Research On The Subseasonal Meridional Activity And Its Mechanism Of The Western Pacific Subtropical High In Summer

Sub-seasonal variability in meridional activity of Western Pacific subtropical high(WPSH)exerts important influences on anomalous weather and climate in East Asian monsoon region.By using the daily reanalysis data from NCEP/DOE,the daily OLR data from NOAA,applying the method of regression and composite analysis,on the basis of the sub-seasonal meridional activity index of WPSH,the characteristics and atmospheric evolutions in association with the sub-seasonal abnormal meridional activity of the WPSH during boreal late summer are investigated.The possible physical mechanism of the low frequency oscillation of the WPSH is further discussed.Finally,the prediction effect of the second version climate forecast system provided by NECP on the intraseasonal meridional activity of the WPSH is evaluated.(1)The abnormal meridional activity of the WPSH exhibits distinctive low frequency oscillation(LFO),with two dominant periods of 10-30 days and 40-50 days.On the 10-30 days LFO,there is adjusting from barotropic structure to baroclinic structure in the middle latitudes during the northward movement of the WPSH,and re-adjusting to barotropic structure during the southward retreat.At the 40-50 days LFO,the vertical structure changing from barotropic to baroclinic at low latitudes when the WPSH advances northward,while the quasi-barotropic structure is maintained in the middle latitudes.(2)The 10-30 days LFO is influenced by wave trains in mid-high latitudes and convection propagating from tropic.For the northward advance of the WPSH,in the high latitudes,teleconnection wave train originating from Atlantic Ocean propagates eastward,then the negative potential vortex anomalies enhance the high-level divergence in Northeast Asia.In the mid-latitudes,the 10-30 day low frequency wave train from North America propagates westward and influences the pressure field along the East Asia coast,whose energy is gained from the westerly jet through barotropic process.In the tropic,on time scale of 10-30 days,anomalous convections over the equatorial western Pacific propagate northwestward and trigger cyclone-anticyclone circulation anomalies along the East Asia coast,which in turn affect the meridional position of the WPSH.(3)The 40-50 days LFO of meridional activity of WPSH is closely related to the convection anomalies under the interaction between Madden-Julian Oscillation(MJO)and monsoon intra-seasonal oscillation(MISO).The tropical MJO propagates eastward from the equatorial Indian Ocean to West Pacific and then propagates northward,in accompany with the northeastward propagation of anomalous convection teleconnection covering the tropical Indian Ocean-North Indian Peninsula-Northwest Pacific,resulting in the anomalous anti-cyclonic circulation over Northwest Pacific,which contributes to the northward advance of the WPSH.For the southward retreat of the WPSH,the contrary is the case in 10-30days atmospheric LFO while relatively weaker 40-50days atmospheric LFO does less contribution to the southward retreat of the WPSH.(4)The NCEP/CFSv2 has a good prediction effect for the 10-30 days LFO of the WPSH,which verifies the existence of the low-frequency westward wave train over the North Pacific and the northward convection in the equatorial western Pacific.The simulation of the second northward jump of the WPSH in 2020 also shows that when the WPSH strengthens,the WPSH will split into several units.Then when the western main body extends westward to the East Asian coast,the western part of the WPSH will break,resulting in the northward lift of the WPSH.The emergence and westward movement of the anomalous center of positive geopotential height located on the east side of Japan can be regarded as an early signal of the northward elevation of the WPSH.