Study on Two Categories of Sudden Stratospheric Warming

Invoking 45-yr daily European Centre for Medium-range Weather Forecasts (ECMWF) reanalysis data, rstly all the sudden stratospheric warming (SSW) events are selected in these years, which can be classified into two categories: downward-propagating event and non-downward-propagating event. And then, based on potential vorticity distribution on isentropic surfaces (IPV), temperature field, and zonal wind field, a detailed description of the SSW occurring during the winter-spring (December and the following January,February, March) in 2000-01 and 2001-02 is given. Finally, the evolvement of polar vortex during warming process and the impact of warming on troposphere are discussed. It is found that (1) there is inter-decadal variation for stratospheric warming phenomenon; (2) the SSW event lasting from late January till early March in 2001 can propagate downward to troposphere; (3) during this SSW, there is zonal-mean easterly winds in both stratosphere and troposphere; (4) the two warming events during December 2001 and March 2002 cannot propagate downward to troposphere, while zonal easterly winds only appear in stratosphere; and (5) in the process of the two types of warming, a long and narrow high-value IPV "tongue" extends out from main polar vortex, which breaks out the gradient of IPV. Compared with the non-downward-propagating stratospheric warming case, the highest value of IPV departs farther from pole and the "tongue" is longer and narrower during the downward-propagating warming event. Pinched by anticyclone from middle latitude,the stratospheric polar vortex will displace, distort or breakdown. By contrast, the change of polar vortex is greater in the course of downward-propagating warming event. Also, troposphere circulation and polar vortex evolve in different degree, and usually both of them go with blocking, but the above evolvement in the downward-propagating warming is more distinct.