North Europe because of reduced surface albedo caused by snow melt could stimulate a Rossby wave train propagating eastwards across Eurasia, which restrains the blocking events around Ural region. In such a context, the incidence of extreme warm (cold) events over East Asia is more (less) frequent. [ABSTRACT FROM AUTHOR]

ABSTRACT The simulated response of the East Asian winter monsoon ( EAWM) to ENSO-like sea surface temperature anomalies ( SSTAs) forcing in three U.S. Climate Variability and Predictability (U.S. CLIVAR) Project models ( GFDL, CCM3, and CCM3.5) are evaluated. The time series of the ENSO-like pattern significantly correlates with the EAWM index for the period 1949-2004 based on the HadISST SST data set and the NCEP atmospheric reanalysis. Their correlation coefficient is −0.32 at the 95% confidence level. The warm (cold) phase of ENSO-like SSTAs is associated with a weakened (strengthened) Siberian High and an eastern-positioned (western-positioned) Aleutian Low, anomalous southerly (northerly) wind at 850 hPa and positive (negative) surface air temperature anomalies over East Asia and western North Pacific, while 300 hPa zonal wind anomalies emerge with a reduced (accelerated) East Asian jet stream. Further model examination reveals that the simulated response over East Asia forced by warm (cold) ENSO-like SSTAs in CCM3 and the three-model ensemble corresponds to the weakened (strengthened) EAWM. The spatial correlation coefficients between the three-model ensemble and observation in the atmospheric circulation and surface air temperature range from 0.15 to 0.80 at the 99% confidence level. This success in simulating relationship between ENSO-like SSTAs and EWAM using three-model ensemble proposes a new idea for the EAWM prediction. [ABSTRACT FROM AUTHOR]

The winter North Atlantic Oscillation (NAO) is a crucial part of our understanding of Eurasian and Atlantic climate variability and predictability. In this study, we developed effective prediction schemes based on the interannual increment prediction method and verified their performance based on the climate hindcasts of the coupled ocean--atmosphere climate models. This approach utilizes the year-to-year increment of a variable (i.e. a difference in a variable between the current year and the previous year, e.g. DY of a variable) as the predictand rather than the anomaly of the variable. The results demonstrate that the new schemes can generally improve prediction skill of the winter NAO compared to the raw coupled model's output. Also, the new schemes show higher skill in prediction of abnormal NAO cases than the climatological prediction. Scheme-I uses just the NAO in the form of year-to-year increments as a predictor that is derived from the direct outputs of the models. Scheme-II is a hybrid prediction model that contains two predictors: the NAO derived from the coupled models and the observed preceding autumn Atlantic sea surface temperature in the form of year-to-year increments. Scheme-II shows an even better prediction skill of the winter NAO than Scheme-I. [ABSTRACT FROM AUTHOR]

ABSTRACT In this study, we investigated the impact of the spring North Pacific Oscillation ( NPO) on typhoon frequency over the Western North Pacific ( WNP, north of the equator and west of dateline). To reduce the statistical uncertainty, all the data we used have been excluded the El Nino/Southern Oscillation signals. We found that the spring NPO positively correlated with the annual typhoon number over the WNP (hereafter TNWNP) during the period 1968-2010, with a correlation coefficient of 0.62 (above the 99% significance level). When the Northern Low and Southern High pattern over the north Pacific weakens, the TNWNP tends to increase. A positive phase of spring NPO is associated with the tropical circulation and sea surface temperature ( SST) anomalies in the North Pacific, which may lead to unfavorable dynamical and thermal conditions for typhoon genesis over the WNP during JJAS. The negative anomaly of SST over the WNP, associated with the positive spring NPO anomaly, is connected to the tropical atmospheric circulations from spring to summer via good oceanic seasonal persistence and air-sea interaction. Thus, the spring NPO-related variability of the tropical atmospheric circulation as well as the SST can affect typhoon activity over the WNP. In addition, the spring NPO can also be adopted as a predictor for the summer rainfall in South China, since the spring NPO can modulate the WNP SST and western Pacific subtropical high from spring to summer. [ABSTRACT FROM AUTHOR]

