Tropospheric Blocking, the Stratospheric Aleutian High and Distorted Polar Vortex, and Cold-air Outbreaks during the Winter Season

Transcription

1 Tropospheric Blocking, the Stratospheric Aleutian High and Distorted Polar Vortex, and Cold-air Outbreaks during the Winter Season Stephen J. Colucci Cornell University

2 While investigating processes that cause blocking for my Ph.D. thesis (1982), I was advised to save a few for the next century. Now we are in the next century. What still don t we know about blocking?

3 Since 1982, we have learned (among other things) that... Tropospheric blocking can precede distortions of the stratospheric polar vortex (Quiroz, JGR 1986 and many others). However, the reverse is not true (suggested by Colucci and Kelleher, JAS 2015). Stratospheric polar vortex distortions can be preceded and followed by lower tropospheric cold-air outbreaks (CAOs) and anomalies (e.g. Kostad et al., QJRMS 2010). Can tropospheric blocking remotely cause CAOs via stratospheric polar vortex distortions? Investigate events during the winter as a pilot study.

4 Surface temperature anomaly (F) averaged over 1 January 31 March 2014

5 Anomalous, near-split 10-mb polar vortex and anomalously cold 850-mb temperatures over North America on 0000 UTC 5 February 2014

6 The vortex distortion was preceded by a strengthening Aleutian High* (left), which in turn was preceded by tropospheric blocking** (right). *Harvey and Hitchman (JAS, 1996) **satisfying the persistent positive anomaly criterion of Dole and Gordon (MWR, 1983)

7 Large, four-day geopotential height rises propagated upwards from the troposphere to the stratosphere with time over the blocking high

8 Investigate geopotential height changes with potential vorticity inversion applied to NCEP/NCAR Reanalysis data q = h g + P é ê ë f 0 g s æ ç è Z P = g f 0 Ñ 2 Z + f - P ì \ Ñ 2 + é í ê î P ë öù ú = q V + q T øû f 0 RT s P Solve for height change ( t) due to each term on the righthand side (RHS) of the above equation. The first term on the RHS is the absolute vorticity tendency ( vorticity forcing ) while the second term is related to the temperature tendency ( thermal forcing ). é ê ë 2 f 0 ùü Z úý s P ûþ t = f 0 g Z ù ú û é ê ë q V t + q T, t ù ú û

9 Thermally forced height rises similarly propagated upwards from the troposphere to the stratosphere with time over the blocking high

10 Vorticity-forced height rises also propagated upwards from the troposphere to the stratosphere with time over the blocking high

11 What caused the large geopotential height falls southwest of the polar vortex?

12 Upward propagating thermally forced height falls (green), but much larger in magnitude vorticity-forced height falls (black) of stratospheric origin

13 Adiabatic temperature change through grid point with maximum 20-mb geopotential height rises (left) and falls (right)

14 Poleward eddy heat fluxes (upward wave propagation) through grid point with maximum 20-mb geopotential height rises (left) and falls (right)

15 Proposed Conceptual Model Poleward heat fluxes helped form a tropospheric blocking high over the Pacific Ocean. These heat fluxes propagated upwards, intensifying a stratospheric Aleutian High. An east-west oriented vertical circulation of air was established in the stratosphere, with subsiding, adiabatically warming air near the Aleutian High and ascending, adiabatically cooling air near the Polar Vortex. The Polar Vortex responded by drifting into the adiabatically cooled air. A CAO ensued over North America. The heat flux patterns suggest upward wave propagation near the blocking/aleutian Highs, and downward wave propagation near the polar vortex.