Tropical Upper-Tropospheric Potential Vorticity Intrusions during Sudden Stratospheric Warmings

Abstract

Abstract The intrusion of lower-stratospheric extratropical potential vorticity into the tropical upper troposphere in the weeks surrounding the occurrence of sudden stratospheric warmings (SSWs) is examined. The analysis reveals that SSW-related PV intrusions are significantly stronger, penetrate more deeply into the tropics, and exhibit distinct geographic distributions compared to their climatological counterparts. While climatological upper-tropospheric and lower-stratospheric (UTLS) PV intrusions are generally attributed to synoptic-scale Rossby wave breaking, it is found that SSW-related PV intrusions are governed by planetary-scale wave disturbances that deform the extratropical meridional PV gradient maximum equatorward. As these deformations unfold, planetary-scale wave breaking along the edge of the polar vortex extends deeply into the subtropical and tropical UTLS. In addition, the material PV deformations also reorganize the geographic structure of the UTLS waveguide, which alters where synoptic-scale waves break. In combination, these two intrusion mechanisms provide a robust explanation describing why displacement and split SSWs—or, more generally, anomalous stratospheric planetary wave events—produce intrusions with unique geographic distributions: displacement SSWs have a single PV intrusion maximum over the Pacific Ocean, while split SSWs have intrusion maxima over the Pacific and Indian Oceans. It is also shown that the two intrusion mechanisms involve distinct time scales of variability, and it is highlighted that they represent an instantaneous and direct link between the stratosphere and troposphere. This is in contrast to higher-latitude stratosphere–troposphere coupling that occurs indirectly via wave–mean flow feedbacks.