ABSTRACT In this study, the atmospheric moisture sources affecting three sub-regions of East China, which are South China (SC), the middle and lower Yangtze River Valleys (YRV), and North China (NC), are identified and compared based on the simulation results of a Lagrangian particle dispersion model, FLEXPART, for the period 2000-2009. The investigation is conducted for 'non-precipitation' and 'precipitation' cases of summertime (May-September) and wintertime (October-April). Furthermore, the contributions of moisture originating from the land area, western Pacific Ocean, and Indian Ocean to the precipitation in the three sub-regions of East China are quantitatively estimated. The results indicate that during summertime, the transport of moisture from the Indian Ocean and western Pacific dominates the precipitation in SC, with almost equivalent contributions; evaporation over the land area seems to be the key contributor to the precipitation in YRV and NC, followed by the evaporation over the western Pacific. During wintertime, the precipitated moisture in SC originates largely from the western Pacific, while evaporation over the land area is the predominant contributor to the precipitation in YRV and NC. The results of this study indicate that the precipitation in the three sub-regions of East China reflect the contributions of the Indian monsoon and East Asian summer monsoon to different degrees. [ABSTRACT FROM AUTHOR]

ABSTRACT The Arctic Oscillation ( AO) and ENSO have dominant influences on the extratropical and tropical climate variabilities in the Northern Hemisphere, respectively. In this paper, we document a strengthening of the AO-ENSO relationship in January after the mid-1990s, when the interannual variability of the previous September Arctic sea ice cover ( ASIC) had consistently increased, by analyzing the AO, ENSO, and associated atmosphere-ocean variability. Our analysis further suggests that the larger ASIC interannual variability may account for the larger scale wave train across the Pacific basin, and the AO is strongly coupled to the circulation in the North Pacific. The strengthened connection between the − AO and Aleutian Low ( AL) may contribute to the identified closer relationship between the − AO and ENSO. [ABSTRACT FROM AUTHOR]

The statistical relationship between observed interannual variability and the corresponding predictability of June-July-August mean precipitation is computed and discussed, using hindcasts from the Development of a European Multimodel Ensemble System for Seasonal to Interanuual Prediction (DEMETER) project and reanalysis datasets of the Global Precipitation Climatology Project and Climatic Research Unit. It is found that there exists a globally positive correlation between the variability and predictability, being prominent in the low latitudes and less evident in the high latitudes. Although this correlation varies with modulated verification datasets, multi-model ensembles and temporal periods, all of which are artificial factors, a significant positive correlation is found over East Asia, Australia, South America, Europe and Africa, while a negative correlation for North America, under most circumstances. Moreover, it is also found that different dynamical processes in the climate system seem to have made different contributions to building this 'larger variability-better predictability' relationship, possibly through enhancing the predictability over domains where there is a larger contribution to variability by the climate signals, while making an uncertain contribution to the predictability over domains that are less influenced by the signals. [ABSTRACT FROM AUTHOR]

Considering the 'Green Great Wall' project and future climate change, the vegetation in the Northwest Hetao Plain (NWHP) of China is expected to undergo considerable changes. In this study, the climatic response to prescribed changes in vegetation is investigated using the Weather Research and Forecasting model. When the vegetation is changed from 'bare or sparsely vegetated' category to 'grassland' in the NWHP, the climate shows both local and remote responses. Locally, the temperature increases in the winter but decreases in the summer. The precipitation level increases substantially in the summer while increases slightly in the winter, which is mainly caused by the increase in the amount of water-vapour and circulation adjustment. Additionally, atmospheric circulation anomalies also lead to remote circulation responses, including a decrease in precipitation over Central North China and an increase in precipitation over Central and South China. The results of this study bring to light the local and remote climatic responses to changes in vegetation in the NWHP using a state-of-the-art regional climate model. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]