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The waveguide modulations

Comparing Figures 1 and 2, it is evident whether an upper stratospheric warm anomaly propagates down depends on how planetary waves interact with stratospheric zonal mean flow. In particular, a continuous wave transport is essential in the period following the upper stratospheric warming. As well-known, large-scale planetary waves originate in the lower troposphere, and the largest wave amplitudes are observed in the subpolar latitudes centered near 60N. Prior to stratospheric sudden warming events, the zonal mean flow is in a preferred state for large waves propagating upward and poleward. However, the condition of zonal mean flow is changed during the warming, which consequently affects the way of wave propagation, because waveguides are modulated by zonal mean flow. In Figures 3 we compare the quasi-geostrophic refractive index for wavenumber 1 stationary wave, in the post-warming period (Day 21-40) for the propagating and non-propagating cases. Note that waves tend to propagate toward large positive values of the index and avoid negative values of the index. The refractive index became very large in the 60N-70N upper troposphere in the propagating case, so that mid-latitude waves were strongly refracted poleward. The increase in the value of refractive index was mainly due to the large decrease in zonal mean zonal wind associated with the reversal of polar westerly flow and the descending of "critical line." In the non-propagating case, however, the refractive index was smaller in the subpolar troposphere, and there was little poleward wave transport.

Figure 3. (a) Average quasi-geostrophic refractive index for wavenumber 1 stationary wave (contours) and E-P flux (vectors) of Day 21 - 40 for the propagating case. (b) Same as (a) except for the non-propagating case. Contour levels are -100, -50, -25, 0, 25, 50, 100, ... and contours greater than 400 are omitted