Monitoring And Prediction Of The Meiyu Subseason Based On The Latitudinal Swing Configuration Of The South Asian High And The Western Pacific Subtropical Hig

Meiyu is a typical stage of the East Asian rainy season,showing substantial subseasonal variation at periods of 10-30 days and 30-60 days,which often leads to extreme precipitation and disastrous flooding over the Yangtze River Basin(YRB).Monitoring and prediction of the subseasonal variation of Meiyu is crucial for disaster prevention and mitigation.This study uses the daily precipitation data provided by the China Meteorological Administration,using lanczos filtering,lead-lag regression analysis,singular value decomposition,composite analysis and other statistical analysis methods,Here,we propose two sets of novel indices for Meiyu monitoring and prediction based on the compound zonal displacements of the South Asia high(SAH)and the western Pacific subtropical high(WPH)at 10-30-day and 30-60-day period.Some conclusions are concluded as below:(1)The MYI is defined as domain-averaged rainfall in the middle and lower reaches of the YRB(26°N-33°N,108°E-123°E)during June and July,which is the location and timing of remarkable climatological and subseasonal variability precipitation,MYI has two significant periods of 10-30 days and 30-60 days.The key circulation system that affects the Meiyu subseasonal variability is the SAH in the upper troposphere and the WPH in the lower troposphere.To describe the zonal shift of the WPH and the SAH,SAH index and the WPH index are defined.When the SAH index is positive,it indicates that the SAH extends to the east,and when the WPH index is positive,it indicates that the WPH extends to the west.(2)For the 10-30-day period of Meiyu,the zonal displacement of the SAH is associated with a mid-latitude Rossby wave train driven by the transient eddy,whereas the WPH is related to the second mode of the boreal summer intraseasonal oscillation(BSISO2).The configuration of the upper-level and lower-level highs associated with Meiyu at the 10–30-day timescale can be considered as an uncoupled mode.For the 30-60-day period of Meiyu,the zonal displacement of the SAH and the WPH are both related to the first mode of the boreal summer intraseasonal oscillation(BSISO1).The convection anomaly of BSISO1 leads to the eastwardpropagated Rossby wave train and influences the zonal variation of the SAH.Meanwhile,the convection of BSISO1 directly influences the zonal displacement of the WPH.The configuration of the upper-level and lower-level highs associated with Meiyu at the 30–60-day timescale can be considered as a coupled mode.(3)Four categories of upper-level and lower-level circulation configurations associated with Meiyu are classified based on the zonal displacement of the SAH and the WPH(i.e.,the inward type,the co-eastward type,the outward type,and the co-westward type).Each type can be further defined as either an SAH-dominated type or a WPH-dominated type.These configurations have different impacts on the subseasonal variation of Meiyu.Meiyu subseasonal variation can be well reconstructed using relationship between these two indices and rainfall anomalies pattern over China.Given that the European Centre for Medium-Range Weather Forecasts(ECMWF)in the Subseasonal to Seasonal(S2S)Project is more skillful in forecasting upper-and lower-level circulation than that in directly forecasting precipitation,a hybrid dynamical-statistical model is conducted to predict the Meiyu subseasonal variation using the ECMWF model forecast indices.The hybrid model outperforms the ECMWF model in predicting the Meiyu subseasonal variation at 17-40-day lead times.