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Tibetan Plateau vortices (TPVs) are important rainfall producers over the Tibetan Plateau in summer and can also influence wide areas east of the Tibetan Plateau when emigrating from the plateau. The Effects of the variations in air temperature over the Tibetan Plateau on the genesis frequency of TPVs at interannual timescales is explored in this work to understand the interannual variations in TPVs and the resultant precipitation. The results indicate that the interannual variations in the genesis frequency of TPVs are significantly related to that in the air temperature at 250 hPa over the Tibetan Plateau. The upper-level air temperature affects the genesis of TPVs by modulating the large-scale circulations at 200 hPa. In warm (cold) years, an anomalous high (low) at 200 hPa is observed over the eastern Tibetan Plateau and central China, and a strong (weak) westerly jet is found north of the Tibetan Plateau. The upper-level westerly jet is considered to have a direct influence on TPVs. Impact of 250-hPa air temperature over the Tibetan Plateau on the westerly jet is further explained from two perspectives. First, the air temperature at 250 hPa regulates the thermal wind between 250 and 200 hPa, thereby affecting the wind at 200 hPa. Second, the air temperature at 250 hPa changes the geopotential height gradient at 200 hPa north of the Tibetan Plateau, which results in variations in the westerly jet there. Consequently, more TPVs are generated over the Tibetan Plateau in warm years, and vice versa for cold years.

The variation in the concentrations of ambient PM2.5 (particles with an aerodynamic diameter less than 2.5 µm ) generally forms a continuous sawtooth cycle with a recurring smooth increase followed by a sharp decrease. The episode of abrupt decay of pollution is mostly meteorological in origin and is controlled by the passage of synoptic systems. One affordable and effective measure for quickly reducing PM2.5 concentrations in northern China is to wait for a strong wind to arrive. However, it is still unclear how strong the wind needs to be and exactly what kind of synoptic system most effectively results in the rapid decay of air pollution episodes. PM2.5 variations over the 28 pollution channel cities of the Beijing region are investigated to determine the mechanisms by which synoptic patterns affect the decay processes of pollution episodes. This work shows more obvious day-to-day variations in PM2.5 concentration in winter than in summer, which implies that wintertime PM2.5 variations are more sensitive to meteorological factors. There were 365 decay processes from January 2014 to March 2020, and 97 of them were related to the effective wet deposition. In total, 26 % – 43 % of PM2.5 pollutant is removed by the wet deposition in different seasons. Two dominant circulation patterns are identified in summer. All the other three seasons have three circulation types (CTs), respectively. The three CTs in spring show the same patterns as those in autumn and winter. The circulation patterns beneficial to the decay processes all exhibit a higher-than-normal surface wind speed, a negative relative humidity anomaly and net outflow of PM2.5 from the domain. In addition, CT1 in spring, autumn and winter is controlled by northeasterly wind and features the most significant horizontal net outflow of air pollutants and effective upward spread of air pollutants to the free atmosphere. CT2 is the most frequent CT in autumn and winter, with the highest wind speed from the northwest, highest boundary layer height (BLH) and lowest relative humidity among the three CTs, all of which are favorable for the reduction of PM2.5 concentrations. In CT3, strong vertical wind shear within the boundary layer enhances the mixing of surface air pollutants, which is the extra cleaning mechanism besides dry and clean air mass inflow. PM2.5 concentrations show significant decreases of more than 37 %, 41 % and 27 % after the passage of CT1, CT2 and CT3, respectively. A dry airflow with a positive BLH anomaly and the effective horizontal outflow of air pollutants are the main reasons for the abrupt decay phase in summer. PM2.5 concentrations after the decay process show a significant decreasing trend from 2014 to 2020, reflecting successful emission mitigation. Emission reductions have led to a 4.3–5.7 µ g m - 3 yr - 1 decrease in PM2.5 concentrations in the 28 pollution channel cities of the Beijing region.

According to the corresponding historical averages of air quality during each month of BF, AF1, and AF2 period (in Fig In general, it shows more notable variations in air quality during the AF1 period than that of the AF2 period [ ]we will go through some detailed air quality variation in the AF1 period here According to the low normalized concentration in the BF period, SO2 concentration shows significant decrease throughout China in recent years, especially over northern China, indicating the importance of coal desulfurization and nationwide emission reduction Considering O3, the concentration of which is driven by two major classes of directly emitted precursors, i e , NOx and volatile organic compounds (VOCs), except for the Guangxi Province, O3 over the other provinces and areas increased significantly with the outbreak of coronavirus

Despite the fact that great efforts have been made to improve the prediction of El Niño events, it remains challenging because of limited understanding of El Niño and its precursors. This research focuses on the influence of South Pacific atmospheric variability on the development of the sea surface temperature anomaly (SSTA) in the tropical Pacific. It is found that as early as in the boreal spring of El Niño years, the sea level pressure anomaly (SLPA) shows a configuration characterized by two significant negative anomaly centers in the north and a positive anomaly center in the south between the subtropics and high latitudes in South Pacific. Such an anomalous SLPA pattern becomes stronger in the following late boreal spring and summer associated with the strengthening of westerly anomalies in the tropical Pacific, weakening the southeasterly trade winds and promoting the warming of tropical eastern Pacific, which is conducive to the development of El Niño events. It is demonstrated that the SLPA pattern in boreal spring revealed in this study is closely associated with boreal summer South Pacific Oscillation (SPO) and South Pacific meridional mode (SPMM). As a precursor in boreal spring, the prediction skill of the South Pacific SLPA in boreal spring for the SSTA in the eastern equatorial Pacific is better than that of the SPMM. This study is helpful to deepen our understanding of the contribution of South Pacific extratropical atmospheric variability to El Niño occurrence.

The Chinese government has made many efforts to mitigate fine particulate matter pollution in recent years by taking strict measures on air pollutant reduction, which has generated the nationwide improvements in air quality since 2013. However, under the stringent air pollution controls, how the wintertime PM2.5 concentration (i.e., the mass concentration of atmospheric particles with diameters less than 2.5 µm) varies and how much the meteorological conditions contribute to the interannual variations in PM2.5 concentrations are still unclear, and these very important for the local government to assess the emission reduction of the previous year and adjust mitigation strategies for the next year. The effects of atmospheric circulation on the interannual variation in wintertime PM2.5 concentrations over the Beijing–Tianjin–Hebei (BTH) region in the period of 2013–2018 are evaluated in this study. Generally, the transport of clean and dry air masses and an unstable boundary layer in combination with the effective near-surface horizontal divergence or pumping action at the top of the boundary layer benefits the horizontal or vertical diffusion of surface air pollutants. Instead, the co-occurrence of a stable boundary layer, frequent air stagnation, positive water vapor advection and deep near-surface horizontal convergence exacerbate the wintertime air pollution. Favorable circulation conditions lasting for 2–4 d are beneficial for the diffusion of air pollutants, and 3–7 d of unfavorable circulation events exacerbates the accumulation of air pollutants. The occurrence frequency of favorable circulation events is consistent with the interannual variation in seasonal mean PM2.5 concentrations. There is better diffusion ability in the winters of 2014 and 2017 than in other years. A 59.9 % observed decrease in PM2.5 concentrations in 2017 over the BTH region could be attributed to the improvement in atmospheric diffusion conditions. It is essential to exclude the contribution of meteorological conditions to the variation in interannual air pollutants when making a quantitative evaluation of emission reduction measurements.

