9.2 Blocking precursors to stratospheric sudden warming events

Thursday, 11 June 2009: 8:40 AM
Pinnacle A (Stoweflake Resort and Confernce Center)
Olivia Martius, ETH, 8092 Zurich, Switzerland; and L. M. Polvani, M. Croci-Maspoli, and H. C. Davies

Both sudden stratospheric warming (SSW) events and blocking events are

major atmospheric flow phenomena. An SSW event severely disrupts the

stratospheric polar night jet, as the polar vortex is either displaced

equatorwards and sheared into a comma shape (a displacement SSW) or

torn into two individual vortices (a splitting SSW). Similarly,

blocking events severely disrupt the extra-tropical circumpolar

tropopause-level jet, which is either displaced poleward of or splits

around the block's core of anomalously low potential vorticity at

tropopause levels.

Here we focus on the dynamical linkage between to two phenomena by

examining their multiple co-occurrence over a climatological time

period, and analyzing specific events. Specifically, we find clear

evidence that blocking events are precursors to SSW events.

Spatial frequency composites of atmospheric blocking are constructed

for the ten-day period preceding splitting and displacement SSW

events, as objectively identified in the ERA-40 data set in an earlier

study. Distinct differences in the location and amplitude of the

blocking frequency distributions are found. Displacement events are

preferentially preceded by blocking in the Atlantic basin, whereas

splitting events are preceded by blocking events occurring in the

Pacific basin or both basins concomitantly. These differences in the

blocking distribution are mirrored in significantly different

planetary wave patterns prior to the warming events.

Analysis of individual events supports the idea of tropospheric

blocking as a key precursor to SSW events, and sheds further light on

the relationship between the two phenomena.

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