Interannual Variability Of Spring Extratropical Cyclones Over The Yellow,Bohai,and East China Seas And Possible Causes

Interannual variability of cyclones that are generated over the eastern Asian continent and passed over the Yellow,Bohai,and East China seas(YBE cyclones)in spring is analyzed using reanalysis datasets for the period of 1979–2017 with an objective tracking algorithm of cyclones.Possible causes for the variability are also discussed.The results are shown as follows: The average number of YBE cyclones is 6 per year,over 80% of which develop when they pass the YBE region.The number of YBE cyclones also exhibits significant interannual variability with a period of 4–5 years.Developing cyclones are further classified into two types: rapidly developing cyclones and slowly developing cyclones.The number of rapidly developing cyclones is highly related to the underlying sea surface temperature(SST)and the atmospheric baroclinicity from Lake Baikal to the Japan Sea.The number of slowly developing cyclones,however,is mainly affected by the North Atlantic Oscillation(NAO)in the preceding winter(DJF);It works in two different ways,both through the upper-level jet stream over Japan and the memory of ocean responses to the atmosphere.Mid-latitude Atlantic Ocean plays an important role in this process.Positive NAO phase in winter is associated with the meridional tripole pattern of SSTA in the North Atlantic Ocean,which persists from winter to the following spring(MAM)due to the thermal inertia of the ocean.The SSTA in the critical mid-latitude Atlantic region in turn act to affect the overlying atmosphere via sensible and latent heat fluxes,leading to an increased frequency of slowly developing cyclones via exciting an anomalous eastward-propagating Rossby wave train.These results are confirmed by several numerical simulations using an atmospheric general circulation model.By analyzing an explosive cyclone in 2016 and performing numerical simulations,we conform that local SST is an important factor in the development of cyclone.