Development of the Ural High blocking pattern during November

Abstract:

Blocking in the mid-troposphere over the Ural Mountain range during November is known to be associated with mid-winter atmospheric variability. Namely, development of this pattern, hereafter referred to as the Ural Block (UB), is associated with variations in the strength of the polar vortex, Barents-Kara sea ice concentration, and continental temperature over Eurasia. The UB is an internal mode of atmospheric variability and it is not clear what large-scale atmospheric circulation features control its development. While sea-ice and continental snow have been reported as drivers of the UB pattern, recent results also show that their variability is influenced by the pattern. Therefore, we seek out (1) other features of the large-scale circulation that lead to the formation of the UB pattern and (2) what impacts the UB has on the mid-winter circulation.

We analyze the output from ten Atmospheric Model Intercomparison Project (AMIP) style simulations, which use a 1979-2016 chronology of sea surface temperature and sea ice concentration variability as boundary forcings. By defining a UB index and compositing on these events, the 19 days preceding UB formation are analyzed. The preliminary results suggest a role for the stratospheric circulation. In particular, it seems that there is downward planetary wave propagation from the stratosphere into the troposphere days before UB formation. While it appears that the waves are modulating the tropospheric flow, more analyses need to be done. An understanding of why the stratospheric circulation may be conducive to downward wave coupling is sought. We also use additional AMIP runs in which observed UB variability in fall is prescribed to the model through nudging. This is part of an effort to understand more broadly how the UB influences the mid-winter circulation and perhaps if predictability can be gained from this.