Abstract. Ozone (O3) in the upper troposphere and lower stratosphere (UTLS) is strongly regulated by upper-level circulation dynamics. Understanding the coupling between UTLS O3 distribution and upper-level circulation dynamics is important not only to understand synoptic processes governing O3 distribution and variability, but also to test the fidelity of chemistry transport models in simulating the stratosphere–troposphere exchange (STE) processes. This study presents the first systematic assessment of observationally constrained UTLS O3 variability associated with upper-level circulation patterns over the Northeast Asia region. By applying the self-organized mapping (SOM) technique to 500, 250, and 100 hPa geopotential height (GPH) data, 12 circulation patterns are quantified and then used to characterize the UTLS O3 distribution in the period 2000–2020 in both four-site (Beijing, Pohang, Tateno, and Sapporo) ozonesonde data and regional-scale satellite products. The underlying dynamic transport mechanism responsible for UTLS O3 responses to different circulation patterns are further explored through correlation analysis between O3 anomalies and transport indicators. The results indicate that although O3 at almost all altitudes shows statistically significant sensitivity to circulation patterns, lower-stratospheric O3 exhibits a far stronger sensitivity when compared with upper-tropospheric O3. Circulation patterns featuring the East Asian Trough (EAT) show clear enhancement of O3 southwest of the trough, and the enhancement zone moves with the eastward propagation of the EAT. Circulation patterns featuring eastward-shedding vortices of the Asia Summer Monsoon Anticyclone (ASMA) show the opposite signal, in which O3 concentrations are decreased, especially at Sapporo, and the negative O3 anomaly zone stretches from South Japan to Sakhalin Island. Each circulation pattern is characterized by distinct transport pathways, which play a determining role in the pattern-specific UTLS O3 response. Positive O3 anomalies are usually associated with post-trough downward and southward transport, whereas negative O3 anomalies are commonly associated with fore-trough upward and northward transport. In the lower stratosphere, the correlation between O3 anomalies and transport indicators is significantly stronger than that in the upper troposphere, and the strongest correlation occurs in the lower stratosphere of Beijing.
The natural wetlands of the Tibetan Plateau (TP) are considered to be an important natural source of methane (CH4) to the atmosphere. The long-term variation in CH4 associated with climate change and wetland loss is still largely unknown. From 1950 to 2010, CH4 emissions over the TP were analyzed using a model framework that integrates CH4MODwetland, TOPMODEL, and TEM models. Our simulation revealed a total increase of 15% in CH4 fluxes, from 6.1 g m−2 year−1 to 7.0 g m−2 year−1. This change was primarily induced by increases in temperature and precipitation. Although climate change has accelerated CH4 fluxes, the total amount of regional CH4 emissions decreased by approximately 20% (0.06 Tg—i.e., from 0.28 Tg in the 1950s to 0.22 Tg in the 2000s), due to the loss of 1.41 million ha of wetland. Spatially, both CH4 fluxes and regional CH4 emissions showed a decreasing trend from the southeast to the northwest of the study area. Lower CH4 emissions occurred in the northwestern Plateau, while the highest emissions occurred in the eastern edge. Overall, our results highlighted the fact that wetland loss decreased the CH4 emissions by approximately 20%, even though climate change has accelerated the overall CH4 emission rates over the last six decades.
Based on four tree-ring chronologies which was analyscd with appropriate collection and accurate dating in the middle Tibetan Plateau,an essential procedure on reconstruction of past climate has been pointed out in this paper.First,the response function of each dendrochronology has been built and used to estimate how ring-width growth responds to variations in monthly climatic change.Second,the climate factors which could be produced with different tree-ring series have been selected.Then,the transfer function equation,including a new set of orthogonal variables,can be used to reconstruct local past temperature or precipitation.It should be emphasized that prior growth has been considered in the relationship between climate factor and tree-ring chronology,Besides,some different periods for calibration and verification have been divided.And some statistics and other kinds of proxy data have been adopted as test approaches.As a result,the variations of air temperature during the last 600 years and precipitation during the last 340 years were reconstructed by combining the same types of tree-ring series in the middle Tibet.
Abstract. Despite frequent foehns in the Beijing–Tianjin–Hebei (BTH) region, there are only a few studies of their effects on air pollution in this region, or elsewhere. Here, we discuss a foehn-induced haze front (HF) event using observational data to document its structure and evolution. Using a dense network of comprehensive measurements in the BTH region, our analyses indicate that the foehn played an important role in the formation of the HF with significant impacts on air pollution. Northerly warm–dry foehn winds, with low particulate concentration in the northern area, collided with a cold–wet polluted air mass to the south and formed an HF in the urban area. The HF, which is associated with a surface wind convergence line and distinct contrasts of temperature, humidity and pollutant concentrations, resulted in an explosive growth of particulate concentration. As the plain–mountain wind circulation was overpowered by the foehn, a weak pressure gradient due to the different air densities between air masses was the main factor forcing advances of the polluted air mass into the clean air mass, resulting in severe air pollution over the main urban areas. Our results show that the foehn can affect air pollution through two effects: direct wind transport of air pollutants, and altering the air mass properties to inhibit boundary layer growth and thus indirectly aggravating air pollution. This study highlights the need to further investigate the foehn and its impacts on air pollution in the BTH region.
