Dynamics of the Stratospheric Aleutian Anticyclone

The "Aleutian High" (AH) is a normal, climatological feature of the wintertime Northern Hemisphere stratospheric circulation. Although the climatology of the AH has been well documented, its existence and dynamics have not been explained. Using MERRA data and a moments technique developed by the second author, an AH was defined by a closed 31.2-km contour on the 10-mb pressure surface, within 40 - 80 N and 120 E - 100 W, 1 October through 31 March, covering 1.84X10**6 km**2 and persisting for at least five days. Sixty-eight events (each separated by at least 15 days) were identified in 35 winters (1980 - 2014), or almost two events per winter season. The onset of each AH event was investigated by inverting the quasigeostrophic potential vorticity tendency to obtain the geopotential height tendency due to mechanical (vorticity) and thermal forcings in the troposphere (1000 - 200 mb) and stratosphere (200 - 10 mb). Analyzed and calculated height tendencies were averaged under each AH and composited over all events. Composite 10-mb height tendencies at AH onset are mostly due to thermal and mechanical forcings in the stratosphere, but there is a non-negligible contribution from mechanical forcing in the troposphere, possibly due to upward propogating blocking anticyclones. Likewise, the demise of composite AH events is mostly due to stratospheric processes but with a non-negligible contribution from mechanical forcing in the troposphere. In most years, the onset of an AH event precedes any distortion (split or displacement) of the stratospheric polar vortex. However, these distortions can occur without an AH event, and an AH can occur without a polar vortex distortion (PVD). Attributes of the AH events (size, duration, intensity and intensification rate) are being investigated to see if they can determine the onset of a PVD. The relationship between tropospheric blocking and AH events is also being investigated and will be reported.