Quality of reanalysis data during stratospheric vortex weakening and intensification events

The quality of five reanalysis data (ERA, JRA-25, NASA-MERRA, NCEP/DOE and NCEP/NCAR) is evaluated for stratospheric vortex weakening (SVW) and intensification (SVI) events in the Northern Hemisphere winter. Both a case study and composite analysis are performed. The case study is carried out for a record-breaking SVW event in 2009 whose spatio-temporal structure is well observed by the COSMIC GPS RO measurements. It is found that most reanalysis data captured this event reasonably well. Although non-negligible biases, locally up to 50 K in potential temperature at 10 hPa, are present, they are mostly random and disappear in the lower stratosphere and troposphere. Composite analysis further shows that the temporal structure of the SVW and SVI events, as measured by the Northern Annular Mode (NAM) index, is almost identical among data set. Remarkably good agreement is found not only in the onset but also in their downward propagation with time. These results indicate that reanalysis data are reliable for examining stratospheric extreme events and their impact on the troposphere in an intraseasonal time scale.