Impact Of Intraseasonal Oscillation On Rainfall Extremes In Southeastern China And Its Predictability In CFSv2

Intraseasonal oscillation(ISO)is the world's strongest low-frequency signal,which has a significant impact on extreme precipitation in the southeastern China(SEC).In this paper,the effect of MJO and BSISO on the extreme precipitation in SEC were studied by using RMM index and BSISO indices combined with the precipitation data from China,and then makes a further analysis of the physical background of this impact,finally,based on the Climate Forecast System version 2(CFSv2),the predictability of extreme precipitation under the influence of ISO was evaluated.The results of this study indicate that:(1)MJO has significant modulation of the probability and spatial distribution of extreme precipitation in the SEC.Among the 8 phases of MJO,the 2–3 and 6–7 phases can be defined as typical“wet”and“dry”phases,respectively.The probability of extreme precipitation in SEC region increased by about 30%–50%and decreased by more than 20%.(2)The response of summer extreme precipitation to BSISO in China showed regional differences.Based on regional differences of the effects of BSISO activities on extreme precipitation,the SEC region can be divided into two sub-regions:South China(SC)and the middle and lower reaches of the Yangtze River Valley(YRV).Phase 4 and 8,phases 3–4 of BSISO1 are typical“wet”phases of the SC and YRV regions,respectively.Where the probability of extreme precipitation increases by about 35%–45%,showing southward propagation with the speed of 1.96~o/phase.Phases 4–5 and 6–7 of BSISO2 are the typical“wet”phases of SC and YRV regions,respectively,which can increase the probability of extreme precipitation occurring over 40%.Interestingly,as a response to the northwest propagation of BSISO2,the region with an increased probability of extreme precipitation presents a steady northward transmission characteristics at phase 4–7,with a phase velocity of 2.75~o/Phase.(3)The diagnosis results of moisture and circulation fields show that the increase in the probability of extreme precipitation is accompanied by the enhancement of horizontal moisture convergence,the vertical motion,and the vertical moisture transport.Our results show that the horizontal moisture advection has littile contribution to the extreme precipitation,and the horizontal moisture convergence and vertical vapor advection may be the key factors in the occurrence of extreme precipitation events.(4)Under the influence of ISO,the predictability of extreme precipitation is evaluated.The results show that the CFSv2 model has a high predictive capability for the central and spatial distribution of winter precipitation in China,but the forecast for the precipitation value of the SEC region is small.The model can effectively predict MJO for more than 20 days,and can capture the MJO's probabilistic modulation of extreme precipitation within two weeks.Similarly,the CFSv2 model also has a high forecasting ability for the central and spatial patterns of summer precipitation in China,but the model significantly weakens the magnitude of the summer extreme precipitation with the etension of the forecast time.The CFSv2 model has a certain predictive ability for BSISO.The effective prediction skill for BSISO1(BSISO2)is 13(10)days.The probability change of extreme precipitation in the SEC region based on BSISO also shows high predictability in the CFSv2 model,which can make accurate predictions within two weeks.The results of the study indicate that the RMM and BSISO indices have broad application prospects for the extendee-range prediction of extreme precipitation in winter and summer in the SEC region.