Understanding the variability in aerosol concentrations (ACs) over China is a scientific challenge and is of practical importance. The present study explored the month-to-month variability in ACs over China based on simulations of an atmospheric chemical transport model with a fixed emissions level. The month-to-month variability in ACs over China is dominated by two principal modes: the first leading monopole mode and the second meridional dipole mode. The monopole mode mainly indicates enhanced ACs over eastern China, and the dipole mode displays a south–north out-of-phase pattern. The two leading modes are associated with different climatic systems. The monopole mode relates to the 3 months leading the El Niño–Southern Oscillation (ENSO), while the dipole mode connects with the simultaneous variation in the North Atlantic Oscillation (NAO) or the Northern Hemisphere Annular Mode (NAM). The associated anomalous dynamic and thermal impacts of the two climatic variabilities are examined to explain their contributions to the formation of the two modes. For the monopole mode, the preceding ENSO is associated with anomalous convergence, decreased planetary boundary layer height (PBLH), and negative temperature anomalies over eastern China, which are unfavorable for emissions. For the dipole mode, the positive NAO is accompanied by opposite anomalies in the convergence, PBLH, and temperature over southern and northern China, paralleling the spatial formation of the mode. This result suggests that the variations originating from the tropical Pacific and extratropical atmospheric systems contribute to the dominant variabilities of ACs over China.

1. College of Global Change and Earth System Science, Beijing Normal University, Beijing, 8 China 9 2. Key Laboratory of Physical Oceanography–Institute for Advanced Ocean Studies, Ocean 10 University of China and Qingdao National Laboratory for Marine Science and Technology, 11 Qingdao 266003, China 12 3. School of Environmental Science and Engineering, Nanjing University of Information Science 13 & Technology, Nanjing, China 14 4. China-ASEAN Environmental Cooperation Center, Beijing, China 15

The manuscript presents analysis of the impacts of NAO and El Niño on the anthropogenic aerosols in China. It uses mostly GEOS-Chem model simulations driven by GEOS-4 reanalysis. Understanding the changes in aerosols is a relevant topic for improving our knowledge of relationship between natural cycle and aerosols. Model simulation show the circulation anomalies during the co-occurrence events of negative NAO and El Niño, and therefore influence on aerosol concentrations over eastern China. However, a sole negative NAO is linked with anomalous aerosols over central China. Overall the manuscript is well written and clear, the figures are also appropriate and clear. After addressing the following minor concerns, I suggest publishing this work.

The high aerosol concentration (AC) over eastern China has attracted attention from both science and society. Based on the simulations of a chemical transport model using a fixed emissions level, the possible impact of the previous autumn North Atlantic Oscillation (NAO) combined with the simultaneous El Niño–Southern Oscillation (ENSO) on the boreal winter AC over eastern China is investigated. We find that the NAO only manifests its negative impacts on the AC during its negative phase over central China, and a significant positive influence on the distribution of AC is observed over south China only during the warm events of ENSO. The impact of the previous NAO on the AC occurs via an anomalous sea surface temperature tripole pattern by which a teleconnection wave train is induced that results in anomalous convergence over central China. In contrast, the occurrence of ENSO events may induce an anomalous shift in the western Pacific subtropical high and result in anomalous southwesterlies over south China. The anomalous circulations associated with a negative NAO and El Niño are not favorable for the transport of AC and correspond to worsening air conditions over central and south China. The results highlight the fact that the combined effects of tropical and extratropical systems play a considerable role in affecting the boreal winter AC over eastern China.

