Extratropical transition of tropical cyclones in global climate models

Abstract:

Globally, approximately half of all tropical cyclones undergo extratropical transition, of which landfalling systems expose populous midlatitude regions to hurricane-force wind speeds and extreme precipitation. The frequency of tropical-origin storms across the midlatitudes is projected to increase under anthropogenic climate change, but multi-model studies are required to reduce projection uncertainties and inform assessments of future risk. We used a Lagrangian feature-tracking algorithm and performed analysis of cyclone-relative thermal winds to identify tropical cyclones undergoing extratropical transition in an ensemble of atmosphere-land-only and fully coupled global model simulations, performed under historical (1950–2014) and future (2015–2050) climate forcing under the CMIP6 HighResMIP experimental protocol. Historical simulations were evaluated against five global reanalysis datasets over the period 1979–2018. Considering all tropical cyclones undergoing transition globally, ensemble-mean biases in track and genesis densities are reduced when nominal horizontal resolution is increased from ~100 to ~25 km. Across most ocean basins, the multi-reanalysis-mean climatological frequency of extratropical transition events per year is also better reproduced at ~25 km than at ~100 km. However, little skill in reproducing historical interannual variability in forced simulations exists, highlighting the challenge posed by complex interplay of sea-surface temperature and midlatitude environment in modulating interannual variability in transition frequency. Cold-core, post-transition intensity distributions are reproduced equally well at low and high resolution across models, but warm-core, pre-transition intensities are markedly improved with increased resolution. Increased resolution also improves the representation of post-transition secondary intensification.