Impacts Of Extratropical Weather Systems On The Frequency Of Tropical Cyclone Landfall On Mainland China During Summer

CMA tropical cyclone best-track data and the NCEP/NCAR reanalysis data from 1949 to2018 and TC information for 2018 provided by the Central Meteorological Typhoon Network are used.Atmospheric circumglobal teleconnection(CGT)and circumglobal teleconnection index(CGTI)is defined.The location and intensity characteristics of the Western Pacific subtropical high(WPSH)from July to August of 1949 to 2016 are analyzed.The relationship between the extratropical weather systems and frequency of TC landfall on mainland China is studied.And the reasons for the anomalous frequency of TC landfall on mainland China in summer of 2018 are analyzed.Obtaining the following main conclusion:(1)There are 5 anomalous centers in middle latitudes of the northern hemisphere.CGTI shows downward trend from 1949 to 2016 and exists periodic variation of two to three years,representatively.CGT is closely related to large-scale circulation anomalies.The result shows that there is positive correlation between CGTI and the frequency of TC landfall.When CGT shows positive phase,The East Asian subtropical westerly jet intensified,and the enhancement of anticyclonic shear on the southern side of the jet stream strengthens the divergence at 200 hPa around TC activity area during landing process.Moreover,the intensification of Rossby wave energy propagating southward in troposphere leads to the convergence of wave flux and therefor easterly anomaly appears in the corresponding region.Both of the above results in the easterly anomalous steering flow and vertical shear of zonal wind,hence the possibility of TC landing in mainland of China would increase.(2)From 1949 to 2016,the area and intensity of the WPSH increased and extended westward.There is a significant negative correlation between the frequency of TC landing in the eastern mid-latitude region of China and the area index as well as intensity index of WPSH.In the negative anomalous year of intensity index,the higher relative vorticity at850 hPa will result in more TC generation frequencies,which will lead to more TC landing in the eastern coastal areas of China at mid-latitude.In addition,due to the relatively weak westerly in the north flank of the 500 hPa WPSH,the westerly steer flow in the mid-latitude region of Eastern China is significantly weaker than that in the positive anomalous year,which is conducive to TC landing in this region.(3)The frequency of TC generation and landing in the Northwest Pacific Ocean in the summer of 2018 is significantly higher than climate state.The mid-latitude weather system which affects the anomalous increase of TC landing frequency in 2018 is mainly the WPSH,and the influence of CGT is very small.In 2018,The higher relative vorticity at 850 hPa will result in more TC generation frequencies,which will lead to more TC landing in the eastern coastal areas of China at mid-latitude.In addition,The steer flow influenced by 500 hPa potential height field is obviously southeast near 25-35?N in eastern China,which leads to TC landing in mid-latitudes.The easterly shear which determined by 200 hPa zonal wind in the middle latitudes of Eastern China is beneficial to the maintenance of TC landing process.