The real-time monitoring of reductions of economic activity by containment measures and its effect on the transmission of the coronavirus (COVID-19) is a critical unanswered question. We inferred 5,642 weekly activity anomalies from the meteorology-adjusted differences in spaceborne tropospheric NO2 column concentrations after the 2020 COVID-19 outbreak relative to the baseline from 2016 to 2019. Two satellite observations reveal reincreasing economic activity associated with lifting control measures that comes together with accelerating COVID-19 cases before the winter of 2020/2021. Application of the near-real-time satellite NO2 observations produces a much better prediction of the deceleration of COVID-19 cases than applying the Oxford Government Response Tracker, the Public Health and Social Measures, or human mobility data as alternative predictors. A convergent cross-mapping suggests that economic activity reduction inferred from NO2 is a driver of case deceleration in most of the territories. This effect, however, is not linear, while further activity reductions were associated with weaker deceleration. Over the winter of 2020/2021, nearly 1 million daily COVID-19 cases could have been avoided by optimizing the timing and strength of activity reduction relative to a scenario based on the real distribution. Our study shows how satellite observations can provide surrogate data for activity reduction during the COVID-19 pandemic and monitor the effectiveness of containment to the pandemic before vaccines become widely available., This work was supported by the National Natural Science Foundation of China (41877506), the Fudan’s Wangdao Undergraduate Research Opportunities Program (18107), the Chinese Thousand Youth Talents Program, the PolEASIA-ANR project (ANR15-CE04-0005), and the Australia-China Centre for Air Quality Science and Management. J.P. and J.S. acknowledge financial support from the Catalan Government grant AGAUR-2020PANDE00117. P.C. acknowledges support from the European Space Agency Climate Change Initiative ESA-CCI RECCAP2 project (ESRIN/4000123002/18/I-NB) and the Observation-based system for monitoring and verification of greenhouse gases project (VERIFY, grant 725546).

We used the APSIM-Maize model to simulate maize potential yield (Yp) and rain-fed yield (Yw) when adaptation options of sowing date and planting density were adopted under Representative Concentration Pathway (RCP) 4.5 and 8.5 in the Guanzhong Plain of China. The results showed that Yp would decrease by 10.6–14.9% and 15.0–31.4% under RCP4.5 and RCP8.5 for summer maize, and 13.9–19.7% and 18.5–36.3% for spring maize, respectively. The Yw would decrease by 17.1–19.0% and 23.6–41.1% under RCP4.5 and RCP8.5 for summer maize, and 20.9–24.5% and 27.8–45.5% for spring maize, respectively. The loss of Yp and Yw could be reduced by 2.6–9.7% and 0–9.9%, respectively, under future climate for summer maize through countermeasures. For spring maize, the loss of Yp was mitigated by 14.0–25.0% and 2.0–21.8% for Yw. The contribution of changing sowing date and plant density on spring maize yield was more than summer maize, and the optimal adaptation options were more effective for spring maize. Additionally, the influences of changing sowing date and planting density on yields become weak as climate changes become more severe. Therefore, it is important to investigate the potential of other adaptation measures to cope with climate change in the Guanzhong Plain of China.

The ‘westerlies-dominated climatic regime’ (WDCR) in the present interglacial period was proposed because precipitation/moisture variations between arid central Asia and mid-latitude monsoonal Asia are out-of-phase or anti-phased on different timescales during the Holocene. In this study. we first review the development of the theoretical framework of the WDCR, and then outline the boundary of its core area: from the Caspian Sea in the west to the western Hexi Corridor in the east, with the northern and southern limits coinciding with the domain of arid central Asia. Next, we present a synthesis of multiple lines of evidence for the occurrence of the WDCR on multi-millennial (‘sub-orbital’ herein) to decadal timescales during the Holocene. Finally, we examine the possible physical mechanisms responsible for the WDCR. We find that external factors (insolation changes induced by orbital factors) generated the WDCR on a sub-orbital timescale, whereas a circum-global teleconnection/Silk Road pattern was the most significant factor responsible for the WDCR on centennial and decadal timescales. The study provides a comprehensive summary of the development of our knowledge of the WDCR over the past several decades, together with a tentative theoretical framework for understanding climatic and environmental changes within its region of influence. In addition, it forms a scientific basis for environmental management and ecological restoration in this arid region in the context of global warming.

Tibetan Plateau vortices (TPVs) are important synoptic systems generated over the Tibetan Plateau, triggering heavy rainfall over southwestern and eastern China when they move off the plateau. In the present work, the precipitation associated with the moving-off TPVs is explored, and two groups of TPVs are selected according to the precipitation intensity (PI) related to the TPVs. The characteristics of these two groups of TPVs and the large-scale circulations during the period after the TPVs move off the plateau are compared. The results show that the impact of TPVs is mainly found in the region to the west of 115° E between 30° N and 36° N, becoming weaker along with the longitude. Sichuan Basin is the area with the greatest influence from the TPVs. PI is attributed to both the TPVs and the large-scale background. The TPVs related to high (low) PI are stronger (weaker) and travel further eastward (travel shorter distance), and the ascending motion associated with the TPVs is stronger (weaker) and deeper (shallower). The Western Pacific subtropical high (WPSH), southwesterlies to the south of TPVs in middle and lower troposphere, the South Asia High (SAH), and upper-level jet stream are crucial systems influencing the PI. Stronger and northwestward stretching WPSH, more intensive southwesterlies and the associated water vapor transportation to the south of TPVs, and stronger upper-level jet stream, as well as the more powerful and eastward extending SAH, correspond to higher PI, and vice versa.

Tibetan Plateau vortices (TPVs) are major rainfall producers generated over the Tibetan Plateau, and the ones moving off the Tibetan Plateau can trigger heavy rainfall over eastern China. The structure characteristics of the moving-off TPVs are investigated based on the final operational global analysis data from the National Centers for Environmental Prediction. Generally, the TPVs show different dynamic and thermodynamic structures before and after they move off the Tibetan Plateau. Specifically, the structures of TPVs at the genesis and peak times before they move off the Tibetan Plateau are distinguishing, and different structure characteristics are also found at the times when the TPVs just move off the Tibetan Plateau and when the intensity reaches the peak after moving off. In addition, the moving-off TPVs are divided into two groups according to their lifespans after moving off the plateau, and the structure characteristics of these two groups of TPVs are further compared. Furthermore, on the basis of the evolution features of the moving-off TPVs, the relationship between the structures of TPVs and their intensity is discussed. It is inferred that the evolution of the TPVs is determined by the structures of TPVs themselves to some extent.

Tibetan Plateau vortices (TPVs) are major rain producer over the Tibetan Plateau, which trigger heavy rainfall in southwestern and eastern China when moving off the plateau. In this work, two groups of TPVs moving off the plateau are selected according to their eastward moving speeds. The features of the atmospheric dynamic and thermodynamic fields associated with the two groups of TPVs are compared, based on the final (FNL) operational global analysis data from the Global Forecasting System of the National Centers for Environment Prediction (NCEP). The results show that the large-scale circulations and heating fields have a close relationship with the moving speed of the TPVs. The TPVs move eastward faster when wider and stronger convergence at 500 hPa, divergence at 200 hPa and the related ascending motion are observed to the east of TPVs. In addition, the stronger and further eastward stretching unstable stratification and water vapor convergence, as well as the more intensive heating field above 500 hPa to the east of TPVs, correspond to larger eastward moving speed of TPVs. Furthermore, the crucial factors modulating the moving speed of TPVs are explored through potential vorticity (PV) budget analyses, in which the physical variables are partitioned into zonal means and disturbances. The convergence of the mean zonal winds and disturbance winds at 500 hPa, as well as the vertical distribution of disturbance heating to the east of TPVs are the crucial factors influencing the eastward moving speed of TPVs, among which the vertical distribution of disturbance heating is the most dominant.

