The Relationship Between East Asia Summer Meridional Teleconnections And Regional Persistent Extreme Precipitation Over The Eastern China

East Asian summer monsoon is a complicated system which has a significant influence on East Asian climate.The meridional teleconnection is one of its major variability modes.Extreme precipitation can cause severe flood disasters and is an important issue for society and economy in East Asia.This thesis starts from meridional teleconnection(atmospheric circulation)and regional persistent extreme precipitation events(precipitation)respectively to discuss the influence of the East Asia meridional teleconnections on the relationship between East Asian summer meridional teleconnections and regional persistent extreme precipitation over the eastern China and its possible mechanisms.The main conclusions are as follows:On the interannual timescale,there are two meridional teleconnections along the coast of East Asia in summer: tropical originated and extratropical originated.They both show a quasi-barotropic structure of westerly wind anomalies that slightly tilted northward with height.The tropical originated meridional teleconnection is triggered by the convection anomaly in the Northwest Pacific and propagates northward.It can strengthen(weaken)the westerly jet and favors the increase(decrease)of precipitation in the Yangtze River Valley.Compared with the East Asia-Pacific/ Pacific-Japan pattern,it can better represent the characteristics of East Asian precipitation.The extratropical originated teleconnection is initiated from the south of the Kara sea and propagates southeastward to the Yangtze River Valley,which is dominated by the North Asia dipole mode on SLP.It is related to the southward(northward)shifted jet and favors a “Southflood-North-Drought”(“South-Drought-North-flood”)pattern of rainfall anomalies over East China.The tropical originated teleconnection is related to the sea surface temperature anomalies in the tropical Pacific-Indian Ocean and ENSO partly contributes to its formation.The extratropical originated teleconnection is related to the soil moisture anomaly to the south of the Kara Sea and the spring sea ice concentration anomalies in Barents Sea.The joint contributions of the two teleconnections are essential for the variability of the East Asian summer monsoon,wherein the tropical originated teleconnection plays a primary role.On the subseasonal timescale(10-30-day),there are also two meridional teleconnections along the coast of East Asia,extratropical originated and tropical originated,respectively.Compared with the stationary teleconnection on the interannual timescale,they propagate with time and also show quasi-barotropic structures in circulation.The extratropical originated teleconnection is related to the soil moisture anomalies in the south of the Kara Sea and Northeast China and starts from the south of the Kara Sea and propagates to the Yangtze River Valley,which results in the southern shift of rain-band from Northeast China to the Yangtze River Valley.The tropical originated teleconnection is associated with the convection anomaly in the Northwest Pacific and BSISO2 and propagates northward from the Northwest Pacific,which favors the northern shift of rain-band from South China to the Yangtze-Huai River valley.Both teleconnections can contribute to the shift of extreme precipitation as the rain-band in summer.Besides,teleconnections on the interannual timescale can modulate the subseasonal teleconnections via their influences on the extreme precipitation.Extreme precipitation will be more often and persistent longer when the interannual and subseasonal meridional teleconnections have the same effect on the extreme precipitation.On the contrary,it will get weak if they have opposite effects.Meanwhile,the interannual signal can influence propagation of the subseasonal signal,including break or enhancement of the propagation,which is not simply linear superposition.Summer regional persistent extreme precipitation events are defined by pentad mean precipitation exceeding its 95 th percentile that covers more than 5% grids over the eastern China,which are then classified into four categories by K-mean cluster analysis: South China,South of Yangtze River Valley,Yangtze-Huai River Valley and North China.The extreme events in South of Yangtze River Valley and Yangtze-Huai River Valley mainly happen in early summer while the other two are rather similar in both early and late summer.The related atmospheric circulation patterns show a cyclone anomaly in the lower level in South China which is more significant in late summer.The other three show an anticyclone anomaly to the south of the extreme precipitation region,with a cyclone anomaly to the north in South of Yangtze River Valley and Yangtze-Huai River Valley,which favors convergence and precipitation anomalies.In the upper level,it appears strengthened westerly anomaly in South China and South of Yangtze River Valley while North China shows a significant anti-cyclone anomaly.The Yangtze-Huai River Valley shows an anticyclone anomaly in early summer and strengthened westerly anomaly in late summer.Generally,the typical circulation pattern is consistent with the subseasonal meridional teleconnection that has the same effect on the extreme precipitation.The typical circulation pattern can be used to represent the extreme events.The extreme precipitations derived from the circulation and the precipitation have 30% of their total amount in common.The vertical velocity at 500 h Pa,the vorticity and the divergence in the lower level are three key dynamic factors to the formation of regional persistent extreme precipitation events.The divergence in upper troposphere also plays an important role in early summer in South of Yangtze River Valley and Yangtze-Huai River Valley and in late summer of North China.The vorticity in upper troposphere contributes little to the extreme precipitation.Apart from the dynamic factors,moisture is also essential to the regional persistent extreme precipitation,especially in South China and South of Yangtze River Valley.However,with sufficient moisture,the dynamic factors are more important in North China.