In Eastern Asia, two categories of cold air outbreaks, ordinary type and extensive and persistent type, usually occur during boreal winter, with obviously different circulation backgrounds. Blocking highs, which play an important role in cold events, are closely related to the evolution of transient eddies in their dynamical mechanism. This paper investigates the characteristics of thermal-dynamical transport of transient eddies associated with blocking events in two cold air events: a low-temperature, snowfall, and freezing event in early 2008 and a cold wave in January 2016. The results suggest that there is a convergence of transient eddy momentum and heat transport around blocking high locations during both cold events, which are beneficial for establishing and maintaining the blocking events. Moreover, the convergence of transient eddy momentum and heat transport is stronger in the 2016 event than in the 2008 event, with a northward shift of the convergence centers. Further analysis shows that the blocking high location is controlled by the convergence of Eliassen-Palm (E-P) flux in both events, which contributes to the westerly wind deceleration and provides a beneficial background for the development of blocking events. Moreover, the anticyclonic Rossby wave breaking (RWB) event occurs two days prior to the onset of blocking and weakens along with the collapse of blocking in both cold events; however, the RWB has a longer duration, stronger intensity, and more north-eastward location in the 2016 event than for the 2008 event. These results indicate that the effect of a synoptic-scale eddy is one important factor influencing different characteristics of the blocking highs and is related to different weather phenomena during the two cold events.

The response of the Hadley circulation (HC) to the sea surface temperature (SST) is determined by the meridional structure of SST and varies according to the changing nature of this meridional structure. The capability of the models from the phase 5 of the Coupled Model Intercomparison Project (CMIP5) is utilized to represent the contrast response of the HC to different meridional SST structures. To evaluate the responses, the variations of HC and SST were linearly decomposed into two components: the equatorially asymmetric (HEA for HC, and SEA for SST) and equatorially symmetric (HES for HC, and SES for SST) components. The result shows that the climatological features of HC and tropical SST (including the spatial structures and amplitude) are reasonably simulated in all the models. However, the response contrast of HC to different SST meridional structures shows uncertainties among models. This may be due to the fact that the long-term temporal variabilities of HEA, HES, and SEA are limited reproduced in the models, although the spatial structures of their long-term variabilities are relatively reasonably simulated. These results indicate that the performance of the CMIP5 models to simulate long-term temporal variability of different meridional SST structures and related HC variations plays a fundamental role in the successful reproduction of the response of HC to different meridional SST structures.

The relationship of the Hadley circulation (HC) to different tropical sea surface temperature (SST) meridional structures during boreal summer is investigated over the period of 1979–2016. After decomposing the variations of the HC into the equatorially asymmetric HC (HEA), zonal-mean equatorially asymmetric SST (SEA), equatorially symmetric HC (HES), and equatorially symmetric SST (SES) components, the ratio of the HEA associated with SEA with respect to the HES associated with SES is around 2 across multiple reanalyses, which is a smaller ratio than in the annual and seasonal cycle. The reduced ratio of the HC to SST is due to the regional SST variation in the Asian summer monsoon (ASM) domain. The first leading mode (EOF1) of the regional SST variability in the ASM domain is dominated by a homogeneous warming pattern. This pattern is associated with an equatorially asymmetric HC, but it has an opposite direction to the climatological HEA and so weakens the HEA. The second dominant mode has an El Niño–like pattern, which resembles the distribution of the principal mode of the SST in the non-ASM region. Both modes are responsible for the variation of HES. However, the SST EOF1 in the ASM domain displays a significant upward trend, favoring a suppressed HEA, and leading to the smaller ratio of the HC to SST during boreal summer. Moreover, the variation of the SST EOF1 is closely linked with the intensity of the ASM, highlighting the potential modulation by the ASM of the relation between the HC and SST during boreal summer.

Using the Goddard Earth Observing System (GEOS)-Chem model driven by GEOS-4 assimilated meteorological data, the potential influence of two La Nina Modoki events on aerosol concentrations over eastern China is explored. The results indicate that the impact of La Nina Modoki on aerosol concentrations differs between strong and moderate events. During the mature phase of the strong event of 1998/1999, an anomalous dipole pattern is observed over eastern China, i.e., with increased aerosol concentrations in the south and reduced concentrations in the north. The dipole pattern in the moderate event of 2000/2001 is reversed, with reduced aerosol concentrations in the south and increased concentrations in north China. Additionally, decreased aerosol concentrations are seen in spring of the decaying phase for the 1998/1999 event, while in the decaying spring of the 2000/2001 event, the dipole is reversed relative to the mature phase. During the decaying summer, aerosol concentrations are found to increase over eastern China in the 1998/1999 event but not in the 2000/2001 event. These anomalous aerosol concentrations are mainly caused by changes in circulation associated with the two La Nina Modoki events. In contrast, the role of wet deposition is observed to be limited during the lifespan of these two events. It is also found that the potential influence of the two La Nina Modoki events on aerosol concentrations can be up to 20% of the climatological mean. This suggests that La Nina Modoki has an important role in determining the distribution of aerosol concentrations over eastern China.

