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-atmosphere observations commenced in 1957. The sudden splitting of the polar vortex, a phenomenon rarely observed even in the Northern Hemisphere, marked this event. While previous studies focused on tropospheric waves and vortex preconditioning, the role of in situ-excited planetary waves (PWs) remains unexplored. The current study addresses this gap by examining the impact of in situ-generated PWs on SSW02 evolution. As the onset approached, the displaced polar vortex elongated and ultimately split into two vortices. The explosive amplification of zonal wavenumber (ZWN) 2 PWs (PW2) at 10 hPa, which split the vortex, was not only driven by upward-propagating PW2 from the lower stratosphere but also by westward-propagating PW2 excited in situ in the mid-to-upper stratosphere, which then descended to 10 hPa. This spontaneous PW2 generation was associated with barotropic–baroclinic instability, triggered as the stratosphere became dominated by easterlies descending from the lower mesosphere. The unusual poleward shift of the polar vortex facilitated easterly development by directing ZWN1 PWs (PW1) into the polar stratosphere, where they deposited strong westward momentum. PW2 amplification via instability occurred through two mechanisms: (1) the breaking of PW1 generated smaller-scale waves through energy cascading while inducing instability that amplified these smaller-scale waves, which could play a role in the local PW2 growth; and (2) over-reflection of upward-propagating PW2. While both mechanisms contributed to the amplification, the latter became dominant as the onset neared.