Itraseasonal Oscillation Of The Summer Monsoon And Its Infulence On Regionally Persistent Heavy Rainfall Over Southern China

Regionally persistent heavy rainfall (RPHR) is one of the main types of disastrousweather that is most likely to cause large-scale severe flooding and endangers the lives andproperty of the public. RPHR events in China are unpredictable, frequent, and persistent.They have been responsible for many catastrophic floods in southern China. As thestability of large-scale circulation is essential for the persistence of rainstorms,atmospheric intraseasonal oscillation (ISO) may also be a potential signal for RPHR.Meanwhile ISO is valuable for the extended range forecast (ERF) of rainfall due to itsquasi-periodicity, time scale, and spatial scale. A further understanding of the relationshipbetween RPHR and ISO would be helpful for improving ERF of persistent heavy rain. Inthe present study, basing the temporal and spatial structures of RPHR, the relationshipbetween ISO and RPHR in southern China was investigated firstly. Then the causes andmechanisms of ISO in the summer rainy season of southern China and its influence onRPHR were studied. Based on the mechanisms and the relationship between ISO andRPHR, using the reforecast from NCEP/CFSv2, a statistics-based ERF method for therainfall ISO was test. The major conclusions are as follows:(1) Defined the RPHR over the southern China by considering its range, intensity andduration. Based on the definition, the statistical characteristics of the RPHR are as follows:①The high incidence areas of RPHR are South China, the south of the Yangtze River,and the Yangtze–Huai River Basin;②The average duration of three areas is4.8days.The duration of RPHR in the south of the Yangtze River is longest;③Most of theRPHRs happen in summer, and the incidence rates are close in three areas during thesummer.(2) Based on the REOF modes of30–60-day bandpass filtered rainfall, southernChina can be divided into three typical regions which are similar to the high incidence areas of RPHR. The ISO rainfall index (IRI) was defined as the average regionalprecipitation in each region. The RPHR events that occurred in the typical regions werefound to be strongly related to the intraseasonal variation in the rainfall; indeed, more than87%of the RPHR events occurred when the IRI was larger than1SD.(3) The mapping of regression coefficients relating U850’ to IRIs indicated that theSouth China Sea summer monsoon (SCSSM) systems play an important role in regulatingintraseasonal rainfall over southern China. EOF analysis was performed on the850-hPazonal wind which was averaged over110°–120°E, to present the main mode of theSCSSM. Then, a pair of ISO modes of the SCSSM was identified by performing Morletwavelet analysis on the leading PCs of EOF1and EOF2. Composite analyses based onPC1and PC2revealed the evolution of the ISO in SCSSM: The accompanyingeastward-propagating MJO, and the east-west oscillation of the Western Pacificsubtropical high (WPSH), producing alternating dry and wet phases over southern China.These alternating periods last approximately40days and modulate the intraseasonalvariation in the rainfall. EOF1of the850hPa zonal wind over the SCS and southern Chinamainly represent the ISO mode controlling the intraseasonal variation of rainfall in thesouth of the Yangtze River, while EOF2leads to the intraseasonal out-of-phase rainfallover South China and the Yangtze–Huai River Basin.(4) The RPHR was found to be closely related to the ISO in SCSSM. In the threetypical regions, more than70%of RPHR events occur during wet phases from April toSeptember. During wet phases, moisture-rich air is transported to southern China at lowerlevels by the southwestward flow to the north of the WPSH. The unstable loweratmosphere coincides with an upper-tropospheric divergence and a low-level positivevorticity center that promotes heavy rainfall. The structures of ISO systems are reversedduring dry phases and suppress the rainfall over the corresponding regions. The region ofRPHR is determined by the zonal position of the WPSH.(5) The meridional-phase structure of the composite ISO modes indicated that thenorthward-propagating ISO and local air–sea interactions over the SCS are two keyfactors that promot the fluctuations in the geopotential height anomaly. Additionalnumerical simulations would be needed to identify the dominant mechanism.(6) Coupling between the ISO in SCSSM and the mid-and high-latitude systems,especially blocking situations, also play important roles in promoting the occurrence ofRPHR over southern China during wet phases. The main modes of the mid-andhigh-latitude ISO systems inclunde “Ural blocking”,“Ural-Okhotak double blocking” and “Okhotak blocking”. These modes provide cold air and stable circulation for theRPHR but do not exactly correspond to the RPHR of a certain region.(7) The SCSSM ISO is also found in the NCEP/CFSv2reforecasts and thecharacteristics of which are similar to the observation. The Real-time SCSSM ISOindeies (RSO1and RSO2) were defined by projecting the NCEP/CFSv2reforecasts onthe EOF spatial patterns of the SCSSM ISO. The statistics-based ERF method for therainfall ISO, by using these indeies, provides useful forecasts of precipitationintraseasonal anomalies up to10to90days of lead times in the southern China.