West China Autumn Rain (WCAR), the last rainy season in China associated with the East Asian summer monsoon, is prone to flooding and secondary disasters and has profound impacts on the economic society. Thus, it is of great interest to understand its variability. Through an analysis with gridded rainfall data and reanalysis data, this study reveals that the variability of WCAR is positively correlated to the boreal summer (austral winter) sea ice concentration (SIC) in the Southern Indian and Pacific Ocean basins. Such a relationship is supported by the SIC-related changes in atmospheric circulations and water vapor transport. Corresponding to a high summer SIC, both the Lake Balkhash trough and the Asian low pressure are deepened, the East Asian jet shifts northward, and the water vapor transport to West China is enhanced during the boreal autumn, providing a favorable background for the increase of WCAR, and vice versa. The potential mechanism of how the boreal summer SIC affects the autumn atmospheric situation is further investigated. Results show that the atmospheric circulation in the high latitudes of the Southern Hemisphere possesses a seasonal persistence from the boreal summer to autumn due to the sea ice persistence. Thus, the boreal summer SIC-related anomalies in the upper-tropospheric winds over the Southern Indian and Pacific Ocean basins can persist to the following autumn and subsequently exert substantial influences on the Eurasian atmospheric circulations via two meridional teleconnections. The counterpart in the lower troposphere contributes to the water vapor transport toward West China. Certainly, this candidate mechanism is a preliminary explanation, and other processes may also act in their linkage.
Abstract. A rapid development of a severe air pollution event at Beijing, China at the end of November 2015 was investigated with observations collected during the Study of Urban Rainfall and Fog/Haze (SURF-15). The analyses indicate that the major pollution source associated with particulate matter of diameter 2.5 μm (PM2.5) was from south of Beijing. On the night of 29 November, the surface stable boundary layer (SBL) was formed northwest of Beijing due to the northwesterly wind downslope of the mountains surrounding Beijing. This relatively cold and less polluted air also diluted the surface air northwest of Beijing while in the southeast of Beijing, the PM2.5 concentration increased continuously through the transport of the surface southwest flow. Around the midnight, the wind above the SBL switched from northerly to southwesterly and transported the heavy polluted air over Beijing. As the daytime convective turbulent mixing developed in the morning of 30 November, turbulent mixing transported the upper polluted air downward, leading to the dramatic increase of the PM2.5 concentration in the urban area. Meanwhile, the daytime weakly northeast-east surface wind led to the horizontal transport of the high PM2.5 air westwards towards Beijing, which further enhanced the PM2.5 increase at Beijing. As a result of both turbulent mixing and advection with possible aerosol growth from secondary aerosol formation under the low wind and high humidity conditions, the PM2.5 concentration reached over 700 μg m−3 at Beijing by the end of 30 November. Contributions of the two transporting processes to the PM2.5 oscillations prior to this dramatic event were also analyzed.
A number of simulation studies show that the sea-breeze fronts (SBFs) that form at the Bohai Bay coast can penetrate far inland through the Beijing-Tianjin-Hebei urban agglomeration (BTH). However, there has been little observational evidence for this claim. Based on dense surface observation networks, we observed a SBF that penetrated 170 km inland from Bohai Bay, interacting with cities in the BTH. The SBF was retarded in urban areas compared to surrounding areas, leading to the front line bending in urban areas. The nocturnal surface air temperature of both urban and rural stations increased temporarily as the SBF passed. Urban roughness effects caused strong updrafts during the passage. Ceilometers observed that the strong updraft of the SBF lifted near-surface aerosols to form sea-breeze heads (SBHs), leading to higher SBHs in urban areas than in rural areas. The uplifted aerosols gradually formed thin aerosol plumes, and produced clouds about four hours after the SBF passed. Our observations will help to improve understanding of the interactions between inland SBFs and cities, and provide a basis for further research on the physical mechanisms of how the inland SBF affects aerosols, clouds, and precipitation in the BTH and elsewhere.
Changes of the winter North Atlantic Oscillation (NAO) variability in response to different climate forcings, and their possible causes, are decomposed and investigated using a set of atmosphere-only timeslice experiments forced by sea surface temperature (SST) from coupled runs. The results indicate that the effects of uniform SST warming and direct CO2 radiative forcing could enhance NAO variability, while SST pattern change could lead to large inter-model difference for model simulations. For the influences of uniform SST warming and the direct CO2 radiative effect, the most significant air temperature increases occur at mid-low latitudes instead of northern polar regions, which produces a greater meridional temperature gradient at mid-high latitudes, thus leading to enhanced westerly winds according to the thermal wind theory. The effects of uniform SST warming and CO2 direct radiative forcing could lead to intensification of winter NAO variability, although this result does not consider ocean-atmosphere coupling. The meridional temperature gradient decreases in most areas of the northern Atlantic under the forcing of SST pattern change, but with a larger inter-model uncertainty, which makes the change of winter NAO variability in response to SST pattern change an open issue.
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