Study On The Difference Of The Influence Of 10-20 Day Low-frequency Oscillation On Summer Drought And Flood Precipitation In Southern Chin

Southern China has experienced frequent droughts and extreme precipitation events against the backdrop of global climate change.Based on the daily precipitation data from 753 meteorological stations provided by the National Meteorological Information Center(China)and the daily reanalysis data from NCEP,among others,during the period from 1980 to 2020,the low-frequency(LF)precipitation characteristics of flood and drought years in southern China and their relation to the 10-20 day LF atmospheric circulation at different latitudes are compared and analyzed,and extended-range forecasting signals are given.A Simple Harmonic Wave Model(SHWM)is established to forecast LF precipitation over an extended period for the entire southern region.Additionally,the predictive performance of the SHWM for summer precipitation during flood,drought,and normal years is assessed.The main conclusions are as follows:(a)In both flood and drought years,summer precipitation in southern China is controlled by 10-20 day oscillation(QBWO).A comparative analysis of the anomalous characteristics of the main 10-20 day atmospheric LF oscillations that affect LF precipitation in flood and drought years at different latitudes is performed,and the LF signal sources and propagation pathways are identified.LF convection is active in southern China in both flood and drought years,but the convective center is southward in flood years,and the vertical meridional circulation is stronger.The key circulation systems of 500 hPa LF height field in flood and drought years include LF “two ridges and one trough” and LF “+”,“-”,“+” East Asia Pacific(EAP)teleconnection wave train in mid-high latitudes of Eurasia.However,the “two ridges and one trough” in flood years are more westward and meridional than in drought years,and the LF Subtropical High is stronger and more extensive,with more significant westward extension.Strong LF Rossby wave sources exist throughout the West Siberian Plain,but in flood years,the position(intensity)is biased to the east(stronger),and more upstream energy is dispersed towards southern China.Under the combined influence of high-and low-latitude LF cyclonic and anticyclonic water vapor circulation,in flood(drought)years,there is northerly and then westerly(central westerly)dry-cold,northeasterly wet-cold,southwesterly,and southeasterly(including southerly across the equator)wet-warm water vapor channels.The sources of dry and wet cold air in flood(drought)years are located near Novaya Zemlya(the eastern West Siberian Plain),the Yellow Sea,the Bohai Sea and the Sea of Japan.Additionally,the sources of wet-warm water vapor include the Arabian Sea,the Bay of Bengal,the western Pacific Ocean,and the sea area of northeastern Australia(the western Pacific Ocean and the northern sea area of Australia).(b)By examining the LF predictive signals and their synergistic effects in the outgoing longwave radiation(OLR)at -11 days and the 500 h Pa height field at -9 days,the negative anomalies of the high-latitude LF 500 h Pa height field in flood(drought)years shift from the north of West Siberia(near the Kamchatka Peninsula)to southern China during the period of -9 to 0 days.Furthermore,when the LF convective inhibition in the tropical region moves from the west of Kalimantan Island(the southern Arabian Sea and the equatorial Indian Ocean)to the South China Sea,this corresponds with a relative increase in LF summer precipitation in southern China.(c)SHWM has shown promising forecasting skills and capabilities in predicting significant precipitation events during the summer extended-range in flood and drought years in the southern region.The concentrated period of future precipitation can be predicted by using the peak days of the LF components forecasted by SHWM.However,there may be a deviation of 1-3 days between the peak days of the forecasted LF components and the actual peak precipitation days.The SHWM forecasting skills are slightly better for drought years than for flood years,and its predictive performance is not very effective for normal years.