Stratospheric Circulation in the Southern Hemisphere: Links to Tropical Winds, Ozone and the Hunga Eruption—Part II: Evidence From a Global Chemistry‐Climate Model

Abstract

The Southern Hemisphere (SH) stratospheric circulation can be categorized based upon the development of a Low‐Latitude Jet (LLJ) in the upper stratosphere during winter months. We analyze the dynamics of the LLJ based on a large ensemble of chemistry‐climate model simulations, supported by reanalysis data. The LLJ is associated with westerly wind anomalies in the middle and upper stratosphere during mid‐winter, coupled with weak planetary wave activity and a slower Brewer‐Dobson circulation. Circulation effects extend into the tropical upper stratosphere, where the LLJ impacts the tropical semi‐annual oscillation (SAO). Additionally, the LLJ is linked to a persistent, strong polar vortex in the lower stratosphere during October–December. This cold, strong vortex is associated with decreased ozone amounts in winter and spring; ozone radiative feedbacks reinforce the cold polar vortex after sunlight returns in October. The 2022 Hunga eruption coincided with an anomalously strong LLJ year, and ensemble simulations show that the Hunga eruption shifted the SH winter circulation toward the LLJ state, although the ensemble‐mean forced Hunga signal is small compared with the observed 2022 anomalies and is embedded within a large amount of stochastic variability. These results establish the LLJ as a robust mode of SH winter stratospheric variability and provide a new insight for interpreting the dynamical response to the Hunga eruption in the context of natural variability.