Stratospheric influences on midtropospheric blocking anticyclones

Midtropospheric (500-mb) anticyclogenesis accompanying the onset of blocking is locally characteterized, in selected cases, by lower tropospheric warming, consistent with previous studies, and by net cooling in the upper troposphere and stratosphere. The stratospheric cooling is predominantly adiabatic in nature and partially offset by advective warming. The stratospheric adiabatic cooling is due to gently ascending air coupled with strong static stability. This ascent in turn has a substantial nonquasigeostrophic component, calculated as a difference between analyzed and quasigeostrophic vertical motions. Nonquasigeostrophic vertical motions are also relatively large in the lower troposphere but yield adiabatic warming through descent. This adiabatic warming is less in magnitude than the stratospheric adiabatic cooling. It is therefore hypothesized that stratospheric waves may contribute to tropospheric blocking anticyclone development if the ascending air associated with these waves becomes aligned with the developing block.