The climatic characteristics of 260 East Asian tropical monsoon depressions (EAMDs) are investigated using the ERA-Interim reanalysis dataset and a tracking dataset of global monsoon low-pressure systems. Most EAMDs form over the South China Sea (SCS) and the western tropical Pacific Ocean in July–October and have an average lifetime of 10 days. The vertical structures of EAMDs are usually upright or tilt slightly westward with height. The warm-over-cold thermal structure is a distinctive characteristic of EAMDs and two potential vorticity (PV) centers are related to the warm core in the upper level and the specific humidity center in the lower level, respectively. We divided the EAMDs into four groups: eastward-moving, westward-moving, turning, and northwestward-moving EAMDs. Most of the eastward-moving EAMDs form over the SCS in May and June, whereas the westward-moving EAMDs form over both the SCS and the western Pacific Ocean in July–October. The turning and northwestward-moving EAMDs are mainly generated over the western Pacific Ocean and have longer lifetimes. The structures of the eastward-moving and turning EAMDs show common characteristics in each stage. Their vertical structures change from upright in the developing and peak stages to northeast tilting with height in the attenuating stage, especially for the specific humidity. By contrast, the structures of westward- and northwestward-moving EAMDs show little change during their lifetime. They are symmetrical relative to the vertical axis of the EAMDs over their whole lifetime and only vary in strength.

Year-to-year variability of surface air temperature (SAT) over central India is most pronounced in June. Climatologically over central India, SAT peaks in May, and the transition from the hot premonsoon to the cooler monsoon period takes place around 9 June, associated with the northeastward propagation of intraseasonal convective anomalies from the western equatorial Indian Ocean. Positive (negative) SAT anomalies during June correspond to a delayed (early) Indian summer monsoon onset and tend to occur during post–El Niño summers. On the interannual time scale, positive SAT anomalies of June over central India are associated with positive SST anomalies over both the equatorial eastern–central Pacific and Indian Oceans, representing El Niño effects in developing and decay years, respectively. Although El Niño peaks in winter, the correlations between winter El Niño and Indian SAT peak in the subsequent June, representing a post–El Niño summer capacitor effect associated with positive SST anomalies over the north Indian Ocean. These results have important implications for the prediction of Indian summer climate including both SAT and summer monsoon onset over central India.

The analysis of observational rainfall shows that the intensity of rainfall intraseasonal oscillation (ISO) and the summer-mean rainfall over the middle-lower reaches of the Yangtze River Basin (YRB) exhibit a significant positive correlation during 1979–2007. A stronger (weaker) ISO variability is often associated with wet (dry) summer in the YRB. The composite ISOs in both the wet and dry summers are further analyzed. In the wet summers, the rainfall ISO in YRB is primarily associated with the northward propagation of a low-level cyclone-anticyclone pair from the tropics. Cyclonic vorticity and associated boundary layer convergence strengthen the rainfall in situ. In contrast, the rainfall ISO in YRB in the dry summers is primarily associated with the westward propagation of an anomalous anticyclone. Southerly flow to the west of the anomalous anticyclone enhances rainfall in YRB through anomalous moisture advection. In addition to the difference in ISO propagation, the background mean state also shows a marked difference. The diagnosis of water vapor flux budget shows that the convergence and advection of seasonal mean moisture play a critical role in maintenance of the intraseasonal rainfall in the YRB. A greater mean ascending motion and associated higher mean moisture in YRB in the wet summers favor greater intraseasonal rainfall variability in situ. The mean state difference is responsible for distinctive vertical structures of boundary layer vertical velocity. A possible feedback of the ISO to the summer-mean rainfall over the YRB is also discussed.

Based on daily precipitation data from the Chinese Meteorological Administration and reanalysis data from the National Centers for Environmental Prediction-Department of Energy, the character of low-frequency precipitation variability during the first rainy season (April–June) over South China and its corresponding atmospheric circulations in the mid-high latitudes are investigated. The results show that the precipitation anomalies during this period exhibit obvious quasi-biweekly oscillation (QBWO) features, with a period of 8–24 days. The influence of wave trains in the mid-high latitudes to low-frequency persistent heavy rain event (PHR-LF event, the 8–24-day filtered precipitation larger than one standard deviation of filtered time series and persisting at least three days over South China) is further discussed. During the first rainy season over South China, there are two low-frequency wave trains in the mid-high latitudes associated with the PHR-LF event—the wave train crossing the Eurasian continent and the wave train along the subtropical westerly jet. Analysis of wave activity flux indicates that the wave energy disperses toward eastern China along these two low-frequency wave trains from north to south and from west to east, and then propagates downward over South China. Accordingly, the disturbance of the relative vorticity of the cyclonic anomalies over eastern China is strengthened, which enhances the meridional gradient of relative vorticity. Owing to the transport of low-frequency relative vorticity and geostrophic vorticity by meridional wind, the ascending motion over South China intensifies and lasts for a long time, triggering a PHR-LF event. In addition, the tropical system is also a key factor to PHR-LF event. The QBWO of the convection over the South China Sea provide moisture for PHR-LF events, maintaining persistent rainfall and vertical ascending motion over South China.

The boreal summer intraseasonal oscillation (BSISO) is simulated by the Climate System Model (CSM) developed at the Chinese Academy of Meteorological Sciences (CAMS), China Meteorological Administration. Firstly, the results indicate that this new model is able to reasonably simulate the annual cycle and seasonal mean of the precipitation, as well as the vertical shear of large-scale zonal wind in the tropics. The model also reproduces the eastward and northward propagating oscillation signals similar to those found in observations. The simulation of BSISO is generally in agreement with the observations in terms of variance center, periodicity, and propagation, with the exception that the magnitude of BSISO anomalous convections are underestimated during both its eastward propagation along the equator and its northward propagation over the Asian–Pacific summer monsoon region. Our preliminary evaluation of the simulated BSISO by CAMS-CSM suggests that this new model has the capability, to a certain extent, to capture the BSISO features, including its propagation zonally along the equator and meridionally over the Asian monsoon region.

Significance Summer rainfall along the Yangtze River in 2020 was the heaviest since 1961, with devastating socioeconomic impacts. While official forecasts based on tropical Pacific state failed, we show that dynamic models, when initialized with ocean observations globally, succeed in predicting the extreme rainfall. Slowly propagating oceanic Rossby waves in the South Indian Ocean are the source of predictability, which are in turn tied to the record-breaking Indian Ocean Dipole in late 2019. The identification of antecedent subsurface conditions of the Indian Ocean as a key predictor represents an important conceptual advance in Asian summer monsoon dynamics, helping improve disaster preparation that saves lives and properties., Heavy monsoon rainfall ravaged a large swath of East Asia in summer 2020. Severe flooding of the Yangtze River displaced millions of residents in the midst of a historic public health crisis. This extreme rainy season was not anticipated from El Niño conditions. Using observations and model experiments, we show that the record strong Indian Ocean Dipole event in 2019 is an important contributor to the extreme Yangtze flooding of 2020. This Indian Ocean mode and a weak El Niño in the Pacific excite downwelling oceanic Rossby waves that propagate slowly westward south of the equator. At a mooring in the Southwest Indian Ocean, the thermocline deepens by a record 70 m in late 2019. The deepened thermocline helps sustain the Indian Ocean warming through the 2020 summer. The Indian Ocean warming forces an anomalous anticyclone in the lower troposphere over the Indo-Northwest Pacific region and intensifies the upper-level westerly jet over East Asia, leading to heavy summer rainfall in the Yangtze Basin. These coupled ocean-atmosphere processes beyond the equatorial Pacific provide predictability. Indeed, dynamic models initialized with observed ocean state predicted the heavy summer rainfall in the Yangtze Basin as early as April 2020.

