The Link Between Rossby Wave Breaking and the Maintenance of Tropical-Extratropical Cloud Bands over the South Pacific

Tropical-extratropical cloud bands are elongated cloud structures bridging tropical and midlatitude regions, and play an important role in the hydrological cycle. While the role of Rossby wave breaking in the formation of cloud bands is established, the extent to which this dynamic precursor governs their formation, duration, spatial distribution, and seasonality has not yet been systematically quantified. In this study, we use an object-based approach applied to reanalysis data to investigate how stratospheric potential vorticity (PV) intrusions, as indicators of Rossby wave breaking, influence cloud band formation and persistence over the South Pacific region. Our climatological analysis confirms a robust statistical link, in which cyclonic PV anomalies steer tropical moisture poleward and eastward and shape the diagonal orientation of the cloud bands. We also reveal that the longevity of cloud bands is modulated by the properties of PV structures: long-lived cloud bands are sustained by persistent PV intrusions that penetrate significantly farther equatorward than those associated with transient events. These findings highlight that equatorward-breaking Rossby waves create a tropospheric environment not only favouring the formation but also the maintenance of tropical-extratropical cloud bands. Consequently, accurately resolving PV intrusion forcing is critical for improving the predictability of cloud band duration and associated precipitation.