Interactions between the tropical moist margin and extratropical Rossby waves for rainfall extremes

Extreme precipitation in the subtropical regions is often influenced by a combination of tropical and extratropical processes. In this work, we examine two-way feedbacks between the tropical moist margin, which is a proxy for heavy rainfall, and extratropical Rossby waves, defined by upper-level potential vorticity (PV). Firstly, cyclonic PV anomalies that approach the moist margin induce strong poleward moisture transport resulting in heavy rainfall, but only if the PV anomaly extends into the low- to mid-troposphere. The enhanced convection and associated upper-level divergence then feeds back onto the upper-level PV field by contributing to ridge building, potentially having downstream impacts. These processes are highlighted in composite analysis and a case study of a subtropical cyclone affecting New Zealand in January 2018. Experiments with modified latent heating in the ACCESS-rAM3 model reveal the critical role of moist processes in such events.