Interdecadal Variabilities And Mechanisms Of Southern China Summer Rainfall

Under the background of global warming,rainfall in southern China during boreal summer(June to August)shows dominant interdecadal variability.Using the monthly rainfall data for the period of 1920-2014,the leading modes of the interdecadal variability of Southern China Summer Rainfall(SCSR)are revealed based on observational analyses and numerical modeling.Meanwhile,the interdecadal changes of spatial patterns of SCSR are uncovered.Finally,the underlying possible mechanisms connecting the sea surface temperature in the Indian Ocean,the North Pacific Ocean and the North Atlantic Ocean and the leading modes of SCSR on interdecadal timescales are analysed.The main results are as follows:(1)The leading modes and corresponding atmosphere circulation anomalies of interdecadal variability of SCSR were revealed.The first two leading modes of the interdecadal variability of SCSR are uncovered:the Southern China Rainfall pattern(SCR)and the Southern China Zonal Dipole pattern(SCZD).The dominance of these two modes experienced apparent decadal change around 1971/1972.During 1925-1971,the dominant mode in the interdecadal variability of SCSR was the SCZD,while was the SCR during 1972-2009.The key regions of sea surface temperature associated with the first two leading modes in each ocean also changed around 1971/1972.The atmospheric circulation anomalies associated with the SCR and the SCZD of interdecadal SCSR are uncovered.Corresponding to the positive(negative)phase of the SCR,an anticyclonic(cyclonic)anomaly is located over South China Sea and northwestern Pacific,which is(not)in favor of moisture transport from South China Sea into southern China.The lower-level anticyclonic(cyclonic)anomaly over Lake Baikal may lead to more southward cold air,which can converge with the moisture transport from South China Sea and is favorable for generating the positive(negative)phase of the SCR.Corresponding to the positive(negative)phase of the SCZD,a positive(negative)center is over Southeastern China,which is favorable for upper-level divergence(convergence)and anomalous ascending(descending)motion.When combined with the lower-level cyclonic(anticyclonic)anomaly,it may result in increased(decreased)rainfall in Southeastern China.Meanwhile,the anticyclonic(cyclonic)anomaly over Indo-china peninsula and the Philippines are favorable(unfavorable)for moisture transport from the Bay of Bengal to Southwestern China,which is in favor of generating the positive(negative)phase of the SCZD.(2)Combined impacts of Southern Indian Ocean(SIO)?Northwestern Pacific Ocean(NWPO)and North Atlantic Ocean(NA)on the SCR of interdecadal SCSR were investigated.Considering the positive(negative)phase of SCR,the combined impacts of the warm(cold)phase of NWPO and the cold(warm)phase of SIO are in favor of an anomalous anticyclone(cyclone)over South China Sea and Northwestern Pacific,which lead to more(less)water vapor to southern China.Meanwhile,the warm(cold)phase of NA is in favor of anomalous anticyclone(cyclone)over eastern Lake Baikal,which is favorable for abundant(deficient)cold air transport into southern China.Therefore,the joint effects of the NWPO,SIO and NA may eventually play a key role in generating the positive(negative)phase of SCR.(3)Combined impacts of Interdecadal Indian Ocean Basin Mode(ID-IOBM)Pacific Decadal Oscillation(PDO)and Atlantic Multidecadal Oscillation(AMO)on the SCZD of interdecadal SCSR were uncovered.Considering the positive(negative)phase of SCZD,the warm(cold)phase of the ID-IOBM is in favor of anomalous anticyclone(cyclone)over the Indo-China peninsula,which reduced(strengthened)the eastward moisture transport from Bay of Bengal to Southwestern China(SWC)and is in favor of decreased(increased)rainfall in SWC.In addition,the Kelvin wave-like pattern,as a response of the warm(cold)ID-IOBM phase,further leads to meridional Pacific-Japan-like(PJ-like)teleconnection pattern.Meanwhile,the warm(cold)phase of PDO is also in favor of the similar PJ-like pattern over East Asia.The lower-level cyclonic(anticyclonic)anomaly associated with the PJ-like pattern over Southeastern China(SEC)is favorable for increased(decreased)rainfall in SEC.The anomalous upper-level divergence(convergence)and ascending(descending)motion over SEC associated with the positive(negative)phase of AMO are in favor of rainfall increase(decrease)in SEC.Therefore,the joint effects of the ID-IOBM,PDO and AMO may eventually play a key role in generating the positive(negative)phase of SCZD.(4)The influence of Indian Ocean sea surface temperature on the interdecadal variability of Southwestern China summer rainfall(SWCSR)was revealed.The SWCSR is markedly impacted by the interdecadal Indian Ocean basin mode(ID-IOBM)of the SST,which may induce anomalous inter-hemispheric vertical circulation.During the cold phase of the ID-IOBM,the anomalous ascending motion associated with the anomalous vertical circulation is in favor of SWCSR increase.In summers during the warm phase of the ID-IOBM,the situation is approximately the same,but with opposite polarity.After the beginning of the 1970s,the impacts of interdecadal Indian Ocean dipole(ID-IOD)on SWCSR is strengthening.The anomalous vertical circulation associated with the positive(negative)phase of ID-IOD is in favor of SWCSR decrease(increase).However,the impacts of ID-IOD on SWCSR is relatively weak before the 1970s,indicating that the ID-IOD is the secondary driver of the interdecadal variability of SWCSR.Modeling results also indicate that the ID-IOBM of SST anomalies is the main driver of interdecadal variability of SWCSR.(5)The influence of Atlantic Multidecadal Oscillation(AMO)on the interdecadal variability of Southeastern China summer rainfall(SECSR)was revealed.The interdecadal variability of SECSR is highly associated to AMO.AMO can trigger a global-scale stationary barotropic wave-like pattern,which was termed the interdecadal circumglobal teleconnection pattern(ID-CGT).The ID-CGT shows zonal wavenumber-5 structures,which can affect the South Asian High(SAH)and East Asian Subtropical Jet Stream(EAJS)by the perturbation associated with it.When in the positive(negative)phase of AMO,the SAH is strengthened(weakened)and shifts southward(northward),the EAJS axis shifts southward(northward)simutaneously.As a result,the SECSR increased(decreased).(6)The collaborative influences of different forcing factors on the interdecadal variability of SCSR were explored.The regression fitting models of the SCR index and the SCZD index were constructed using the associated indices of forcing factors.The NWPO,SIO and NA can have an impact on the SCR of SCSR collaboratively,while the ID-IOBM,PDO and AMO can have an impact on the SCZD of SCSR collaboratively.The results show that the more factors are considered,the better the models perform.Therefore,it is meaningful to construct different regression fitting models during the corresponding period.To clarify the mechanisms associated with the interdecadal variability of SCSR,it is necessary to consider the combined impacts of different forcing factors.After 1971/1972,the most significant SSTA signals were transformed from ID-IOBM,PDO and AMO into SIO,NWPO and NA,which means that the forcing factors of interdecadal variability of SCSR experienced decadal change.As a result,the dominant mode of interdecadal SCSR experienced decadal change simultaneously,which were transformed from SCZD into SCR.