The Influences Of Soil Moisture Over Eastern China On Chinese Climate

Soil moisture is one of the most fundamental parameters in land surface processes,which can “remember” the antecedent climatic anomalies(i.e.precipitation)and influence the subsequent atmosphere slowly.This long-tem “memory” makes SM a new implication for short-term climate predictions.Based on the observed and reanalysis data,the effect of soil moisture on the relation between spring and summer precipitation is investigated.Also,the characteristics of summer precipitation anomalies over northeastern China and its causes are analyzed.As for the local effect of soil moisture,the relationship of LAI to soil moisture in different soil types is mainly investigated.The physical causes for the discrepant LAI-SM correlations in different soil types are also explored from the aspects of evapotranspiration(ET)and soil water retention.The effect of autumn soil moisture on winter monsoon is also explored and the influence chain is revealed.The main conclusions are drawn as follows:The spring precipitation over YRNC(30-43°N,100-120°E)region can affect the summer precipitation over eastern China significantly through soil moisture.When spring precipitation in the YRNC region is higher(lower),more(less)summer precipitation occurs in northeastern China and the lower and middle reaches of the Yangtze River valley,and less(more)in the Hetao region and southeastern China.Since the memory of atmosphere is not so long,the physical mechanism is analyzed from the aspect of soil moisture.The result showed that higher(lower)spring precipitation in the YRNC region is closely related to wet(dry)spring soil moisture,which decreases(increases)the surface temperature and sensible heat flux in late spring(May).Because the memory of spring soil moisture in the YRNC region reaches about 2.4 months,the surface thermal anomaly lasts into the subsequent summer,resulting in the relative small temperature difference between land and sea,which lead to the weak(strong)East Asian summer monsoon.A weak East Asian summer monsoon associated with more spring precipitation over the YRNC region corresponds to anomalous anticyclone and cyclone over southeastern and northeastern China,respectively,in lower troposphere.The anomalous anticyclone depresses the summer precipitation in south-eastern China and the anomalous cyclone promotes precipitation over northeastern China.The abnormal northerly and southerly winds associated with the anomalous cyclone and anticyclone,respectively,converge in the lower and middle reaches of the Yangtze River valley,inducing more summer precipitation there.The simulations of WRF verify the variation of summer circulation induced by the antecedent soil moisture anomalies.As mentioned above,the spring precipitation over YRNC region can affect the summer precipitation over northeastern China.Accompanied with the abnormal precipitation,there is an abnormal cyclone lower troposphere and the surface is colder and wetter over northeastern China.The causes are further investigated.The result showed that the colder and wetter air induced by the wetter spring soil in the YRNC region is transported to northeastern China by the effect of horizontal advection,which also makes the surface colder and wetter there.The cyclone anomaly over northeastern China is associated with the EAP pattern.Namely,more spring precipitation in the YRNC region enhances the western pacific subtropical high,which depresses the development of convection near Philippines.Thus the EAP pattern turns into the negative phase,corresponding to the cyclone anomaly over northeastern China.As important parameters in the land-atmosphere system,both soil moisture(SM)and vegetation play a significant role in land-atmosphere interactions.Using observational data from clay and sand stations over central eastern China,we investigated the relationship between the leaf area index(LAI)and SM(LAI-SM)in different types of soil.The results show that the LAI-SM correlation is significantly positive in clay but not significant in sand.The physical causes for the discrepant LAI-SM correlations in different types of soil were explored from the perspectives of evapotranspiration(ET)and soil water retention.In clay stations,increasing LAI is associated with greater soil water-retention capacity.Although the increasing LAI corresponds to increasing ET,the impact of ET on SM is weak because of the small particle size of soil.Consequently,the LAI-SM relationship in clay is significantly positive.In sand stations,ET is negatively correlated with SM owing to the large soil particle size,resulting in a negative LAI-SM correlation in sand.However,soil water retention is weakened by the increased LAI,which may be an important factor causing the insignificant LAI-SM correlation in sand.In view of the significant effect of antecedent soil moisture on precipitation,the influence of autumn soil moisture on winter monsoon is also explored.It is found that when the autumn soil is wetter in the YRNC region,the winter precipitation in the most of eastern China is less.The reason is investigated from the circulation and surface.The wetter soil in autumn over the YRNC region also makes the latent heat larger.Furtherly,the latent heat anomaly can last into the followed winter,when the evaporation also becomes larger.The larger evaporation causes the air becomes colder in the lower troposphere which in turn enlarges the land-sea temperature difference and strengthens the winter monsoon.Corresponding to the strong winter monsoon,there are northerly wind anomalies in the lower troposphere,which leads to the less winter precipitation.In addition,the strong northerly wind induced by the wetter soil in the YRNC region weakens the western pacific subtropical high and promotes the development of convection near Philippines.Then the strong convection enhances the meridional circulation,which may furtherly strength the winter wind.