Lockdown measures are essential to containing the spread of coronavirus disease 2019 (COVID-19), but they will slow down economic growth by reducing industrial and commercial activities. However, the benefits of activity control from containing the pandemic have not been examined and assessed. Here we use daily carbon dioxide (CO2) emission reduction in China estimated from statistical data for energy consumption and satellite data for nitrogen dioxide (NO2) measured by the Ozone Monitoring Instrument (OMI) as an indicator for reduced activities consecutive to a lockdown. We perform a correlation analysis to show that a 1% day−1 decrease in the rate of COVID-19 cases is associated with a reduction in daily CO2 emissions of 0.22% ± 0.02% using statistical data for energy consumption relative to emissions without COVID-19, or 0.20% ± 0.02% using satellite data for atmospheric column NO2. We estimate that swift action in China is effective in limiting the number of COVID-19 cases, We are grateful for the provision of funds from the National Natural Science Foundation of China ( 41877506 ), the Fudan’s Wangdao Undergraduate Research Opportunities Program ( 18107 ), the Chinese Thousand Youth Talents Program , and the Australia-China Centre for Air Quality Science and Management.

In recent decades, unprecedented extreme summer heat waves have occurred in Europe, and they have exhibited an increasing trend since 1970s. Although previous studies have suggested that these recent hot European summers could have been instigated by the underlying surface thermal conditions, the possible influence of shrinking Arctic sea ice and Eurasian snow cover on heat waves are not well understood. Herein, we present evidence obtained via observational analyses and numerical experiments indicating that the interdecadal increase in European heat waves is closely linked to the reductions in Arctic sea ice concentration (ASIC) and Eurasian snow cover fraction (EASC) across mid–high latitudes via the excitation of the anomalous Eurasian wave train. The combined effects of declined ASIC and EASC, accompanied by the drier soil and the stronger heat flux, tend to weaken the poleward temperature gradient at mid–high latitudes and affect the midlatitude jet stream and transient eddy activities. These dynamic and thermodynamic circulations increase the likelihood of more persistent European blocking events that favor frequent and strengthened heat waves. Further projection analysis of simulations from 13 CMIP5 climate models suggests that Europe may experience more hot summers as the ASIC and EASC continue to decline over the next century.

The modulation of the intensity of nascent Tibetan Plateau vortices (ITPV) by atmospheric quasi-biweekly oscillation (QBWO) is investigated based on final operational global analysis data from the National Centers for Environmental Prediction. The spatial and temporal distributions of the ITPV show distinct features of 10–20-day QBWO. The average ITPV is much higher in the positive phases than in the negative phases, and the number of strong TPVs is much larger in the former, with a peak that appears in phase 3. In addition, the maximum centers of the ITPV stretch eastward in the positive phases, indicating periodic variations in the locations where strong TPVs are generated. The large-scale circulations and related thermodynamic fields are discussed to investigate the mechanism by which the 10–20-day QBWO modulates the ITPV. The atmospheric circulations and heating fields of the 10–20-day QBWO have a major impact on the ITPV. In the positive QBWO phases, the anomalous convergence at 500 hPa and divergence at 200 hPa are conducive to ascending motion. In addition, the convergence centers of the water vapor and the atmospheric unstable stratification are found in the positive QBWO phases and move eastward. Correspondingly, condensational latent heat is released and shifts eastward with the heating centers located at 400 hPa, which favors a higher ITPV by depressing the isobaric surface at 500 hPa. All of the dynamic and thermodynamic conditions in the positive QBWO phases are conducive to the generation of stronger TPVs and their eastward expansion.

Based on the Final operational global analysis data from the Global Forecasting System of the National Centers for Environment Prediction and the radiosonde data, the development and eastward movement mechanisms of 15 Tibetan Plateau vortices (TPVs) after they move off the plateau are investigated. The results show that the convergence to the east of the TPVs at 500 hPa, the divergence associated with the westerly jet stream at 200 hPa, as well as the corresponding ascending motion provide favorable conditions for the development and eastward movement of the TPVs. The spatial structures of the atmospheric apparent heat source (Q1) and the apparent moisture sink (Q2) are studied, showing that the heating centers of Q1 at 400 hPa mainly sourced from the condensation latent heat are beneficial to the eastward movement of the TPVs, while the horizontal distribution of Q1 at 500 hPa goes against that. The development and eastward movement mechanisms of the TPVs after they move off the plateau are further discussed through diagnosing the potential vorticity (PV) tendency equation. It is revealed that the horizontal PV flux convergence to the east of the TPVs related to the convergence at 500 hPa plays as the dominant role, exerting positive contribution to the PV tendency. Meanwhile, the heating fields induce feeble PV tendency, indicating more important effect of the dynamic factor. The development and eastward movement mechanisms of the TPVs after they move off the plateau are different from those before the TPVs move off, and the dynamic effect is vital in the former stage while the effect of Q1 is revealed as the dominant influencing factor in the latter.

The rectification of intraseasonal wind forcing on interannual sea surface temperature anomalies (SSTA) and sea level anomalies (SLA) associated with El Nino–Southern Oscillation (ENSO) during 1993–2016 are investigated using the LICOM ocean general circulation model forced with daily winds. The comparisons of the experiments with and without the intraseasonal wind forcing have shown that the rectified interannual SSTA and SLA by the intraseasonal winds are much weaker than the total interannual SSTA and SLA in the cold tongue, due to the much weaker rectified than the total interannual Kelvin and Rossby waves in the equatorial Pacific Ocean. The dynamics of the rectification are through the nonlinear zonal and vertical advection by the background currents, which produces downwelling equatorial Kelvin waves during El Nino. The meridional advection is much smaller than the zonal and vertical advection, suggesting that the rectification is not induced by the Ekman dynamics or the thermocline rectification. The rectified interannual Kelvin waves are found to be much smaller than reflected at the Pacific western boundary and those forced by the interannual winds, suggesting that the latter two play a much more important role in ENSO dynamics than the intraseasonal winds. The results of this study suggest an unlikely significant role of oceanic nonlinear rectification by intraseasonal winds during the onset and cycling of El Nino.

Based on data diagnoses, in this study the interdecadal changes in asymmetric impacts of ENSO on wintertime East Asian climate are investigated. It is found that the Pacific Decadal Oscillation (PDO) can significantly modulate the asymmetry in the responses of the East Asian climate to El Nino and La Nina events. In positive PDO phase the response is asymmetric with a strong anomalous anticyclone over western North Pacific (WNP) and significant positive rainfall anomalies over southern China during El Nino winters, but a weak anomalous cyclone over WNP and insignificant negative rainfall anomalies over southern China during La Nina winters. However, such asymmetric responses do not appear in negative PDO phase, with comparable amplitudes of anomalous circulations over WNP and rainfall over southern China in El Nino and La Nina winters. Further analyses reveal that the warm background of the tropical Pacific in positive PDO phase causes significant difference in the amplitudes of convection anomalies over tropical western Pacific, resulting in asymmetric responses of the WNP circulation and rainfall over southern China to El Nino and La Nina events. Nevertheless, the cold background in negative PDO phase reduces the amplitude difference between El Nino and La Nina winters. A comparable convection anomalies appear in tropical western Pacific in El Nino and La Nina winters and the asymmetric responses do not occur.

The Sendai Framework for Disaster Risk Reduction 2015–2030 shifts the focus from managing disasters to reducing risks. Such a shift requires a better understanding of risk in all its dimensions of environment, hazards, exposure, and vulnerability; a disaster risk governance that ensures disaster risk is factored into planning and development at all levels across all sectors, as well as into disaster pre-paredness, rehabilitation, recovery, and reconstruction; and cost–benefit analyses to support the prioritization of investments in disaster risk reduction (DRR) for long-term resilience.

