Tropospheric Blocking with and without Stratospheric Sudden Warming

It is well known that most Northern Hemisphere stratospheric sudden warmings (SSWs), defined by a reversal of the zonal mean zonal flow from westerly to easterly at 60N and 10 mb, are preceded by tropospheric blocking, defined by a locally persistent reversal of the midtropospheric flow from westerly to easterly. However, the converse is not true because tropospheric blocking events are far more frequent (several each winter) than SSWs (less than one per winter). Cases of tropospheric blocking in a reanalysis data set are therefore being diagnosed for insight into features that might distinguish blocking events that are followed by SSWs from those that are not. Most of the blocking cases, and all of those followed by a SSW, are characterized by poleward heat fluxes, and therefore upward stationary wave activity fluxes, that are persistently and anomalously large when averaged around 45N – 75N at 200 mb. Other investigators have found this feature in composites of SSWs, but it appears from the case studies to be a necessary but not sufficient condition for blocking to be followed by a SSW.

The cases of blocking that are each followed by a SSW are associated with a maximum in 20-mb anticyclonic potential vorticity (PV) tendency (exceeding 200 PVU per five days in magnitude) that is poleward of the 20-mb cyclonic vortex. In each of these cases, there is a well defined and intensifying Aleutian High at 10 mb. The cases of blocking that are not followed by a SSW are each associated with a maximum in anticyclonic vorticity tendency at 20 mb that is equatorward of the 20-mb polar cyclonic vortex. The Aleutian High at 10 mb is weak or absent in these cases. Similar results are found in an extended GFDL model run, including an unusual SSW case not associated with blocking. It is therefore hypothesized that a SSW occurs, with or without tropospheric blocking, when the latitude of the most negative (anticyclonic) PV tendency at 20 mb is greater than the latitude of the lowest 20-mb geopotential height.