Impact Of Boreal Autumn Antarctic Oscillation On Following Winter Wet-cold Weather In Southern China

On the background of global warming,extreme weather events occur frequently,such as freezing rain,heavy snowfall accompanied by extremely cold weather,which has a great impact on the national economy and people’s lives and property.It is a work of practical and theoretical innovation to take a deep understanding of the spatiotemporal variations and causes of such wet-cold weathers,as to improve their prediction and warning capability.Based on the combination of station observations,reanalysis data diagnosis and numerical simulation,we analyze the spatiotemporal distributions and circulation background of winter wet-cold weather in Southern China from 1980 to 2018 on the interannual timescale.It is found that there is a relationship between the preceding boreal autumn Antarctic oscillation（AAO）and the following winter wet-cold weather in Southern China.Furthermore,the modulations of southern extratropical sea surface temperature（SST）,Antarctic sea-ice and stratospheric atmosphere between AAO and wet-cold weather in southern China are analyzed,and the mechanisms of the across-equatorial and across-seasonal influence of the‘sea-ice-air’system in the southern hemisphere on the winter wet-cold weather in Southern China are revealed.The main conclusions are as follows:（1）Wet-cold weather usually occurs from January to February.In the winter when the wet-cold weather is strong（weak）,the Mongolian High is anomalous strengthened（weakened）,the near-surface cold air flow activity is strong（weak）,the South Branch Trough is deepened（weakened）,the moisture in the middle and lower troposphere is active（inactive）.Southern China is controlled by cyclonic（anticyclonic）anomalies in the middle troposphere,with anomalous convergence（divergence）in the lower layer and anomalous divergence（convergence）in the upper layer,when invaded by the strong（weak）moisture and the strong（weak）cold air flow.Correspondingly,the geopotential anomaly over the North Atlantic presents a dipole structure of positive（negative）in the north and negative（positive）in the south,which is similar to the negative（positive）phase of the North Atlantic Oscillation（hereafter refer to as NAO-like）,while the AAO in the southern hemisphere presents the negative（positive）phase.The winter wet-cold weather related AAO can be traced back to the early autumn from September to October.When the September-October AAO is strengthened（weakened）,the NAO-like is in positive（negative）phase,the positive（negative）potential anomaly dominates the middle latitudes of East Asia,the weakening（deepening）of the South Branch Trough and the weakening（strengthening）of the Mongolian High are established in the late January-February,unfavorable for the formation of wet-cold weather in Southern China.At the same time,the precipitation anomaly of the Inter-tropical Convergence Zone（ITCZ）in Atlantic-South America（West Pacific）is in the positive（negative）phase,and the South Atlantic and South Indian Ocean SST show a meridional gradient.They serve as the modulator for the impact of boreal autumn AAO on the following winter wet-cold weather in Southern China.（2）September-October AAO signal is stored in the underlying surface ocean through the thermal and dynamical process,triggering the SST dipole mode of the Southern Ocean（SOD）.Through the‘air-sea’interaction,SOD persists into late January-February,and affects the wet-cold weather in southern China through two processes:firstly,it regulates the local meridional-vertical circulation in the South Atlantic,affects the tropical-subtropical North Atlantic anomalous convective precipitation,and then affects the eastward propagational Rossby wave over North Atlantic-Eurasian,thus modulating the atmospheric circulation in East Asia.Secondly,SOD regulates the local meridional vertical circulation in the Southeast Indian Ocean and directly affects the vertical movement over South China and the Yangtze Huaihe River Basin.When SOD is strengthened（weakened）,the Ural Blocking High is strengthened（weakened）,the East Asian Trough is deepened（weakened）,the Mongolian High is strengthened（weakened）,the local Hadley circulation in East Asia is strengthened（weakened）,inducing（reducing）the southward invasion of cold air flow,reducing the northward invasion of moisture from the Bay of Bengal（inducing the northward invasion of moisture from West Pacific）,which is unfavorable（favorable）for the formation of wet-cold weather in Southern China.（3）Stratospheric process plays a role in the relationship between SOD and wet-cold weather.The strengthening（weakening）of January-February SOD corresponds to the development of the Quasi-biennial Oscillation of the tropical zonal wind in the middle and lower stratosphere(30-70 h Pa,mainly 50 h Pa,refer to as QBO₅₀)to the westerly（easterly）phase.The westerly（easterly）phase of QBO₅₀and the strengthened（weakened）SOD correspond to the increase（decrease）of the downward propagation of stratospheric planetary waves at northern high latitudes.When QBO₅₀develops to the westerly（easterly）phase,the Arctic polar vortex is deepened（weakened）,the South Branch Trough is deepened（weakened）,and the Mongolian High is strengthened（weakened）,which is favorable（unfavorable）for the formation of wet-cold weather in Southern China.Therefore,QBO₅₀plays a modulation role in the process of SOD affecting the wet-cold weather in Southern China.（4）The Antarctic Sea-ice Dipole Pattern（ADP）also plays a role in modulating the impact of AAO to wet-cold weather in southern China.September-October AAO affects the ADP sea-ice anomaly in the Weddell Sea and Amundsen-Ross Sea.The sea ice anomaly persists into late January-February,in turn affects the wet-cold weather in southern China.The excitation of ADP by AAO is independent of SOD.The response of atmospheric circulation to ADP in January-February shows an obvious wave train pattern in the Southern Ocean.One branch of wave train originates from the Southeast Pacific-South Atlantic,propagates northward and reaches the tropical North Atlantic,causing convective precipitation anomalies and affecting the variation of convective precipitation of ITCZ in Atlantic-South America.The other branch also originates from ADP region but propagates eastward and northward along the South Atlantic and South Indian Ocean,and finally affects ITCZ in West Pacific.The variation of ITCZ further affects the East Asian atmospheric circulation through Rossby wave dispersion.When ADP is strengthened（weakened）,the precipitation anomaly of ITCZ in Atlantic-South America is positive（negative）while ITCZ in West Pacific is negative（positive）,the geopotential anomaly over East China-West Pacific is negative（positive）,East Asian Trough is weakened（deepened）,resulting the weakened（strengthened）invasion of wet-warm and cold air flows into Southern China,subsequently unfavorable（favorable）for the formation of wet-cold weather.（5）ADP can also lead to the anomalous response of stratospheric polar vortices,and well correlated to QBO in the middle stratosphere(30 h Pa,refer to as QBO₃₀)through the dispersion of planetary waves.QBO₃₀regulates the intensity of East Asian Winter Monsoon and subtropical jet by affecting the deep convection over the Atlantic and further modulates the impact of ADP on the wet-cold weather in Southern China.When QBO₃₀is in the westerly phase and ADP is strengthened（weakened）,the Ural Blocking High is weakened（strengthened）,the South Branch Trough is weakened（deepened）,and the Mongolian High is weakened（strengthened）,which is unfavorable（favorable）for the formation of wet-cold weather in Southern China.When QBO₃₀is in the easterly phase,the correlation between ADP and East Asian Winter Monsoon is insignificant,subsequently the insignificant response of wet-cold weather in Southern China to the variation of ADP.That is,when QBO₃₀is in the westerly phase,ADP has a more significant impact on the wet-cold weather in Southern China.