Eurasian Cold Surges Triggered by the Nonlinear Reflection of Stratospheric Planetary Waves in December 2012

Eurasia experienced severe cold weather in December 2012. Particularly strong cold spells occurred in two regions: one over eastern Eurasia in early December, and the other in central to western Eurasia in mid-December. These extreme events were produced in association with blocking phenomena triggered by reflection or downward propagation of planetary waves from the stratosphere. This event was unusual in that dominant component of upward and downward propagating wave is different: zonal wavenumber 2 component is dominant in the upward propagation; due to the amplification of zonal wavenumber 1 component in the upper stratosphere, zonal wavenumber 1 component becomes important in the subsequent downward propagation in the polar region. Identification of the tropospheric impact of the downward propagation of planetary waves is usually not easy. In the present case, however, the difference in the dominant zonal wavenumber between the upward and downward propagation facilitates to separate upward and downward propagation processes, and to isolate the amplification process of planetary waves in the upper stratosphere.