After the quick decaying of the 2015 super El Niño, the predicted La Niña unexpectedly failed to materialize to the anticipated standard in 2016. Diagnostic analyses, as well as numerical experiments, showed that this ENSO evolution of the 2015 super El Niño and the hindered 2016 La Niña may be essentially caused by sea surface temperature anomalies (SSTAs) in the subtropical Pacific. The self-sustaining SSTAs in the subtropical Pacific tend to weaken the trade winds during boreal spring–summer, leading to anomalous westerlies along the equatorial region over a period of more than one season. Such long-lasting wind anomalies provide an essential requirement for ENSO formation, particularly before a positive Bjerknes feedback is thoroughly built up between the oceanic and atmospheric states. Besides the 2015 super El Niño and the hindered La Niña in 2016, there were several other El Niño and La Niña events that cannot be explained only by the oceanic heat content in the equatorial Pacific. However, the questions related to those eccentric El Niño and La Niña events can be well explained by suitable SSTAs in the subtropical Pacific. Thus, the leading SSTAs in the subtropical Pacific can be treated as an independent indicator for ENSO prediction, on the basis of the oceanic heat content inherent in the equatorial region. Because ENSO events have become more uncertain under the background of global warming and the Pacific decadal oscillation during recent decades, thorough investigation of the role of the subtropical Pacific in ENSO formation is urgently needed.

The western North Pacific anomalous anticyclone (WNPAC) is an important atmospheric circulation system that conveys El Nino impact on East Asian climate. In this review paper, various theories on the formation and maintenance of the WNPAC, including warm pool atmosphere–ocean interaction, Indian Ocean capacitor, a combination mode that emphasizes nonlinear interaction between ENSO and annual cycle, moist enthalpy advection/Rossby wave modulation, and central Pacific SST forcing, are discussed. It is concluded that local atmosphere–ocean interaction and moist enthalpy advection/Rossby wave modulation mechanisms are essential for the initial development and maintenance of the WNPAC during El Nino mature winter and subsequent spring. The Indian Ocean capacitor mechanism does not contribute to the earlier development but helps maintain the WNPAC in El Nino decaying summer. The cold SST anomaly in the western North Pacific, although damped in the summer, also plays a role. An inter-basin atmosphere–ocean interaction across the Indo-Pacific warm pool emerges as a new mechanism in summer. In addition, the central Pacific cold SST anomaly may induce the WNPAC during rapid El Nino decaying/La Nina developing or La Nina persisting summer. The near-annual periods predicted by the combination mode theory are hardly detected from observations and thus do not contribute to the formation of the WNPAC. The tropical Atlantic may have a capacitor effect similar to the tropical Indian Ocean.

The relation of spring (March–May) to summer (July–August) precipitation in eastern China is examined using observed data. It is found that when spring precipitation from the lower and middle reaches of the Yangtze River valley to northern China (the YRNC region) is higher (lower), more (less) summer precipitation occurs in northeastern China and the lower and middle reaches of the Yangtze River valley, and less (more) in southeastern China. The analysis of physical mechanism showed that higher (lower) spring precipitation in the YRNC region is closely related to wet (dry) spring soil moisture, which decreases (increases) the surface temperature and sensible heat flux in late spring. Because the memory of spring soil moisture in the YRNC region reaches about 2.4 months, the surface thermal anomaly lasts into the subsequent summer, resulting in a weak (strong) East Asian summer monsoon. A weak East Asian summer monsoon corresponds to an anomalous anticyclone and a cyclone over southeastern and northeastern China, respectively, in the lower troposphere. The anomalous anticyclone depresses the summer precipitation in southeastern China, and the anomalous cyclone promotes precipitation over northeastern China. The abnormal northerly and southerly winds associated with the anomalous cyclone and anticyclone, respectively, converge in the lower and middle reaches of the Yangtze River valley, inducing more summer precipitation there.

In this study, the variations of annual land surface air temperature (SAT) over Eurasia and the northern part of Africa (0 degrees-180 degrees E, 0 degrees-90 degrees N) were investigated using monthly SAT data from the Climatic Research Unit, University of East Anglia for 1901-2014 and the simulations from the Geophysical Fluid Dynamics Laboratory coupled model. The observed results suggested that the SAT variations exhibited robust non-uniform spatial features at multi-time scales. For the variations in inter-annual to decadal time scales (IDV), the intensity generally increased from south to north, with the strongest intensity being around Siberia and four times that of the weakest intensity found around China. The IDV leading pattern showed a north-south dipole across 40 degrees N. The simulated results suggested that the north-south dipole and the northwards increase of the IDV were due to internal interactions within the complex nonlinear climate system, but the natural and greenhouse gas forcings could intensify the IDV. The warming trend of the SAT was generally homogeneous, but it showed distinctive multi-decadal fluctuations in different regions. The linear secular trends and robust multi-decadal variation around Siberia and China corresponded to the considerable acceleration and deceleration in the warming over the two regions, respectively. The warming around Siberia was mainly caused by greenhouse gases but its modulation due to natural forcing was also considerable because of the robust multi-decadal variations. Around China, the multi-decadal variation, contributed by the natural forcing, can explain more than half the variances in the warming. The warming trend around central Asia was intense and parabolic, and the multi-decadal variation over there was weak and showed few modulating effects.

In this study, the year-to-year variability of surface air temperature (SAT) over China in winter was investigated during 1961–2014. The results indicate that the year-to-year SAT variability can explain more than 30% of the SAT variance over most parts of China, with up to 60% variance over southern China, northeastern China and northwestern China. The leading pattern of year-to-year SAT exhibits homogeneous variability over most parts of China, except in small areas over the Tibetan Plateau and southwestern China. The circulation over the northern Pacific is a key factor of this homogeneous variability pattern. An anomalous anticyclonic circulation and weakening midlatitude westerly jet in the middle and high troposphere over the northern Pacific are associated with the homogeneous warmth over China. The second pattern shows a south–north dipole, with variability in northeastern China opposite to that south of 25°N in China and over the Tibetan Plateau. The south–north dipole pattern is part of a global year-to-year SAT anomaly pattern because it exhibits a significant relationship with the year-to-year SAT over large parts north of 50°N over the Eurasian landmass. The north-cold/south-warm pattern is accompanied by a significant weakening of the Arctic Oscillation. In comparison, the atmospheric circulation anomalies associated with the homogeneous variability pattern are much weaker than those with the south–north dipole pattern. The anomalous Indian Ocean dipole in the previous autumn and the snow cover around China in November are the two key causes of the homogeneous variability pattern. Many factors, such as the tropical central Pacific sea surface temperature (SST), stratospheric quasi-biennial oscillation (QBO), Okhotsk sea ice and western Siberia snow cover, can significantly influence the south–north dipole pattern. Compared to the tropical Pacific SST, the impact of the Indian Ocean SST on the winter SAT over China is much more important.

In this study, modulation of the atmospheric quasi-biweekly oscillation (QBWO) on diurnal variation of the occurrence frequency of Tibetan Plateau vortices (TPVs) during May–August of 2000–2009 was investigated. The diurnal variations of the occurrence frequency of the TPVs (OFTPVs) and the related dynamic and thermodynamic features in the positive and negative phases of QBWO were compared. In both the positive and negative phases, the OFTPVs reaches the maximum from evening to midnight (18–00 LT, LT indicates the local time), and minimum from early morning to noon (06–12 LT). At 18 LT, there is strongest convergence at 500 hPa and ascending motion, as well as the most abundant net water vapor budget over the Tibetan Plateau, which is in favor of the precipitation and the related condensation latent heat release, corresponding to the maximum of OFTPVs in 18–00 LT. On the contrary, in the early morning at 06 LT, the conditions are most unfavorable for genesis of TPVs in 06–12 LT. QBWO leads to stronger convergence at 500 hPa, ascending motion as well as more massive water vapor in the positive phases than those in the negative phases, resulting in larger numbers of TPVs occur in all of the four periods of a day (00–06 LT, 06–12 LT, 12–18 LT, and 18–00 LT) in the former. The TPVs generating from the early morning to noon (06–12 LT) are weaker and more sensitive and fragile to the disadvantageous background, while the TPVs occurring from evening to midnight (18–00 LT) are stronger and seem to be well tolerated, leading to more remarkable contrast between the OFTPVs in the negative and positive phases in 06–12 LT than in 18–00 LT.

