Effect Of The Variation Of East Asian Westerly On The Rainfall Changes In Yangtze-Huaihe River Valley And North China

Droughts and floods occur frequently in the eastern China containing Yangtze-Huaihe River Valley(YHRV)and North China(NC,mainly Hebei and Shanxi Province)in recent years,and the changes in the summer rainfall in these two regions and their causes draw much attention from the climate community.Many researchers focus on the relationship of the rainfall anomalies in eastern China with the variation of the East Asian westerly jet(EAWJ),and they found that the changes in the meridional position and the intensity of EAWJ have significant impact on the rainfall changes.However,the effect of the jet's intensity on interannual variation in summer rainfall in eastern China has not been systematically studied.The dynamic mechanism for the impact of the changes in EAWJ on the rainfall anomalies in eastern China has also not been resolved yet.Asymmetric relationship exitsts widely among the climate variabilities over East Asia and whether the influence of EAWJ's variation on the rainfall in YHRV and NC is asymmetric has not been analyzed in the previous studies.The present research aims to study the above three scientific issues using reanalysis and observational data and statistical methods such as correlation analysis.The results are listed as follows.1)The intensity of EAWJ is closely correlated with the rainfall anomalies in YHRV.Associated with anomalously strong EAWJ,there is increased rainfall in YHRV but decreased rainfall in NC in early summer(from June 16 th to July 15th);and vice versa.In midsummer(from July 16 th to August 14th),decreased rainfall is observed in YHRV associated with anomalously strong EAWJ;and vice versa.The relationship between the rainfall in eastern China and the meridional position of EAWJ is similar to the results in previous studies: the rainfall in YHRV is negatively correlated with the latitudes of EAWJ's axis in both early summer and midsummer,while the rainfall in NC is positive correlated with the latitudes of EAWJ's axis in midsummer.Further analysis suggests that at least part of the correlation between the rainfall anomalies in eastern China and the variation in the intensity and meridional position of EAWJ is the result of the impact of the variation in EAWJ on the rainfall changes.2)Both the 30-day-mean and the day-to-day variability of midtropospheric horizontal temperature advection have significant impact on the rainfall anomalies in eastern China.In both early summer and midsummer,the midtropospheric 30-day-mean warm advection anomalies can result in the increased rainfall in NC and YHRV through the adiabatic ascent motion and the convective activities associated with the moisture feedback;and vice versa.The anomalously strong day-to-day variability of midtropospheric horizontal temperature advection causes increased rainfall in most part of NC and YHRV through “lower-bound effect”(daily rainfall amount can not become negative)in early summer and midsummer;and vice versa.3)The anomalously northward displacement of EAWJ is associated with the weakened midtropospheric westerly wind in YHRV and strengthened midtropospheric southerly wind(or weakened midtropospheric northerly wind)in NC through the anomalously southward displacement of midtropospheric jet over East Asia;and vice versa.These horizontal circulation anomalies in turn cause midtropospheric warm advection anomalies in NC and midtropospheric cold advection anomalies in YHRV since the climatological-mean midtropospheric temperature is high in the west(south)and low in the east(north)in YHRV(NC);and vice versa.The anomalously northward displacement of EAWJ is also associated with the increased day-to-day variability in NC and decreased day-to-day variability in YHRV of midtropospheric horizontal temperature advection in early summer by the jet's trapping transient eddies since EAWJ's axis is located north of YHRV and over southern NC.These day-to-day variability anomalies have the same signs with 30-day-mean temperature advection anomalies in both YHRV and NC,which causes increased rainfall in NC and decreased rainfall in YHRV.Via the jet's trapping transient eddies decreased day-to-day variability of midtropospheric horizontal temperature advection is associated with the anomalously northward displacement of EAWJ in both YHRV and NC for midsummer since EAWJ's axis is located north of NC.These anomalies and 30-day-mean temperature advection anomalies have the same sign in YHRV but have the opposite sign in NC,which leads to the decreased rainfall in YHRV but no obvious rainfall changes in NC.4)The anomalously strong EAWJ is associated with midtropospheric warm advection anomalies over YHRV and cold advection anomalies over NC through the strengthened westerly wind and the strengthened zonal temperature contrast in the middle troposphere over YHRV and strengthened northerly wind over NC in early summer.In midsummer the anomalously strong EAWJ is associated with the weakened meridional temperature contrast over YHRV and thus the weakened warm advection anomalies over YHRV through the meridional teleconnection pattern along the eastern coast of China.Associated with the anomalously strong EAWJ,enhanced day-to-day variability of midtropospheric horizontal temperature advection is observed around the axis of EAWJ.The anomalies of the day-to-day variability and the 30-day-mean of midtropospheric temperature advection leads to the increased rainfall in YHRV and decreased rainfall in NC in early summer and decreased rainfall in YHRV in midsummer associated with the intensified EAWJ.5)The meridional displacement of EAWJ has asymmetric influence on summer rainfall changes in eastern China.As EAWJ moves northward from its southernmost limit thorough the climatological-mean position to its northernmost limit,the rainfall in northern(southern)YHRV first drastically increases(decreases)and then drastically decreases(slightly increases).For midsummer in the process of EAWJ's northward movement the rainfall in NC(YHRV)first increases(decreases)and then decreases(keeps unchanging).The results for the relationship between the jet's variation in the middle troposphere and rainfall anomalies in NC and YHRV are similar to those for the EAWJ in the upper troposphere,which suggests that at least part of the asymmetric relationship between the rainfall anomalies in eastern China and the variation in the meridional position of EAWJ is the result of the asymmetric influence of the variation in EAWJ on the rainfall changes.6)As EAWJ moves northward from its southernmost limit to its northernmost limit,the inflow into the midtropospheric jet over eastern East Asia from the north flank of Tibetan Plateau strengthens,while the one from the southern flank of Tibetan Plateau weakens in early summer.The changes in the two inflows lead to the asymmetric influence of EAWJ's meridional displacement on the rainfall anomalies in southern YHRV through the zonal temperature gradient anomalies and the associated 30-day-mean midtropospheric temperature advection changes.For midsummer,as EAWJ moves northwards the directions of the two inflows into the midtropospheric jet are changed,which results in the asymmetric influence of EAWJ's meridional displacement on the midtropospheric 30-day-mean temperature advection anomalies and thus the rainfall anomalies in the northern YHRV.7)In midsummer,as EAWJ moves northward from its southernmost limit to its northernmost limit the position of midtropospheric horizontal circulation and temperature anomalies associated with EAWJ's meridional displacement changes,which causes the asymmetric influence of EAWJ's meridional displacement on midtropospheric 30-day-mean horizontal temperature advection and thus the asymmetric influence on the rainfall anomalies in YHRV.EAWJ's meridional displacement exerts asymmetric influence on the meridional wind anomalies through the changes in the position of midtropospheric jet,which causes the asymmetric influence on 30-day-mean horizontal temperature advection and thus the asymmetric influence on the rainfall anomalies in NC.