Downscaled Climate Projections of Tropical and Ex-Tropical Cyclones Over the Southwest Pacific

Abstract

Reliable projections of tropical cyclones (TCs) and associated impacts remain hampered by both climate model resolution and simulation length. To address this, here we present updated projections of TCs for the southwest Pacific from a high-resolution downscaled ensemble of CMIP6 models. The downscaling implements a variable-resolution atmospheric model enhancing resolution over the southwest Pacific and New Zealand (∼12–30 km). We assess future changes in TC frequency, changes in large-scale environmental conditions, and associated extreme precipitation and winds across tropical and ex-tropical storm phases. Changes in TC track pathways are also investigated through cluster analysis. Across the downscaled simulations, robust changes in TC frequency were not found, including for a high-emissions scenario at end-of-century. Projections of the background environmental conditions are shown to be a significant source of uncertainty, owing to diverging projections of relative SST and tropical convection across the region in the host GCMs. However, very strong TCs (category 4 and above) show greater consensus for an increase in frequency, with 16 of 18 simulations across models and scenarios projecting an increase. Cluster analysis of TC tracks indicates a slight decrease in tracks that often impact northern parts of Australia. Extreme precipitation associated with TCs under a high-emissions scenario is projected to increase by ∼30%–35% averaged across models, both for storms in the tropics and ex-TCs impacting New Zealand. This increase exceeds Clausius-Clapeyron scaling in five of six simulations. These projected increases in associated extreme precipitation pose significant societal risks despite the remaining uncertainty in TC frequency changes.