Impact Of The NAO On The Dust And The Synoptic Causes In North China

China has always been suffered from dusty weather.In recent years,the disasters caused by dusty weather still cannot be ignored under the background of global warming.Previous studies have focused on the dust source areas,including Xinjiang and most of the northern parts of China,indicating the causes of dusty weather are complex in China.As a non-dust-source area,North China,a densely populated region and an important economic development center in China,is affected by dust hazards seriously.However,the exact causes of dusty weather in the region require further research.The North Atlantic Oscillation(NAO)has been confirmed to be closely related to weather and climate in China,but its mechanism for the formation and influence of regional dust events in China has been less explored.By using MERRA-2atmospheric aerosol reanalysis data,China National Meteorological Centre dust station data,ERA5 atmospheric reanalysis data and ERSSTv5 SST data during 1980-2018,this paper explores the climatic characteristics of dust aerosols in China,especially in North China.Then the relationship between NAO and dusty weather in North China is analyzed,and a seasonal prediction model is constructed for dust aerosol concentration.Finally,under the modulation of NAO,the role of transient eddies is investigated on the evolution of the dusty weather situation during the dust events.The main conclusions are as follows.(1)By using the dust station data and MERRA-2 atmospheric aerosol reanalysis data,the climatic characteristics and variations of dusty weather and dust aerosols in China are statistically analyzed.The results are shown that the two types of data sets are in good agreement,especially in the characteristics of spatiotemporal distribution of dust aerosol concentration and its variation situation.The dusty weather mainly shows obvious characteristics of seasonal distribution in the dust source areas in China(mainly including Xinjiang and Inner Mongolia).Among them,the frequency of dusty weather occurrence in the dust source area is higher in spring,and gradually is lower in summer and autumn,with the impact of dusty weather in winter slightly expands compared with that in autumn,and the dust aerosol concentration and its standard deviation show high values in spring and summer.In the non-dust-source area,the dusty weather in North China(30°N-40°N,105°E-120°E)demonstrates higher dust aerosol concentration and stronger inter-annual variability in spring.The results of EOF analysis manifest that the first mode in North China in spring accounted for 71.38%and 69.24% for the frequency of dusty weather occurrence and dust aerosol concentration,respectively.The spatial vector field is consistent throughout the region,that is,the frequency of dusty weather occurrence and dust aerosol concentration have a consistent trend of variation in North China in spring.By the wavelet analysis,it is shown that the spring frequency of dusty weather occurrence and dust aerosol concentration have a 2-3 and 5-7 years periodic signal from 1980 to 2018 in North China.(2)The correlation distribution between the spring dust aerosol concentration in North China and the previous height field of the lower,middle and upper tropospheric potentials in the North Atlantic demonstrates that the dipole structure has a significant north-south inverse variation,including positive in Iceland and negative in the Azores,indicating a typical NAO negative phase characteristics.Further analysis shows that the spring dust aerosol concentration in North China is significantly correlated with the previous NAO index,with the correlation coefficients of-0.38,indicating that the NAO has a predictive role for the occurrence of dust events in North China.By using the regional mean sea surface temperature anomalies of four high-related areas in the tropical Pacific associated with dust aerosol concentration in North China and the NAO in previous winter as predictors,the dust seasonal prediction model is established.Not only the important influence of NAO on dusty weather is verified,but also the prediction model demonstrates suitable results with a model fit rate of 0.69 and variance contribution of 47%,as well as the reliability and stability of the model are verified using cross validation.(3)The evolution of synoptic systems at various levels of the troposphere is studied by selecting 9 spring dust events in North China influenced by the strong negative NAO signal in the previous winter.During the dust events,the 200 h Pa zonal winds over the dust source area and North China gradually increase and diminish,respectively,which facilitates the occurrence of windy weather in the dust source area and the maintenance of dust aerosols arriving in North China through the action of vertical circulation.At 500 h Pa,the intensity of the Ural ridge is enhanced,with the establishment,strengthening and southward movement of the Siberian High(SH)and Mongolian Cyclone(MC)on the sea level pressure field.The northward airflow in front of the dust source area will guide the cold air at high latitudes southward,which is conducive to the uplift and transmission of dust aerosols to the downstream North China.It is also noted that the south side of North China is dominated by southerly winds,which have a certain blocking effect on the northward airflow carrying dust aerosols and are favorable for the outbreak of dust events.(4)Transient eddies play significant role in the development of the synoptic situation of dust events modulated by the NAO negative signal.During the dust events,the transient momentum(heat)flux over the dust source area is mainly convergence(divergence),while the temperature anomaly is manifested as positive south and negative north,the temperature gradient is enhanced,as well as the thermal wind is increased.Therefore,the wind speed above the dust source area will be enhanced under the dual effect of high-altitude momentum transmission and thermal wind,promoting the occurrence of the dust-uplifting.The transient eddy of momentum(heat)flux in the range of the Ural ridge manifests the characteristics of divergence(convergence),both of which are important for the establishment and development of the meridional circulation here,which is conducive to the development of the Ural ridge.Simultaneously,the changes in upstream transient eddy flux transport cause both energy and mass divergence in North China,resulting in diminishing winds during dust events,which will facilitate the maintenance of dust aerosols here and promote the outbreak of dust events.The dynamical mechanism of dust event formation is verified by the quasigeostrophic theory,and the role of transient waves in it is further explained.