Extratropical Transition of Tropical Cyclones in a Multiresolution Ensemble of Atmosphere-Only and Fully Coupled Global Climate Models

Abstract

Tropical cyclones undergo extratropical transition (ET) in every ocean basin. Projected changes in ET frequency under climate change are uncertain and differ between basins, so multimodel studies are required to establish confidence. We used a feature-tracking algorithm to identify tropical cyclones and performed cyclone phase-space analysis to identify ET in an ensemble of atmosphere-only and fully coupled global model simulations, run at various resolutions under historical (1950–2014) and future (2015–2050) forcing. Historical simulations were evaluated against five reanalyses for 1979–2018. Considering ET globally, ensemble-mean biases in track and genesis densities are reduced in the North Atlantic and Western North Pacific when horizontal resolution is increased from ∼100 to ∼25km. At high resolution, multireanalysis-mean climatological ET frequencies across most ocean basins as well as basins’ seasonal cycles are reproduced better than in low-resolution models. Skill in simulating historical ET interannual variability in the North Atlantic and Western North Pacific is ∼0.3, which is lower than for all tropical cyclones. Models project an increase in ET frequency in the North Atlantic and a decrease in the Western North Pacific. We explain these opposing responses by secular change in ET seasonality and an increase in lower-tropospheric, pre-ET warm-core strength, both of which are largely unique to the North Atlantic. Multimodel consensus about climate-change responses is clearer for frequency metrics than for intensity metrics. These results help clarify the role of model resolution in simulating ET and help quantify uncertainty surrounding ET in a warming climate.

Can low-resolution CMIP6 ScenarioMIP models provide insight into future European Post-Tropical Cyclone risk?

“…Despite clear model biases, the selected CMIP6 models represent many features of the observed TC climatology, with spatial patterns and frequencies in qualitative agreement with observations. TC frequency, seasonal cycle, and spatial distribution in these selected CMIP6 models are comparable to those found in higher-resolution modelling studies, such as Climate-SPHINX (Vidale et al, 2021), UPSCALE (Roberts et al, 2015) and HighResMIP-PRIMAVERA (Roberts et al, 2020a;Haarsma et al, 2016;Baker et al, 2022), which used the same tracking and identification scheme. However, as expected, TC intensities are considerably lower in the CMIP6 models than identified in high-resolution studies.…”

“…Reduction in vertical wind shear (VWS) and increases in PI (Figure 6), seen across the models, are also consistent with enhanced longevity, implying increased likelihood of recurvature. A similar change in seasonality of extratropical transition was found across HighResMIP models (Baker et al, 2022). Climatologically, midlatitude baroclinicity increases throughout hurricane season and so in the future, many recurving TCs in HadGEM encounter a more favourable midlatitude environment.…”

“…S2), and so the original TC identification method is retained. The TC identification method used here has been used in numerous studies based on reanalyses (Hodges et al, 2017;Baker et al, 2021) and climate models (e.g., Baker et al, 2022;Roberts et al, 2015;Vidale et al, 2021). It has been shown that PTC impacts over Europe in the present climate are similar whether this objective TC identification method, or objective track matching with observational data, are used (Sainsbury et al, 2020).…”

“…Both cyclones later impacted Europe as PTCs, and Ophelia was responsible for Ireland's highest-recorded wind gust of 53ms -1 (119mph). Projected increases in sea surface temperature (SST) and the range of latitudes occupied by TCs (Studholme et al, 2022), combined with the observed trend in TC lifetime maximum intensity latitude (Kossin et al, 2014) opens the possibility for more cyclones to form-and attain high intensities-further polewards and eastwards in the basin, closer to Europe (Baker et al, 2022;Haarsma et al, 2013;Haarsma, 2021). Additionally, as TCs attain greater intensities they may become more resilient to hostile environmental conditions such as decreasing SSTs and increasing wind shear (Baker et al, 2022;Michaelis and Lackmann, 2019), increasing their likelihood of both recurving (Sainsbury et al, 2022a), and reaching Europe (Sainsbury et al, 2022b).…”

“…Projected increases in sea surface temperature (SST) and the range of latitudes occupied by TCs (Studholme et al, 2022), combined with the observed trend in TC lifetime maximum intensity latitude (Kossin et al, 2014) opens the possibility for more cyclones to form-and attain high intensities-further polewards and eastwards in the basin, closer to Europe (Baker et al, 2022;Haarsma et al, 2013;Haarsma, 2021). Additionally, as TCs attain greater intensities they may become more resilient to hostile environmental conditions such as decreasing SSTs and increasing wind shear (Baker et al, 2022;Michaelis and Lackmann, 2019), increasing their likelihood of both recurving (Sainsbury et al, 2022a), and reaching Europe (Sainsbury et al, 2022b). Diabatic processes have also been shown to be important in case studies of high-impact PTCs (Rantanen et al, 2020), implying a future increased PTC risk in a warmer atmosphere which is capable of holding a larger amount of moisture (Haarsma, 2021).…”