The role of Rossby wave breaking in the formation and maintenance of tropical-extratropical cloud bands over the South Pacific

Abstract

Tropical-extratropical cloud bands are elongated cloud structures bridging tropical and midlatitude regions that act as a primary source of regional precipitation. While the role of Rossby wave breaking in the formation of cloud bands is established, the extent to which this dynamic forcing governs cloud band characteristics, their entire lifecycle, their spatial distribution and seasonality has not yet been systematically quantified. In this study, we apply an object-based approach to reanalysis data to investigate how stratospheric potential vorticity (PV) structures, 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 structures steer tropical moisture poleward and eastward, shaping the diagonal orientation of the cloud bands. We also find that cloud band duration is modulated by the properties of PV structures: long-lived cloud bands are distinguished by a systematically higher frequency of upstream PV structures and are sustained by persistent PV structures throughout their lifecycle, which favour a more zonal orientation of the cloud systems. Categorizing by cloud band duration reveals distinct seasonal regimes: while short-lived events occur year-round, persistent cloud bands are strictly confined to the austral warm season. Furthermore, long-lived cloud bands are associated with PV structures that reside significantly farther equatorward prior to genesis compared to those of short-lived events. These findings highlight that breaking Rossby waves create a tropospheric environment favouring not only the formation but also the maintenance of these cloud bands. Consequently, accurately representing Rossby wave dynamics in weather and climate models is critical for simulating cloud band characteristics and their influence on climate variability.