At a Glance
The objective of this research from the Vecchi group is to better understand all aspects and variations in the statistics of tropical cyclone (TC) activity and other climate impacts over the past few centuries, as well as the coming one. An equally salient objective is to better understand the likely range of equilibrium and transient climate sensitivity, such as how much warming to expect from a doubling of atmospheric CO2 . Key tools in these studies are climate and atmospheric models. These, along with analyses of the observed record, help researchers to distinguish whether observed multi-decadal to centennial changes in TC activity have been driven by largescale factors such as ocean temperature changes, greenhouse gases, volcanic eruptions, or the El Niño, as opposed to random atmospheric fluctuations.
Understanding tropical cyclone frequency remains a challenge for the tropical cyclones community (Knutson et al., 2021; Sobel et al., 2021). The Vecchi group has worked in recent years to build a consistent and physically grounded framework to understand the mechanisms controlling tropical cyclone frequency (Vecchi et al., 2019; Hsieh et al., 2020). Through this effort they have developed a new paradigm, which considers the change in the number of pre-tropical cyclone vortices (or “TC seeds”). The paradigm also looks at the impact of largescale environmental conditions on the probability of cyclones arising from these seeds. Once one accounts for the distinct climate dependence of seeds and genesis probability, one can accurately predict the sensitivity of TC frequency to a wide range of global forcing. Prior to this work, the literature on TC frequency had focused primarily or solely on the role of climate on genesis probability.
The seed-probability framework for understanding TC genesis was originally developed to understand the response of TC frequency to climatic changes and to accurately capture the observed and modeled annual cycle (Yang et al., 2021; see Figure 13.1). The North Atlantic hurricane season is very narrow, with the vast majority of hurricanes occurring between August and October. However, the extremely sharp transition between the inactive and active periods during the year cannot alone be explained by the environmental conditions that favor tropical cyclone genesis (“genesis probability” in the left panel of Figure 13.1) or the frequency of pre-TC “seeds” (center panel). However, the combined effect of variations in genesis probability and seeds yields a sharp annual cycle that accurately explains hurricane climatology across both climate models and observations (right panel). The ability of this theoretical framework to explain the annual cycle of hurricanes provides an observational test of the hypothesis, developed in climate simulations, that the combined role of seeds and genesis probability in the climate impacts hurricane frequency.
A related study explores the impact of TC seeds on genesis across a broad suite of climates and climate model configurations and develops a theory to link seed frequency to large-scale environmental factors (Hsieh et al., 2022, submitted). This work has shown that inter-model spread in TC genesis sensitivity to changing climate is largely driven by differences in the response of pre-TC synoptic disturbances. The climatological changes in these disturbances can be understood in terms of changes to large-scale aspects of the atmosphere (such as the amount of energy converging in the atmosphere and the rate at which air ascends). The researchers are now working to connect observed and modeled changes in TC frequency to large-scale climatic parameters using first principles (e.g., conservation of energy, mass and momentum), in an effort to build a theoretical constraint on tropical cyclone frequency.
The Vecchi team also explored impacts of uncertainties in ocean temperature reconstructions and changes in hurricane monitoring as a way to assess past changes in hurricane activity. They found that improvements in ocean temperature estimates over the 20th century allow for a reproduction of multi-decadal historical tropical cyclone frequency changes with high-resolution atmospheric models. This has enhanced confidence in the models’ ability to reproduce future changes in TC frequency (Chan et al. 2021). The researchers have also extended a methodology to account for the impact of past changes in hurricane monitoring on tropical cyclone frequency estimates. This has allowed the researchers to build a homogenized record of major (Category 3-5, the most destructive storms) hurricanes in the Atlantic from 1851-2020. This new homogenized record suggests that expected century-scale increases in major hurricane frequency have been masked by a combination of multi-decadal climate variability and late-20th century aerosol forcing. The Vecchi team is currently developing high-resolution climate model simulations to test this hypothesis.
In addition to developing a new understanding of the climatetropical cyclone connection, the team have worked to review the state of the literature and highlight key open challenges. This work has appeared in two review articles, one on tropical cyclones and global warming (Knutson et al., 2021) and the other on tropical cyclone frequency more generally (Sobel et al., 2021).
Relevance to bp
bp has long been interested in tropical cyclone risk because of the vulnerability of its coastal and offshore infrastructure, and because increases in the severity or frequency of tropical cyclones is an important driver of public opinion in support of the energy transition. The most interesting conclusion of the highlighted studies is that the 20th century increase in the frequency of major hurricanes would now be higher if not for past and ongoing anthropogenic aerosol emissions (primarily from coal combustion), which will continue to decrease during the energy transition. In the near term, we should thus expect more rapid increases in the frequency of major hurricanes than would be expected because of elevated greenhouse gases alone.
Chan, D., G.A. Vecchi, W. Yang, and P. Huybers, 2021. Improved simulation of 19th- and 20th-century North Atlantic hurricane frequency after correcting historical sea surface temperatures. Science Advances 7: eabg6931. (https://doi.org/10.1126/sciadv.abg6931).
Hsieh, T. L., G.A. Vecchi, W. Yang, I.M. Held, and S.T. Garner, 2020. Large-scale control on the frequency of tropical cyclones and seeds: a consistent relationship across a hierarchy of global atmospheric models. Climate Dynamics 55: 3277-3196. (https://doi.org/10.1007/s00382-020-05446-5).
Hsieh, T.L., W. Yang, M. Zhao and G.A. Vecchi, 2022. Environmental propensity for tropical cyclone seeding across climate perturbations and models. (submitted)
Knutson, T. R., M.V. Chung, G. Vecchi, J. Sun, T.L. Hsieh, and A.J.P. Smith, 2021. ScienceBrief Review: Climate change is probably increasing the intensity of tropical cyclones. In: Critical Issues in Climate Change Science, edited by: Corinne Le Quéré, Peter Liss & Piers Forster. (https://doi.org/10.5281/zenodo.4570334).
Sobel, A., S.J. Camargo, C.Y. Lee, C. Patricola, M. Tippett, G.A. Vecchi, and A.A. Wing, 2021. Tropical cyclone frequency. Earth’s Future 9(12): e2021EF002275. (https://doi. org/10.1029/2021EF002275).
Vecchi, G.A., et al., 2019. Tropical cyclone sensitivities to CO2 doubling: Roles of atmospheric resolution and background climate changes. Climate Dynamics 53:5999–6033. (https://doi. org/10.1007/s00382-019-04913-y).
Vecchi, G.A., C. Landsea, W. Zhang, G. Villarini and T.R. Knutson, 2021. Changes in Atlantic major hurricane frequency since the late-19th century. Nature Communications 12:4054. (https://doi.org/10.1038/s41467-021-24268-5).
Yang, W., T.L. Hsieh and G.A. Vecchi, 2021. Hurricane annual cycle controlled by both seeds and genesis probability. Proceedings of the National Academy of Sciences 118(41): e2108397118. (https://doi.org/10.1073/pnas.2108397118).