Comparison of the Methods in Defining the Onset of Stratospheric Final Warming Event during Boreal Spring

This paper compares three methods in defining the onset of the boreal spring stratospheric final warming (SFW) events, which are based on the final reversal of daily circumpolar zonal wind direction (referred to as UD0), the reversal of zonal wind direction linearly interpolated from the monthly data (referred to as UM0), and the reversal of the meridional gradient of zonalmean temperature between the North Pole and 60°N (referred to as TG0), respectively.The SFW onset date identified by the UM0 method is basically consistent with that by the UD0 method.However, in the presence of major stratospheric sudden warming events in late winter and early spring, considerable discrepencies are discovered between the results of the two methods.The SFW onset date identified by TG0 tends to be earlier than that by UD0.At 30 hPa, The mean date of SFW by the TG0 method is~16 days earlier than that by the UD0 method.As the UD0 method is more common in practical use, the TG0 method is revised with this method as a reference.The research results show that if the threshold of the TG0 method is lifted from 0 K to 3 K, the SFW onset date in this method has excellent agreement with that by the UD0 method.