Effects Of The Concurrent Variation Between The East Asian Subtropical And Polar Front Jets On Precipitation During Meiyu Season

As an essential part of the East Asian summer monsoon system, the East Asian jet stream has an important influence on the weather and climate of East Asia. Observational analysis indicates that the East Asian jet stream consists of two separate branches, i.e.,the East Asian subtropical jet (EASJ) and polar front jet (EAPJ).Based on the daily NCEP-NCAR reanalysis dataset and daily gauge observations from 738 China stations, the relationship between the EAPJ and mid-high latitude circulation systems and the related cold air activity path is revealed. Effects of different configurations of the EASJ and EAPJ on precipitation during Meiyu season are investigated, and the possible mechanism responsible for this phenomenon is discussed. Furthermore, we discuss the influence of East Asia-Pacific teleconnection and Eurasian teleconnection on the concurrent variation between EASJ and EAPJ,and the role of East Asian jet stream in persistent heavy rainfall events during Meiyu season.1 The relationship between the East Asian polar front jet and mid-high latitude circulation systemsThe meridional component of the East Asian polar front jet is very important even though it is a westerly jet stream on the average during Meiyu season. Accompanied with the strong meridional wind in the EAPJ region, trough and ridge systems develop in mid-high latitude, mid-high latitude circulation changes from zonal type to meridional type. The EOF analyses are carried out on meridional wind at 300hPa over the domain of the EAPJ region. The result shows that there are four centers of meridional wind anormaly in the EAPJ region:80°E,100°E,120°E,140°E. When strong northerly wind appears in 80°E, the anormalies in 500hPa geopotential height show "+,-,+" pattern from west to east, the anormaly centers are located at 60°E,100°E and 140°E, the Okhotsk Sea blocking high is stronger than the Ural blocking high, the cold airmass from mid-high latitude comes down with northwesterly wind. When strong northerly wind appears in 100°E, mid-high latitude circulation shows ridge-trough-ridge pattern, and the Ural blocking high is stronger .than the Okhotsk Sea blocking high, the cold airmass from mid-high latitude comes down along the central path. When strong northerly wind appears in 120°E, mid-high latitude circulation shows ridge- trough pattern, the ridge is located at 100°E and the trough is located at 140°E, blocking high mainly occurred in 90-110°E, the cold airmass from mid-high latitude comes down with northeasterly wind, effecting the weather in Yangtze-Huai River Basin. When strong northerly wind appears in 140°E, a positive anomaly center of 500hPa geopotential height appears in 120°E, and blocking high concentrated in 120°E, the East China is affected by cold air coming from northwest and northeast.2 Effects of different configurations of the East Asian subtropical and polar front jets on precipitation during Meiyu seasonThe intensity and location of precipitation are associated with different configurations of the EASJ and EAPJ during Meiyu season. Precipitation intensity increases with intensification of the EASJ and EAPJ. Rain-band is located to the north of Meiyu region when the EASJ intensifies and the EAPJ weakens.Further analyses indicate that the intensity changes of the EASJ and EAPJ are linked to the cold and warm airmass activities. For cases with strong EASJ and EAPJ, both the warm-moist airmass and cold airmass are active. When the warm-moist airmass and cold airmass meet near 30°N, abundant precipitation occurs in the Yangtze-Huai River Basin (YHRB).For cases with weak EASJ and EAPJ,both the cold and warm-moist airmasses are inactive, and no significant precipitation occurs in the YHRB.For cases with strong EASJ and weak EAPJ, warm-moist airmass moves northward while the cold airmass is weak. Precipitation concentrates to the north of YHRB.For cases with weak EASJ and strong EAPJ, cold air extends farther south while the warm-moist airmass is inactive. Precipitation occurs to the south of YHRB. 3 The influence of atmospheric teleconnections on the concurrent variation between EASJ and EAPJ and the role of East Asian jet stream in persistent heavy rainfall eventsThe East Asia-Pacific teleconnection (EAP) is related to the meridional positions and intensities of the EASJ and EAPJ.In the positive EAP phase, the EAPJ is unusually strong and moves more northward, the EASJ is intensified and moves southward. In the negative EAP phase, the EASJ and EAPJ are weak; meanwhile, the EAPJ moves more southward. There is a negative correlation between Eurasian teleconnection (EU) and the zonal locations of the EASJ and EAPJ. Positive EU phase is accompanied by the westward-intruding EASJ and EAPJ,negative EU phase is accompanied by the eastward-intruding EASJ and EAPJ.The concurrent variation between EASJ and EAPJ is the reflection of EAP and EU.The atmospheric teleconnections and related East Asian jet stream play important roles in the persistent heavy rainfall events (PHR) during Meiyu season.In the background of EAP events, the EASJ and EAPJ are enhanced significantly during PHR, the sudden enhancement/ weakening of two jets marks the beginning/end of PHR. The northerly wind in the EAPJ region is not strong. Persistent anomalies appear in low latitude circulation, accompanied with the activity of warm airmass. During PHR under the EU events, the strength of two jets do not change obviously, the meridional component of EAPJ is strong, the northerly wind anomaly center located at 50-800°E, showing the characteristic of persistent anomaly. The cold airmass activity is active with the persistent anomalies in mid-high latitude circulation.