The climatological features of the pole-ward, equator-ward edges, and the width of the tropical Hadley Cell (HC) during the annual cycle in each hemisphere are studied based on six reanalyses datasets (i.e., NCEP/NCAR, NCEP-DOE, ERA40, JRA25, ERA-Interim, and CFSR). The result indicates that the amplitude of HC in the Northern Hemisphere (NH) is much intense than that in the Southern Hemisphere (SH). And the amplitude of equator-ward edge is much larger than that of the pole-ward edges in both hemispheres. In addition, it is found that the ERA40, CFSR (for the pole-ward edge of HC in the SH) and JRA25 (for the pole-ward edge of the HC in the NH) show big inconsistency comparing with the other datasets in depicting the locations of pole-ward edges of HC.

The potential influence of El Niño Modoki event 1994/1995 on tropospheric aerosol concentrations was investigated for southern China using the GEOS‐Chem model with meteorological data collected from 1986 to 2006. The results suggest that aerosol concentrations increase during mature phases of El Niño Modoki event 1994/1995 (boreal winter) and decrease during decay phases (boreal spring, summer, and autumn). Aerosol concentrations anomalies were found to be due to circulation changes associated with El Niño Modoki, whereby both the horizontal and vertical transport of aerosol mass fluxes determined the pattern of aerosol concentrations associated with the event. In contrast, the role of wet deposition appeared to be limited. Furthermore, these results suggest that the amplitude of anomalous aerosol concentrations associated with El Niño Modoki event 1994/1995 have the potential to reach approximately 30% of the climatological mean, indicating that El Niño Modoki plays an important role in influencing aerosol concentrations over southern China.

The influence of El Niño on aerosol concentrations is investigated for eastern China over period 1986–2006. Result suggests that the influence of El Niño on aerosol concentration differs between events. The 1987/1988 El Niño event decreased the aerosol concentrations during the mature and decay spring of the event; however, an anomalous northern‐increase and southern‐decrease dipole structure occurred during the decay summertime over eastern China. In contrast, the event of 1997/1998 was associated with an evident increase in aerosol concentration over its lifespan. These anomalous aerosol concentrations are mainly caused by circulation changes associated with El Niño events.

The tropical Hadley circulation (HC) plays an important role in influencing the climate in the tropics and extra-tropics. The realism of the climatological characteristics, spatial structure, and temporal evolution of the long-term variation of the principal mode of the annual mean HC (i.e., the equatorially asymmetric mode, EAM) was examined in model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The results showed that all the models are moderately successful in capturing the HC’s climatological features, including the spatial pattern, meridional extent, and intensity, but not the spatial or temporal variation of the EAM. The possible reasons for the poor simulation of the long-term variability of the EAM were explored. None of the models can successfully capture the differences in the warming rate between the tropical Southern Hemisphere (SH) and Northern Hemisphere (NH), which is considered to be an important driver for the variation of the AM. Most of the models produce a faster warming in the NH than in the SH, which is the reverse of the observed trend. This leads to a reversed trend in the meridional gradient between the SH and NH, and contributes to the poor simulation of EAM variability. Thus, this aspect of the models should be improved to provide better simulations of the variability of the HC. This study suggests a possible reason for the poor simulation of the HC, which may be helpful for improving the skill of the CMIP5 models in the future.

