The Synergistic Effect Of The Systems In The Mid-high Latitudes And Tropics On Persistent Heavy Rainfall During The First Rainy Season Over South China

Based on reanalysis data and daily precipitation data from the Chinese Meteorological Administration,the persistent and low-frequency characteristics of precipitation and its corresponding large-scale circulation in the first rainy season(April–June)over South China are discussed.Focused on the quasi-biweekly time scale,the influence of wave trains in midhigh latitudes and the air-sea interaction in the tropics on persistent heavy rainfall in the first rainy season over South China and associated physical mechanisms are further analyzed.On the basis,by defining the low-frequency circulation indices,the relative contribution and synergistic effects of different circulation patterns on persistent heavy rainfall in the first rainy season over South China are discussed quantitatively.The main conclusions are summarized as follows:According to the statistics of precipitation anomalies in the first rainy season over South China from 1981 to 2010,the total precipitation anomalies are mainly contributed by the persistent precipitation anomalies(lasting 3 days or more).On the analysis of large-scale circulation,the cooperation of two wave trains in the mid-high latitudes(the wave train crossing the Eurasian continent in the high latitude and the wave train along the subtropical westerly jet)and abundant supply of water vapor in the low latitude,as well as the secondary circulation formed by convective anomalies over the South China Sea(SCS)are all related to the persistence of precipitation in the first rainy season over South China.The results of power spectrum analysis show that the precipitation anomalies during this period exhibit obvious quasi-biweekly oscillation(QBWO)features,with a period of 8–24 days.Taking one standard deviation of the climate state as the threshold,the heavy rainfall events are defined.It is found that when the amplitude of low-frequency precipitation is strong and the duration is long,the real persistent heavy rainfall events often occur.It can be seen there is a good correspondence between persistent heavy rainfall and QBWO in the first rainy season over South China.It is found that when low-frequency heavy rainfall events persist at least three days,the corresponding circulations are more stable.Therefore,a low-frequency persistent heavy rainfall event(PHR-LF event)is defined as the 8–24-day filtered precipitation larger than one standard deviation of filtered time series and persisting at least three days during the first rainy season over South China.During the PHR-LF events in the first rainy season over South China,the spatial distribution of anomalous circulation and low-frequency field is very similar,and there are two low-frequency wave trains in the mid high latitudes,which are also similar to the circulation aforementioned,indicating that the low-frequency oscillations of circulation contribute a lot to the anomalous circulations related to persistent heavy rainfall.By describing the structure and propagation characteristics of the low-frequency wave trains through the three-dimensional wave active flux,it is found that western Europe and northern Mediterranean are Rossby wave sources,and the wave energy disperses toward eastern China along these two low-frequency wave trains from north to south and from west to east,and then propagates downward over South China,which intensify the disturbance and cause the anomalies of circulation over South China.The evolution of the two wave trains show that the anomalous variation of wave train in the subtropics before that of the precipitation may trigger the heavy rainfall,while the wave train in high latitudes varying with the occurrence and development of the heavy rainfall may maintain the heavy rainfall.The cooperation of the two wave trains enhances the cyclonic anomalies disturbance over eastern China and is conducive to the development and maintenance of the circulation anomalies.The diagnostic analysis of the vertical motion using the quasi-geostrophic equation shows that the wave trains in the mid-high can affect the vertical motion over South China through both dynamic and thermal effects.The results show that the advection of the low-frequency relative vorticity by the meridional wind is the main factor that causes the low frequency variation of vertical motion over South China.The perturbation anomalies over eastern China caused by wave trains through wave energy dispersion can cause the variation of relative vorticity of this region,increasing the meridional gradient of vorticity over South China,thus triggering the upward movement over South China.Furthermore,with strengthening of the wave trains,the meridional gradient of vorticity increases,which further intensifies and maintains the ascending motion and finally leads to the occurrence of persistent heavy rainfall during the first rainy season over South China.The low-frequency oscillation of the atmospheric circulation in the tropics also contributes a lot to the anomalous circulation related to the persistent heavy rainfall during the first rainy season over South China.The QBWO of the convection over SCS can affect the persistence of heavy rainfall during the first rainy season over South China by affecting the low-frequency variation of water vapor and vertical motion over South China.On the quasi-biweekly scale,the convection and sea surface temperature(SST)of the SCS form a negative feedback process,which is closely related to the persistent heavy rainfall during the first rainy season over South China.The low-frequency cold SST anomaly in the SCS can inhibit the development of the low-frequency convection over SCS,triggering the low-frequency local anticyclone and the local descending motion.Furthermore,due to the increase of solar radiation and the decrease of the upward sensible heat flux,as well as the accumulation of warm SST caused by the southerly winds,the low-frequency SST in the SCS gradually turns into a warm anomaly.At this time,there is ascending motion over South China in the direct vertical circulation driven by thermal force,and South China is controlled by cyclonic circulation,and the moisture transport is strengthened.During the process of the low-frequency variation of the convection over SCS,the direct thermodynamical circulation and the moisture transport are constantly strengthened,which makes the precipitation intensity over South China strong and lasting for a long time,and eventually leads to the occurrence of persistent heavy rainfall.The lowfrequency warm SST in the SCS will further stimulate convection and promote the generation of the local cyclone over this region.At this time,the low-frequency vertical circulation becomes the reverse phase,and the circulation conditions are not conducive to the precipitation over South China.The feedback process of air-sea interaction on the subseasonal scale over SCS and its related secondary circulation is the physical process and mechanism of how the tropical systems affect the persistent heavy rainfall during the first rainy season over South China.Based on the study of the influence of wave trains in the mid-high latitudes and the tropical systems on persistent heavy rainfall in the first rainy season over South China,the lowfrequency wave train index(WT)and convection index(SCS_OLR)are defined,which takes into account the activities of the two wave trains and represents the intensity of the convection over SCS,respectively.The low-frequency circulation indices are used to define the lowfrequency persistent strong circulation events,during which the persistence and intensity of the circulation are required at the same time.The statistical results show that the circulation pattern under the synergistic effect of the low-frequency wave trains in the mid-high latitudes and convection over SCS has the highest probability of occurrence of the PHR-LF events,and the average intensity of precipitation is the strongest.Under the influence of the single signal,the intensity of precipitation is greatly reduced.The convection over SCS is the necessary condition of the occurrence of heavy rainfall,and the superposition of the two signals can intensify the precipitation.Therefore,the synergistic effect of the low-frequency wave trains in the mid-high latitudes and convection over SCS has an important impact on the intensity and persistence of precipitation during the first rainy season over South China.Finally,according to the above results,the following forecast ideas can be put forward: a single circulation signal can provide the background of a persistent rainfall or a heavy rainfall,however,in order to satisfy the persistence and intensity of precipitation at the same time,the synergistic effect of multiple signals is very critical,especially the intensity of precipitation.