Regularity And Prediction Method Of The Influence Of Boreal Summer Tropical Subseasonal Signals On Summer Rainfall Over China

Subseasonal oscillations are widely found in the global atmosphere,among which the tropical atmospheric subseasonal oscillations are the most prominent and have significant impacts on the rainfalls over China.There are obvious intraseasonal characteristics in summer rainfall over China.The tropical subseasonal oscillation is one of important factors leading to the intraseasonal variations of summer precipitation over China.As a "bridge" linking weather forecasting and seasonal climate prediction,subseasonal forecast plays an important role in forecasting operations.Developing subseasonal forecast and improving subseasonal predictability of intraseasonal summer rainfall are significant for the development of forecasting operations in China,and also the major demands for people's life and social production.In this paper,researches have been carried out on regularity and prediction method of the influence of boreal summer tropical subseasonal signals on summer rainfall over China.The anomalous relative tendency(ART)method is used to extract the subseasonal components of meteorological elements,then the EOF analysis is used to extract the main subseasonal signals in the boreal summer tropics.The regularity of the influence of the subseasonal signals on the rainfall over China has been revealed.On the basis of the regularity,a subseasonal forecast model of summer precipitation over China has been constructed with linear regression using the main subseasonal signal captured.Through hindcast and cross validation,the intraseasonal rainfall prediction skills of the subseasonal forecast model have been evaluated for the YangtzeHuaihe region during the meiyu period(including the record-breaking Meiyu rainfall around the Yangtze River in 2020).The main results are as follows:(1)Regularity of the influence of boreal summer tropical subseasonal signals on summer rainfall over China.The anomalous relative tendency(ART)method is used to effectively extract the subseasonal components of the main boreal summer tropical signals(Outgoing Longwave Radiation,OLR),and the most dominant boreal summer tropical subseasonal oscillation is exhibited through EOF analysis.The results show that the first two EOF modes of the ART for OLR are orthogonal in phase,which together reflect the BSISO(Boreal Summer Intra-Seasonal Oscillation).Originated from India Ocean,the BSISO travels eastward to the central and eastern Pacific,and spreads northward to around 20�N,with a period of about 40-50 days.The results of regression analysis show that the BSISO leads to anomalies in the low-level wind field and geopotential height field in the local and its north area,consequently affects the convergences and divergences of water vapor in the local and its north area,which,as a result,have impacts on the intraseasonal droughts and floods over China,especially the southern China,and to some extent reflect the northward propagations of anomalous rain belts in China.The record-breaking Meiyu rainfall occurred around the Yangtze River in 2020.Our analysis shows that the precipitation during the Meiyu period of2020 features significant subseasonal variation,which is mainly manifested as the north-south swings of the anomalous rain belt,and the north-south oscillation of the rain belt is due to the north-south swings of the western Pacific subtropical high.The OLR anomalies over the South China Sea have an important impact on the north-south swings of the subtropical high,and the BSISO extracted in this paper is an important reason for the OLR anomaly over the South China Sea.(2)Prediction method for the influence of boreal summer tropical subseasonal signals on summer rainfall over China.Based on anomalous relative tendency(ART),the extracted BSISO is used as a predictor to construct a subseasonal forecast model with multiple linear regression for the prediction of summer intraseasonal rainfall over China.The coefficients are obtained by projecting the ART of observed OLR onto the first two EOF modes.By bringing the projection coefficients into the subseasonal forecast model,the ART of precipitation is predicted.The forecast of intraseasonal anomalous precipitation is finally obtained by combining the predicted ART and observed recent anomalous precipitation background.Based on the method mentioned above,a subseasonal forecast model has been constructed with one ten-day,two tenday,and three ten-day lead,respectively.The hindcast and cross-validation of the 40-year samples have been made and results show that the subseasonal forecasts are skillful with one ten-day lead for precipitation over southern China,and even they are skillful up to three ten-day lead for the precipitation over the Yangtze-Huaihe region during the Meiyu period.The constructed subseasonal forecast model also exhibits a considerable performance in the hindcast of the Meiyu rainfall in 2020.The regularity of the influence of boreal summer tropical subseasonal signals on summer rainfall over China revealed in this study is of great significance to the understanding of the causes of intraseasonal variations of summer rainfall over China.The prediction method presented in this study for the influence of boreal summer tropical subseasonal signals on summer rainfall over China has applicative value for the subseasonal droughts and floods prediction in eastern China.