Role of in situ-excited planetary waves in polar vortex splitting during the 2002 Southern Hemisphere sudden stratospheric warming event

Abstract. On 25 September 2002, the Southern Hemisphere experienced its first and only major sudden stratospheric warming (SSW02) since routine upper-atmospheric observations commenced in 1957. This event was marked by a sudden splitting of the polar vortex. While previous studies focused on tropospheric waves and vortex preconditioning, the role of in situ-excited planetary waves (PWs) remains unexplored. The current study addressed this gap by examining the impact of in situ-generated PWs on SSW02. As the onset approached, the displaced polar vortex elongated and split into two. The explosive amplification of zonal wavenumber (ZWN) 2 PWs (PW2) at 10 hPa was driven not only by upward-propagating PW2 from below but also by westward- and downward-propagating PW2 excited in the mid-to-upper stratosphere. This spontaneous PW2 generation was associated with barotropic–baroclinic instability, triggered by easterlies descending from the polar lower mesosphere. An unusual poleward shift of the polar vortex facilitated the development of easterlies by directing ZWN1 PWs (PW1) into the polar stratosphere, where they deposited strong westward momentum. This poleward displacement was attributed to anomalous easterlies in the equatorial upper stratosphere. PW2 amplification via instability involved two mechanisms: (i) PW1 breaking generated smaller-scale waves through a nonlinear energy cascade while inducing instability that amplified those waves, and (ii) over-reflection of upward-propagating PW2. A nonlinear energy cascade initiated PW2 amplification, with over-reflection contributing immediately before the onset. Instability-driven wave growth and equatorial easterlies in the upper stratosphere also appear in other vortex-splitting SSWs, suggesting their broader significance in SSW dynamics.