24th Pacific Science Congress (PSC-24)

Cyclone Gabrielle passed along the northern coast of Aotearoa New Zealand in February 2023, producing historic rainfall accumulations and consequent impacts. The Gabrielle storm -- an ex-tropical cyclone stalling and re-energising off the north coast -- approached a worst-case rainfall scenario for the northeast of the North Island. Here we report on a comparison of the actual forecast of the storm against forecasts under conditions representative of past and future anthropogenic climate change. We find that regional total rainfall accumulations from a Gabrielle-class storm have increased by about 10% under the historical 0.96°C of regional warming, and will increase by a similar amount under similar future warming. This attributable increase is driven by a 20% (past) to 30% (future) rise in peak rainfall rates, which in turn is driven by a more concentrated column-integrated moisture flux under past warming, but equally a more concentrated moisture flux and an increased precipitation efficiency under future warming.

This method of comparing weather forecasts against perturbations of those forecasts representing past or future conditions under climate change is well suited for understanding climate change risk for some types of events but not others. In this presentation we will also discuss the strengths and weaknesses of this approach, and how it might be deployed more widely for extreme weather across the Pacific.