Growth of planetary waves and the formation of an elevated stratopause after a major stratospheric sudden warming in a T213L256 GCM

Abstract

Recovery processes after a major stratospheric sudden warming (SSW) with the formation of an elevated stratopause and a strong polar‐night jet are investigated using a gravity‐wave‐resolving GCM. The major SSW that occurred in the GCM bears a strong resemblance to observations in January 2006 and January 2009. The recovery phase of the SSW in the GCM is divided into two stages. In the first stage during about five days just after the SSW, a large positive Eliassen‐Palm (E‐P) flux divergence associated with the growth of planetary waves contributes to the quick recovery of eastward wind above 2 hPa (about 42 km), which is likely due to baroclinic and/or barotropic instabilities. In the second stage over the next three weeks, a prolonged westward wind in the lower stratosphere blocked upward propagation of gravity waves with westward intrinsic phase velocities. It reduces the deceleration of eastward wind in the upper mesosphere and raises the breaking height of gravity waves. Since the height of westward gravity wave forcing also rises, the polar stratopause created by the gravity‐wave‐driven meridional circulation is formed at an elevated height (about 75 km) compared to that before the SSW (55–65 km). In addition, the weaker westward gravity‐wave forcing in the upper mesosphere drives weaker downwelling around 1 hPa and forms a cold layer. Consequently, the strong polar‐night jet forms at a higher altitude than before the SSW as a result of adjustment toward the thermal wind balance. This indicates that these two stages provide eastward acceleration in different ways.