SWVs are important summer rain-producing systems over southwestern China, and even wider areas in eastern China when they move eastward. Based on the final analyses data (FNL) of the Global Forecasting System of the National Centers for Environment Prediction, the role played by Tibetan Plateau vortices (TPVs) in the genesis processes of vortices in southwest China, referred to as southwest vortices (SWVs) is investigated. Since the TPVs and SWVs are most active in summer, the cases in May-August are studied. It is revealed that the TPVs moving off the Tibetan Plateau (moving-off TPVs) can exert significant effects on the genesis of SWVs through both dynamic and thermodynamic processes. The moving-off TPVs are favorable for the generation of SWVs through strengthening the cyclonic vorticity, convergence and ascending motion. Diagnoses of the potential vorticity budgets reveals that the condensational latent heat has the greatest contribution to the generation of SWVs. Analysis of the water vapor budget indicates that the water vapor is mainly transported from south of the genesis region of SWVs associated with strong southerlies. It is demonstrated that the southerlies and associated water vapor transport are another prominent factor affecting the genesis of the SWVs.

The spring snow water equivalent (SWE) over Eurasia plays an important role in East Asian and Indian monsoon rainfall. This study evaluates the seasonal prediction capability of NCEP Climate Forecast System version 2 (CFSv2) retrospective forecasts (1983–2010) for the Eurasian spring SWE. The results demonstrate that CFSv2 is able to represent the climatological distribution of the observed Eurasian spring SWE with a lead time of 1–3 months, with the maximum SWE occurring over western Siberia and Northeastern Europe. For a longer lead time, the SWE is exaggerated in CFSv2 because the start of snow ablation in CFSv2 lags behind that of the observation, and the simulated snowmelt rate is less than that in the observation. Generally, CFSv2 can simulate the interannual variations of the Eurasian spring SWE 1–5 months ahead of time but with an exaggerated magnitude. Additionally, the downtrend in CFSv2 is also overestimated. Because the initial conditions (ICs) are related to the corresponding simulation results significantly, the robust interannual variability and the downtrend in the ICs are most likely the causes for these biases. Moreover, CFSv2 exhibits a high potential predictability for the Eurasian spring SWE, especially the spring SWE over Siberia, with a lead time of 1–5 months. For forecasts with lead times longer than 5 months, the model predictability gradually decreases mainly due to the rapid decrease in the model signal.

Seasonal prediction of the East Asian (EA) winter monsoon (EAWM) is of great significance yet a challenging issue. In this study, three statistical seasonal prediction models for the EAWM are established using three leading modes of the Eurasian snow cover (ESC), the first leading mode of sea surface temperature (SST) and the four leading modes of the combination of the ESC and SST in preceding autumn, respectively. These leading modes are identified by the partial-least square (PLS) regression. The first PLS (PLS1) mode for the ESC features significantly anomalous snow cover in Siberia and Tibetan Plateau regions. The ESC second PLS (PLS2) mode corresponds to large areas of snow cover anomalies in the central Siberia, whereas the third PLS (PLS3) mode a meridional seesaw pattern of ESC. The SST PLS1 mode basically exhibits an El Nino-Southern Oscillation developing phase in equatorial eastern Pacific and significant SST anomalies in North Atlantic. A strong EAWM tends to emerge in a La Nina year concurrent with cold SST anomalies in the North Atlantic, and vice versa. After a 35-year training period (1967–2001), three PLS seasonal prediction models are constructed and the 11-year hindcast is performed for the period of 2002–2012, respectively. The PLS model based on combination of the autumn ESC and SST exhibits the best hindcast skill among the three models, its correlation coefficient between the observation and the hindcast reaching 0.86. This indicates that this physical-based PLS model may provide another practical tool for the EAWM. In addition, the relative contribution of the ESC and SST is also examined by assessing the hindcast skills of the other two PLS models constructed solely by the ESC or SST. Possible physical mechanisms are also discussed.

The relationship between cross-equatorial flow (CEF) during winter and spring and the South China Sea summer monsoon (SCSSM) onset is investigated by data diagnoses for the period of 1979–2013 and numerical experiments. The SCSSM onset is found to have a significant negative correlation with the CEF nearby the Philippines in preceding January, February and March prior to SCSSM onset. A strong CEF during January, February and March tends to be succeeded by an early onset of SCSSM, whereas a weak CEF is likely to be followed by a late onset. The CEF in preceding months links with the SCSSM onset through the subtropical high over South China Sea and western North Pacific in April. The subtropical high in April is weak and eastward shifted (strong and westward shifted) when the CEF in preceding months is strong (weak). The weak and eastward shifted (strong and westward shifted) subtropical high favours (does not favour) the convection developing (depressing) over South China Sea, resulting in an early (late) onset of SCSSM. The anomalies of CEF nearby the Philippines in preceding months and its close relationship with subsequent subtropical high are caused by the sea surface temperature anomalies (SSTA) and atmospheric heating source over tropical western Pacific. The difference of SSTA between south and north of equator in tropical western Pacific and the corresponding atmospheric diabatic heating result in the anomalies of CEF in January, February and March, and impact the activity of subtropical high in April. The positive SSTA in tropical western Pacific, combining with the wet and warm climatic background over northern South China Sea and coastal areas of Southern China in April are favourable for a cyclonic circulation anomaly occurring over northern South China Sea. As a result, the subtropical high is weak and eastward shifted, and the SCSSM onset is early.

In this study, the relationship between Eurasian spring snow decrement (SSD) and East Asian summer precipitation and related mechanisms were investigated using observational data and the Community Atmospheric Model, version 3.1 (CAM3.1). The results show that a west–east dipole pattern in Eurasian SSD anomalies, with a negative center located in the region between eastern Europe and the West Siberia Plain (EEWSP) and a positive center located around Baikal Lake (BL), is significantly associated with East Asian summer precipitation via triggering an anomalous midlatitude Eurasian wave train. Reduced SSD over EEWSP corresponds to anomalously dry local soil conditions from spring to the following summer, thereby increasing surface heat flux and near-surface temperatures. Similarly, the increase in SSD over BL is accompanied by anomalously low near-surface temperatures. The near-surface thermal anomalies cause an anomalous meridional temperature gradient, which intensifies the lower-level baroclinicity and causes an acceleration of the subtropical westerly jet stream, leading to an enhanced and maintained Eurasian wave train. Additionally, the atmospheric response to changed surface thermal conditions tends to simultaneously increase the local 1000–500-hPa thickness, which further enhances the Eurasian wave train. Consequently, significant wave activity flux anomalies spread from eastern Europe eastward to East Asia and significantly influence the summer precipitation over China, with more rainfall over northeastern China and the Yellow River valley and less rainfall over Inner Mongolia and southern China.

This paper presents a review on the impact of El Nino on the interannual variability of atmospheric circulations over East Asia and rainfall in China through the anomalous anticyclone over western North Pacific (WNPAC). It explains the formation mechanisms of the WNPAC and physical processes by which the WNPAC affects the rainfall in China. During the mature phase of El Nino, the convective cooling anomalies over western tropical Pacific caused by the weakened convections trigger up an atmospheric Rossby wave response, resulting in the generation of the WNPAC. The WNPAC can persist from the winter when the El Nino is in its peak to subsequent summer, which is maintained by multiple factors including the sustained presence of convective cooling anomalies and the local air-sea interaction over western tropical Pacific, and the persistence of sea surface temperature anomalies (SSTA) in tropical Indian and tropical North Atlantic. The WNPAC can influence the atmospheric circulations over East Asia and rainfall in China not only simultaneously, but also in the subsequent summer after an El Nino year, leading to more rainfall over southern China. The current paper also points out that significant anomalies of atmospheric circulations over East Asia and rainfall over southern China occur in El Nino winter but not in La Nina winter, suggesting that El Nino and La Nina have an asymmetric effect. Other issues, including the impact of El Nino diversity and its impact as well as the relations of the factors affecting the persistence of the WNPAC with summer rainfall anomalies in China, are also discussed. At the end of this paper some issues calling for further investigation are discussed.

