Shape and Chemical Diagnostics of the Polar Vortices in WACCM

The structure of the polar vortices is diagnosed using 35 years (1979-2014) of output from the Specified Dynamics version of the Whole Atmosphere Community Climate Model (SDW). Moment diagnostics characterize vortex geometry in the lower and middle stratosphere. Trace gas-based definitions complement the wind-based diagnostics in the upper stratosphere and mesosphere. The polar vortices in SDW are compared to the polar vortices defined using geostrophic winds derived from Microwave Limb Sounder (MLS) observations of temperatures. SDW is nudged up to 40 km by the Modern Era Retrospective Reanalysis (MERRA) and in this region the polar vortices are well represented. In the upper stratosphere and mesosphere, however, there are significant differences between the polar vortices in the model and observations. Particular attention is paid to the Arctic vortex structure during the elevated stratopause (ES) events in 2004, 2006, 2009, 2012, and 2013. The evolution of vortex centroid longitude and latitude, area, ellipticity, and vortex splitting is documented as a function of altitude in both the model and in the observations. One month prior to ES onset, the vortex centroid moves poleward and the shape of the vortex becomes less elliptic in the upper stratosphere. Several days before ES onset, the area of the vortex doubles over a deep vertical range. A distinction from major SSWs, the large vortex area is observed in the middle and then in the lower stratosphere as the event evolves. The Arctic vortex structure in WACCM is well reproduced during ES events. The strong descent associated with these events provides ideal conditions to define the upper stratospheric vortex chemically.