Impacts Of Climate Anomalies On The Interannual Variability Of Frequency Of Autumnal Haze Days In The Beijing-Tianjin-Hebei Region And Associated Physical Mechanisms

In recent years,the haze weather in the Beijing–Tianjin–Hebei(BTH)region has been frequently occurred,and its negative effects have become more prominent.Haze is often accompanied with air pollution and low visibility,which is a real problem that hampers local socio-economic development and is therefore widely concerned by the government,academia and the public.The autumnal haze days in the Beijing–Tianjin–Hebei(AHD_BTH)are second only to the winter season,owning a very significant interannual variability as in winter.However,autumn is a transitional season from summer to winter.The autumn climate is more complex and variable,and studies on the physical mechanisms of how climate anomalies affect the haze frequency are fewer.As such,this paper systematically explores the physical mechanisms of how climate anomalies affect the interannual variability of frequency of AHD_BTH by using a variety of observational data and reanalysis data,with low-level meteorological elements,atmospheric circulations and associated external forcing factors as the main line.The key meteorological factors and circulation systems related to the interannual variability of AHD_BTH as well as external forcing signals and associated physical mechanisms are revealed.The main results are as follows:1)The near-surface meteorological factors significantly related to the interannual variability of the number of AHD_BTH are clarified.On the interannual timescale,the anomalous near-surface meteorological conditions lead to abnormal number of AHD_BTH.A higher AHD_BTH is related to the higher ground relative humidity and temperature and positive stable atmospheric stratification at lower levels,and also closely related to the low surface wind velocity and planetary boundary layer height(PBLH);and vice versa.2)The Northeast Asian anticyclonic anomaly(NEAACA for short)is revealed as the key circulation system regulating the interannual variability of the number of AHD_BTH.The NEAACA,which is centered over the Sea of Japan,is the key circulation system affecting the interannual variability of the frequency of AHD_BTH.The western flank of the NEAACA prevails with southerly wind anomalies in the middle and lower troposphere,indicating that the cold air is inactive and weakened(weakened surface northerly wind anomalies).As such,the surface wind velocity and PBLH are decreased,along with the enhanced relative humidity and atmospheric stratification at lower levels,which is conducive to the accumulation of pollutants and warm and humid water vapor in the BTH region.In such a scenario,the atmospheric stagnant days are more than normal,and the positive feedback loop effect of meteorological elements and atmospheric particulate matter in the near-surface layer in the BTH region is marked.Their synergistic effect creates environmental conditions conducive to the development of the haze weather,which can increase the frequency of autumnal haze weather.3)It is found that the autumnal sea surface temperature(SST)anomalies can modulated the concurrent NEAACA via triggering atmospheric teleconnections,which would remotely impact the variability of AHD_BTH.On the interannual timescale,the climate variability of the frequency of AHD_BTH is closely related to the quasi-barotropic Scandinavia–central Siberia–Western Pacific teleconnection pattern(SCSWP).The positive phase of the SCSWP helps to enhance the simultaneous NEAACA.Moreover,a higher AHD_BTH is closely correlated with the simultaneous SST warming in the subtropical North Atlantic(R1;22°–32°N,90°–40°W)and the tropical western North Pacific(R2;10°–30°N,108°–140°E).The SST warming in the R1 region can stimulate a zonal planetary-scale Rossby teleconnection wave train(SCSWP is an important component)that propagating downstream,which facilitates the development and strengthening of NEAACA;the SST warming in the R2 region can enhance in-situ ascending motion to reinforce the NEAACA through local meridional overturning circulation.4)It is unveiled that the October–November snowpack anomalies can modulated the concurrent NEAACA via triggering atmospheric teleconnections,which would remotely impact the variability of October–November haze days in the BTH region.Snow cover in the Eurasian and North American regions from October to November has significant impacts on the interannual variability in the frequency of concurrent haze days in the BTH region.The eastern Europe-Siberian plain(50°–60°N,40°–80°E)is a key snow-cover area in the Eurasia,where the localized positive snowpack anomalies could excite a large-scale zonal quasi-barotropic Rossby wavetrain from the area around the Scandinavia through the above key area to Northeast Asia,modulating the climate variability in the concurrent NEAACA,a key circulation system affecting the simultaneous frequency of haze days.Moreover,the interannual variability of frequency of haze days in the BTH region from October to November is significantly negatively correlated with snow cover in the south of Hudson Bay in North America(48°–52°N,105°–90°W),and positively correlated with snow cover in the south of the Great Lakes in North America(35°–40°N,100°–85°W).This dipole-pattern snow cover anomaly can also induce the wavetrain to remotely modulate the NEAACA,so that it can occur,develop and strengthen.5)The physical causes of the highest number of haze days in the autumn of October-November 2014 over the BTH region are elucidated.The NEAACA with stronger barotropic structure,which was centered over the northwest side of the Japan Sea,was the key modulating system.This NEAACA intensity is abnormally strong,and its spatial structure is very conducive to a higher number of concurrent haze days.Influenced by this NEAACA,the middle and lower layers of troposphere were controlled by anomalous southerly winds,and the cold air activity was weak.The BTH region,especially its southern portion,was dominated by lower wind speed,and higher atmospheric stability and humidity,which was beneficial to the localized accumulation of aerosolsmore,leading to quite frequent haze days in situ.The pronounced positive SST anomaly over the Northwest Atlantic SST,centering around areas adjoining Newfoundland to Bermuda(70°–50°W,30°–50°N),could modulate this key system by exciting a planetary-scale Rossby wavetrain.Moreover,the remote modulation of positive snow anomalies in key region of Eurasia may also be significant.Therefore,it can be deemed that the synergistic effects of the above-mentioned circulation and external forcing factors may be the important reason for this extreme haze frequency in October-November 2014.