In the present study, the relationship between the atmospheric quasi-biweekly oscillation (QBWO) and Tibetan Plateau vortices (TPVs) moving off the Tibetan Plateau was investigated based on the radiosonde and reanalysis data. It is found that the number of TPVs moving off the Tibetan Plateau (moving-off TPVs) has the distinct feature of the 10–20-day QBWO. 77% of the moving-off TPVs occur in the positive phases of the 10–20-day filtered 500 hPa vorticity over eastern Tibetan Plateau. Besides, distributions of the zonal and meridional components of E-vectors coincide well with the trajectories of TPVs, indicating the moving-off TPVs are well related with the propagation of the QBWO energy. The atmospheric circulations and related thermodynamic fields are discussed to reveal the mechanism of the effect of 10–20-day QBWO on the moving-off TPVs. It is found that the atmospheric circulations and heating fields of 10–20-day QBWO have major impact on the moving-off TPVs. In positive QBWO phases, at 500 hPa over eastern Tibetan Plateau, there appear negative geopotential height anomalies and anomalous cyclonic wind shear; the anomalous jet stream and positive geopotential heights at 200 hPa lying over the northeast of the Tibetan Plateau stretch eastward gradually, benefiting for the upper level divergence and ascending motion. The condensation latent heat is released and shifts eastward with the heating centers located at 400 hPa, which depresses the isobaric surface at 500 hPa. All these conditions are in favor of the maintenance and eastward movement of TPVs in the positive QBWO phases.

Multi-pollutants in smoke particulate matter (SPM) were identified and quantified for the biomass burning of five major agricultural residues (wheat, rice, corn, cotton, and soybean straw) in China by an aerosol chamber system combined with various measurement techniques. The primary emission factors (EFs) for PM1. 0 and PM2. 5 are 3.04–12.64 and 3.25–15.16 g kg−1. Organic carbon (OC), elemental carbon (EC), water-soluble inorganics (WSIs), water-soluble organic acids (WSOAs), water-soluble amine salts (WSAs), trace mineral elements (THMs), polycyclic aromatic hydrocarbons (PAHs), and phenols in smoke PM1. 0/PM2. 5 are 1.34–6.04/1.54–7.42, 0.58–2.08/0.61–2.18, 0.51–3.52/0.52–3.81, 0.13–0.64/0.14–0.77, (4.39–85.72/4.51–104.79) × 10−3, (11.8-51.1/14.0-131.6) × 10−3, (1.1–4.0/1.8–8.3) × 10−3, and (7.7–23.5/9.7–41.5) × 10−3 g kg−1, respectively. Black carbon (BC) mainly exists in PM1. 0; heavy-metal-bearing particles favour residing in the range of smoke PM1. 0−2. 5, which is also confirmed by individual particle analysis. With respect to the five scenarios of burning activities, the average emissions and overall propagation of uncertainties at the 95 % confidence interval (CI) of SPM from agricultural open burning in China in 2012 were estimated to be 1005.7 (−24.6, 33.7 %), 901.4 (−24.4, 33.5 %), 432.4 (−24.2, 33.5 %), 134.2 (−24., 34.0 %), 249.8 (−25.4, 34.9 %), 25.1 (−33.3, 41.4 %), 5.8 (−30.1, 38.5 %), 8.7 (−26.6, 35.6 %), 0.5 (−26.0, 34.9 %), and 2.7 (−26.1, 35.1 %) Gg for PM2. 5, PM1. 0, OC, EC, WSI, WSOA, WSA, THM, PAHs, and phenols , respectively. The emissions were further spatio-temporally characterized using a geographic information system (GIS) in different regions in the summer and autumn post-harvest periods. It was found that less than 25 % of the total emissions were released during the summer harvest, which was mainly contributed by the North Plain and the centre of China, especially Henan, Shandong, and Anhui, which are the top three provinces regarding smoke particle emissions. Flux concentrations of primarily emitted smoke PM2. 5 that were calculated using the box-model method based on five versions of emission inventories all exceed the carcinogenic-risk permissible exposure limits (PEL). The health impacts and health-related economic losses from the smoke PM2. 5 short-term exposure were assessed. The results show that China suffered from 7836 cases (95,% CI: 3232, 12362) of premature mortality and 7 267 237 cases (95 % CI: 2 961 487, 1 130 784) of chronic bronchitis in 2012, which led to losses of USD 8822.4 million (95 % CI: 3574.4, 13 034.2) or 0.1 % of the total GDP. We suggest that the percentage of open-burnt crop straw in the post-harvest period should be cut down by over 97 % to ensure a reduction in carcinogenicity risk, especially in the North Plain and the northeast, where the emissions should decrease at least by 94 % to meet the PEL. With such emission control, over 92 % of the mortality and morbidity attributed to agricultural fire smoke PM2. 5 can be avoided in China.

An interannual variability mode in the southeast Pacific with a physical interpretation similar to that of the Pacific meridional mode (PMM) in the North Pacific was recently identified. Both modes have been shown to influence the subsequent development of El Niño–Southern Oscillation (ENSO) events. This study investigates the relationship between ENSO and the two PMMs using observational and reanalysis data. The results show that the South Pacific meridional mode (SPMM) mainly favors the development of sea surface temperature anomalies (SSTAs) in the eastern equatorial Pacific, whereas the North Pacific meridional mode (NPMM) mainly favors the development of SSTAs in the central equatorial Pacific. Both of the meridional modes are considered to be analogous in terms of their physical interpretation and can be important predictors of ENSO when considering different flavors of ENSO. Neither the NPMM nor the SPMM can be precluded as accurate indicators when forecasting particular flavors of ENSO.

Diagnostic analyses are performed to investigate the relationship between the upper-level Asian westerly jet stream (AWJS) and the associated rainfall pattern over the AWJS region in boreal summer on interannual time scales. Results show an out-of-phase rainfall variation over central Asia (CA) and north China (NC), which is closely related to the southeast–northwest (SE–NW) shift of the AWJS. The physical mechanisms on the relationship between the AWJS and the rainfall pattern are revealed by exploring the effects of the Indian summer monsoon (ISM) and the South Asian high (SAH). It is found that the SE–NW shifts of the AWJS and SAH associated with the ISM lead to the anomalous circulations over the midlatitudes in the AWJS region and cause rainfall anomalies over CA and NC. A weak ISM results in a southeastward shift of SAH, which is responsible for a southeastward shift of the AWJS. The anomalous atmospheric circulation associated with the southeastward located SAH produces anomalous updrafts (downdrafts) over the western (eastern) AWJS region, resulting in increased rainfall over CA and decreased rainfall over NC. It is proposed that the upper-level system SAH plays a crucial role in the teleconnection among the summer rainfall over the midlatitude AWJS region and ISM region.