This study examines the relationships among the monsoon-like southwest Australian circulation (SWAC), the Southern Annular Mode (SAM), and southwest Western Australia winter rainfall (SWR), based on observed rainfall, reanalysis datasets, and the results of numerical modeling. By decomposing the SWAC into two components using a linear model, i.e. the component related to SAM (RSAM) and the component unrelated to SAM (SWACI*), we find it is the SWACI* that shows a significant influence on SWR. Similarly, it is the component of SAM associated with SWAC that exhibits an impact on SWR, whereas the component unrelated to SAM. A similar result is obtained in terms of the circulation associated with SWAC and the SAM. These facts suggest the SAM plays an indirect role in influencing SWR, and raise the possibility that SWAC acts as a bridge between the SAM and SWR, by which the SAM passes its influences onto SWR. This is due to the fact that the variations of SWAC are closely linked to the thermal contrast between land and sea across the southern Indian Ocean and southwest Australia. By contrast, the SAM does not significantly relate to this thermal structure, particularly for the component unrelated to SWAC. The variations of surface sea temperature over the southern Indian Ocean contribute to the favored rainfall circulation patterns. This finding is supported by the numerical modeling results. The strong coupling between SWAC and SWR may be instrumental for understanding the interactions between SWR and the southern Indian Ocean, and provides another perspective in examining the variations in SWR.

China has been experiencing increased concentrations of aerosols, commonly attributed to the large increases in emissions associated with the rapid economic development. We show by using a chemical transport model driven by the assimilated meteorological fields that the observed decadal‐scale weakening of the East Asian summer monsoon also contributed to the increases in aerosols in China. We find that the simulated aerosol concentrations have strong negative correlations with the strength of the East Asian Summer monsoon. Accounting for sulfate, nitrate, ammonium, black carbon, and organic carbon aerosols, the summer surface‐layer PM 2.5 concentration averaged over eastern China (110°–125°E, 20°–45°N) can be 17.7% higher in the weakest monsoon years than in the strongest monsoon years. The weakening of the East Asian Summer monsoon increases aerosol concentrations mainly by the changes in atmospheric circulation (the convergence of air pollutants) in eastern China.

Based on the climatological daily mean NCEP/NCAR reanalysis data, NOAA outgoing longwave radiation (OLR) data, and pentad NOAA CMAP precipitation from 1979 to 2006, the variation of the western Pacific subtropical high (WPSH) ridge during late spring and early summer (LSES) and its relationship with the onset of the Asian summer monsoon is discussed from a climatological perspective. It is found that a remarkable southward retreat process (SRP) of the WPSH during LSES appears at both lower and higher levels of the troposphere, with a lifespan of approximate two weeks. Afterwards, the first northward jump of the WPSH occurs. The end date of the WPSH SRP in the upper troposphere is about 10 days earlier than the beginning of the WPSH SRP in the lower troposphere, showing a meaningful leading signal for predicting the WPSH SRP in the lower troposphere and the subsequent northward jump of the WPSH. The WPSH SRP at lower levels happens simultaneously with a notable eastward shift of the WPSH. After the WPSH SRP at lower levels comes to the southernmost position around the end of May, the WPSH ridge axis inclines northward rather than southward with altitude due to the change of the meridional gradient of air temperature.The Asian summer monsoon onset and associated variations in strong convection and rainfall in Asia are closely related to the variations of WPSH SRP during LSES. In the mid-late period of the higher-level WPSH SRP, around the end of April, the summer monsoon onset takes place in the Andaman Sea and the Bay of Bengal. Following the start of the lower-level WPSH SRP, the South China Sea (SCS) summer monsoon breaks out (May 14-15). By the end of the lower-level WPSH SRP, in the beginning of June, the Indian summer monsoon kicks off. Upon the end of the lower-level WPSH return stage, the East Asian summer monsoon begins. The commencement of each component of the Asian summer monsoon system corresponds nicely to a particular stage of the WPSH SRP in the lower or higher troposphere. This offers valuable information for monsoon onset prediction in different sectors of Asia.In addition, it is found that there is a typical wet-dry-wet sandwich precipitation pattern, with two rainfall belts in the regions south and north to the WPSH main body, and a dry belt under it. The variation of this rainfall pattern is related to the shift of the WPSH ridge.