Slow Eastward-Propagating Planetary Waves prior to Sudden Stratospheric Warmings

Recent studies observed slow eastward-propagating planetary waves (EPWs) in the lower mesosphere prior to the sudden stratospheric warming (SSW) of January 2009. Although these EPWs were seen in conjunction with upward-propagating stratospheric planetary waves (PWs), the formation of instability in the lower mesosphere suggests that they were also generated in situ. Expanding the context to SSWs in general, the present study investigates the source and characteristics of these precursory EPWs using a long simulation (1978 to 2013) of the Whole Atmosphere Community Climate Model in the specified dynamics configuration. EPWs are analyzed from the perspective of instabilities in zonal-mean flows. EPWs with a phase speed of 10 m s^-1 are found as both wavenumber-1 and wavenumber-2 perturbations; these perturbations are amplified in regions of instability. These regions of instability coincide with the formation of a double-jet structure prior to SSW. Although the location, time of occurrence, and magnitude of EPW growth varies, a composite of split SSWs shows a robust response of wavenumber-2 EPW growth inside a region of instability in the lower mesosphere; the associated Eliassen-Palm flux divergence accelerates the background wind between the two cores of the double-jet structure.