The seafloor has a significant influence on sound propagation in deep water in the context of an incomplete sound channel. Using data collected during an experiment in the South China Sea, we study seafloor reflection effects on sound propagation in a range dependent environment. The experiment phenomenon is different from the convergence phenomenon in the deep sound channel, also referred to as the SOFAR channel. We observe that the spatial variation of bathymetry contributes to the formation of the seafloor reflection convergence zone in advance, and the sound intensity in some areas of the shadow zone is significantly increased. Due to seafloor reflection, there are two seafloor reflection convergence zone within 60 km of the propagation track, in which the gains of the acoustic energy can exceed 10 dB. High sound intensity areas are also observed in the shadow zone near 11 km and 51 km at some depths. In addition, the gain of the second convergence zone is higher than that of the first convergence zone when the receiving depth is the same as the source depth. In the first convergence zone, as the receiving depth increases, the arrival structure tends to become complicated, and the multipath effect becomes more obvious. Numerical analysis based on the parabolic equation and ray theory is carried out to explain the physical mechanism of the seafloor reflection convergence zone. The study result is meaningful for the performance analysis of sonar in complex deep-water environments.*

As important parameters in the land–atmosphere system, both soil moisture (SM) and vegetation play a significant role in land–atmosphere interactions. Using observational data from clay and sand stations over central eastern China, the relationship between leaf area index (LAI) and SM (LAI–SM) in different types of soil was investigated. The results show that the LAI–SM correlation is significantly positive in clay but not significant in sand. The physical causes for the discrepant LAI–SM correlations in different types of soil were explored from the perspectives of evapotranspiration (ET) and soil water retention. In clay stations, increasing LAI is associated with greater soil-water-retention capacity. Although the increasing LAI corresponds to increasing ET, the impact of ET on SM is weak because of the small particle size of soil. Consequently, the LAI–SM relationship in clay is significantly positive. In sand stations, ET is negatively correlated with SM owing to the large soil particle size, resulting in a negative LAI–SM correlation in sand. However, soil water retention is weakened by the increased LAI, which may be an important factor causing the insignificant LAI–SM correlation in sand.

The sensitivity of the East Asian summer monsoon to soil moisture anomalies over China was investigated based on ensembles of seasonal simulations (March–September) using the NCEP GCM coupled with the Simplified Simple Biosphere Model (NCEP GCM/SSiB). After a control experiment with free-running soil moisture, two ensembles were performed in which the soil moisture over the vast region from the lower and middle reaches of the Yangtze River valley to North China (YRNC) was double and half that in the control, with the maximum less than the field capacity. The simulation results showed significant sensitivity of the East Asian summer monsoon to wet soil in YRNC. The wetter soil was associated with increased surface latent heat flux and reduced surface sensible heat flux. In turn, these changes resulted in a wetter and colder local land surface and reduced land–sea temperature gradients, corresponding to a weakened East Asian monsoon circulation in an anomalous anticyclone over southeastern China, and a strengthened East Asian trough southward over Northeast China. Consequently, less precipitation appeared over southeastern China and North China and more rainfall over Northeast China. The weakened monsoon circulation and strengthened East Asian trough was accompanied by the convergence of abnormal northerly and southerly flow over the Yangtze River valley, resulting in more rainfall in this region. In the drier soil experiments, less precipitation appeared over YRNC. The East Asian monsoon circulation seems to show little sensitivity to dry soil anomalies in NCEP GCM/SSiB.

Using observational data for 1979–2010, the temporal and spatial variations of wintertime (December–February) snow water equivalent (SWE) over China and their relations to the November SWE, wintertime snowfall, snow evaporation, sublimation and melting are investigated. The results revealed that the SWE is principally distributed in four snow regions of northwestern (NW) China, northeastern (NE) China, the Tibetan Plateau (TP) and the Yangtze–Huaihe River valley (YH). It is found that the impact of the November SWE is confined to the north of 42°N, while the impact of snowfall are relevant over all China, and the impacts of snow evaporation and sublimation are limited to NW, NE and the TP. The effect of snow melting is large in the YH but weak in other regions. On the inter-annual timescale, the snow mass balance shows different features in different snow regions. The wintertime SWE is in line with snowfall. The November SWE has a major effect on the wintertime SWE in NW and NE but not in the TP and YH. The effect of meteorological factors on snow evaporation, sublimation and melting differs in different snow regions. Evaporation is mainly affected by vapour pressure deficit in NW and wind speeds in NE and the TP. Sublimation is controlled by air temperature in NW, wind speeds in NE, vapour pressure deficit in YH and air temperature, pressure and vapour pressure deficit in the TP. Melting is closely related with air temperature in the TP and net radiation in the YH.

This study examined the relationship between the western North Pacific monsoon (WNPM) and the tropical cyclone (TC) activity over western North Pacific during the summer (June to August) in a period from 1977 to 2013. According to the western North Pacific monsoon index (WNPMI), 10 years with high indices (referred to as the positive WNPMI phase) and 10 years with low indices (referred to as the negative WNPMI phase) were chosen. TCs frequently formed in the southeast quadrant of the subtropical western North Pacific in the positive WNPMI phase, and in the northwest quadrant in the negative phase. With respect to the TC track, TCs tended to move northward from the distant sea off the east coast of the Philippines towards Korea and Japan past the East China Sea in the positive WNPMI phase, whereas the characteristic pattern of the negative phase was the TC movement from the Philippines to southern China and the Indochinese peninsula over the South China Sea. As for the TC intensity, the TCs were stronger in the positive WNPMI phase than in the negative WNPMI phase. The anomalous cyclonic circulations in the south of the 30°N signified strong convection, causing the high TC genesis frequency in the positive WNPMI phase. In addition, the East China Sea, Korea and Japan were shown to be influenced by the anomalous southeasterly, which contributed as the anomalous steering flows and was favourable for the TCs to move into these regions in the positive WNPMI phase. Meanwhile, the positive anomaly of 600 hPa relative humidity and sea surface temperature as well as the negative anomaly of vertical wind shear presented in most regions of the subtropical western North Pacific, creating an environment that facilitated the generation of TCs in the positive WNPMI phase.

Tibetan Plateau vortices (TPVs), generated over the Tibetan Plateau, are important rainfall triggers in southwestern and eastern China when moving off the plateau. The heavy rainfall event that occurred over southwestern China in early July 2018 caused severe floods and landslides, leading to substantial damage to normal production and life. In this work, the roles of TPVs in the rainfall are investigated based on NCEP FNL (final) operational global analysis and forecast data from the Global Data Assimilation System (GDAS). The results show that TPVs over the eastern Tibetan Plateau favor ascending motion to the east of the plateau, which is conducive to precipitation and therefore the genesis of southwest vortices (SWVs). Generally, the ascending motion in southwestern China is stronger when TPVs exist, compared with the other scenarios in which the TPVs are absent. Ascending motion is always observed on the east side of TPVs, which is greatly attributed to the positive vertical vorticity and warm centers in the TPVs. In this fashion, the influence of TPVs is not merely limited around their centers, but further eastward. Therefore, the effects of TPVs may have been underestimated in previous investigations; the TPVs located over the eastern Tibetan Plateau that do not move off the plateau should also be seriously considered in future precipitation predictions.

In this study the interdecadal variations of haze pollution during recent 34 years from 1980 to 2013 over eastern China are investigated. It is found that opposite interdecadal variations of wintertime haze days (WHDs) between the North China Plain region (NCP) and the Yangtze River Delta region (YRD) occurred in this period, i.e., a decreased interdecadal trend of anomalous WHDs appeared in NCP and an increased trend in YRD. Meteorological factors driven the opposite interdecadal variations of anomalous WHDs are revealed. In the phase of interdecadal variations with more WHDs in NCP and less in YRD before 1996, there were weaker northerly winds in the lower troposphere over eastern China accompanied with a weakened Siberia High and mid-latitude westerly jet. The decreased northerly winds are conductive to the local accumulation of air pollutants in NCP and inhibition of air pollutants to be transported from North China to YRD. Consequently, more WHDs appeared in YRD and less in NCP. On the contrary, in the phase of interdecadal variations with less WHDs in NCP and more in YRD after 1996, stronger northerly winds associated with strengthened Siberia High and westerly jet, appeared over eastern China. The increased northerly winds are favorable for the dispersion of air pollutants in NCP and transport more pollutants from North China to the downstream area in YRD, resulting in less WHDs in NCP and more in YRD.