Characteristics Of Intraseasonal Variabilities In Forming The Persistent Extreme Precipitation Events Over The Eastern China

Based on daily precipitation observations at 756 Chinese stations archived by the China Meteorological Administration, atmospheric reanalysis datasets from NCEP/NCAR and sea surface temperature (SST) data from Hadley Center during the period 1951-2010, the characteristics and mechanisms of summer persistent extreme precipitation events (PEPE) occurred in the eastern China are investigated from the aspect of subseasonal variability. Furthermore, role of air-sea coupling between the Indian Ocean and the western Pacificin the northward propagation of the Boreal Summer Intraseasonal Oscillation (BSISO) is revealed. The main conclusions are as follows:The different roles of 30-60-day and 10-30-day oscillations in forming the PEPE occurred during June 12-27 in 1998 over the Yangtze River Basin (YRB) are revealed. According to the definition given by Chen and Zhai (2013), the 1998 PEPE ranks first in measures of lasting time and affecting area in recent 60 years. In this period, the YRB rain belt features significant intraseasonal oscillation (ISO) with two position migrations, among which the 10-30-day ISO mainly determines the south-north swing of the YRB rain belt while the 30-60-day ISO generally maintains its whole shape and intensity. The 30-60-day intraseasonal rainfall is generated by a meridional dipole mode of low-level water vapor flux straddling the YRB over the eastern China, which is in turn caused by the positive SST anomalies in the North Indian Ocean via local convection-induced diabatic heating. The two position migrations of the 10-30-day intraseasonal rainfall divide the PEPE into 3 stages. At Stage 1 (2), because of the northwestward propagation of the 10-30-day ISO originating from the Northwest Pacific, suppressed (enhanced) convective activities occur over the Philippine Sea, where the corresponding diabatic cooling (heating) stimulates a downstream northeastward-propagating Rossby wave train extending from the South China Sea (SCS) into the mid-latitude North Pacific, therefore, an anomalous low-level anti-cyclone (cyclone) forms over south of YRB, and it then constantly transports moisture from the SCS (western Pacific) to the YRB. However, at Stage 3, the significantly enhanced convective activities appear over the central SCS, which mainly come from northeastward propagating 10-30-day ISO initiated in the southwestern-most SCS, thus a similar Rossby wave train forms as that at Stage 2, and the southeasterly wind brings moisture from both the SCS and western Pacific into the YRB, inducing a southwest-northeast-oriented YRB rain belt. In the Northwest Pacific, both the air-sea coupling and the western North Pacific summer monsoon play essential roles in the northwestward propagation of 10-30-day ISO.Three subseasonal schematic models of the formation of PEPEs over eastern China are summarized. There are totally 25 PEPEs in the eastern China during the period 1951-2010 shown by Chen and Zhai (2013), which mainly occur in the early summer (June-July) and exhibit significant subseasonal oscillations at 3 subseasonal time scales:10-40-day oscillation, qusi-biweekly oscillation and qusi-30-day oscillation. On the 10-40-day timescale, the anomalous low-level (850 hPa) anticyclone over the north region of SCS and the northwest Pacific transports warm moisture to the YRB. Such anomalous anticyclone is formed by combination of the anticyclone generated locally in SCS and the one propagated northwestward from the tropicalwest Pacific, and its period is about 16 days. In the upper level (200 hPa), the anomalous cyclone to the north of YRB deepens the East Asian trough, which favors the intrusion of cold air from north into the YRB. This anomalous cyclone moves clockwise around the Tibetan Plateau and shows an oscillation period of 30 days. On the qusi-biweekly (10-20-day) timescale, the anomalous low-level anticyclone propagates northwestward from the tropical western Pacific to the south of YRB. And the anomalous upper-level cyclone moves southeastward along the great circle over the Eurasian continient to the northeast of YRB. The warm moisture conveyed by the low-level southwesterly from SCS meets with the cold air transported by the upper-level northwesterly from the high latitude, inducing PEPEs in the YRB. The quasi-30-day (20-40-day) PEPEs are mainly related to the BSISO. The anomalous anticyclone over the northwest Pacific and the northern SCS conveys abundant warm moisture to the YRB. The strong upper-level anticyclone that comes from the southeastward propagation along the great circle over the Eurasian continient resides right above the YRB, inducing upper-level divergence. The low- and upper-level atmospheric anomalies form the thermodynamical and dynamical conditions of PEPEs in the YRB, respectively. Furthermore, associated with the PEPEs, the early summer mean oceanic and atmospheric anomalies are also favorable for the occurrence of persistent extreme YRB rainfall. Therefore, both the background states of the atmosphere and ocean and subseasonal disturbances have great contributions to the significant PEPEs in the YRB.A mechanism model for the northward propagationg of BSISO is proposed from the perspective of air-sea coupling between Indian Ocean and western Paciifc. The BSISO initiated in the tropical Indian Ocean also plays an important role in regulating the PEPEs over the eastern China. It propagates in two directions, i.e., eastward propagation from the equatorial Indian Ocean to the equatorial western Pacific; and northward propagation over both the Indian Ocean and the western Pacific to the Asian continent. In this study the air-sea coupling between the Indian Ocean and the western Pacific is proven to play an essencial role in the northward propagation of BSISO. In the Indian Ocean, the reduced surface wind speed by the anomalous easterly wind to the north of the anomalous convection leads to the northward movement of BSISO signal via wind-evaporation-sea surface temperature (WES) feedback. Among which, the easterly wind anomaly is one of the key factor. It originates from two major sources:the response of Rossby wave pack to the convective diabatic heating in the Indian Ocean and the easterly wind response to the convective diabatic cooling in the northwest Pacific, wherein, the latter dominates the easterly wind anomaly during the whole period of northward propagation of BSISO. Concurrently, the convective diabatic cooling in the north west Pacific propagates northwestward as well, which results from two processes, too, i.e., the Kelvin wave response to the convective diabatic heating in the Indian Ocean, which triggers an anticyclone over northwest Pacific and sets up a favorable environment for the suppressed convection over there; the anticylonic Rossby wave response to the convective diabatic cooling induced by the suppressed convection in northwest Pacific, which locates northwest to the diabatic cooling and leads the suppressed convetion to propagate northwestward.