At a Glance
Tropical cyclones (TCs) impact society and ecosystems through extreme wind, rain and surge. A better understanding of TC frequency, track, and wind and rainfall intensity are key to building strategies to mitigate their damages for the public and private sectors. The goal of the Vecchi group is to understand the mechanisms behind tropical cyclone activity changes over recent and future decades. The researchers’ main tools of study are climate and atmospheric models. The researchers combine modeling studies with analyses of the historical record (i.e., weather data over a period of years) to help distinguish the extent to which observed multi-decadalto-centennial changes in TC activity have been driven by large scale factors. These factors include ocean temperature changes, greenhouse gases, volcanic eruptions and El Niño oscillations, as opposed to random atmospheric fluctuations.
Tropical cyclones (TCs) are of profound societal and economic significance. TC characteristics, such as their track, frequency, wind and rainfall intensity, exhibit variations on a range of timescales. Predicting these variations with greater accuracy requires improved understanding of the character of and mechanisms behind these changes. Throughout this year, the Vecchi group has worked to understand the climatic controls on TC frequency (Hsieh et al., 2022, 2023), track (Kortum et al., 2023), rapid intensification (Bhatia et al., 2022) and rainfall (Liu et al., 2022).
All TC impacts are fundamentally modulated by TC frequency. Understanding TC frequency—the number of storms that form in a region or globally over a period of time, such as a season— remains a challenge for the scientific and forecasting community (Knutson et al., 2021; Sobel et al., 2021). In recent years, the researchers have developed a physics-based framework to understand the mechanisms controlling tropical cyclone frequency (Vecchi et al., 2019; Hsieh et al., 2020). This has led to a new paradigm in which the change in the number of pre-tropical cyclone vortices (or “TC seeds”) is a main driver of changes in TC frequency. Guided by this paradigm, the Vecchi group has developed the groundwork to understand the climatic drivers of the changes in TC seeds (Hsieh et al., 2020, 2022, 2023). This work has enabled the researchers to build a physically consistent framework that connects changes in atmospheric energy balance in locations across the globe.
A study published this past year (Hsieh et al., 2022) explores the impact of TC seeds on cyclone genesis across a broad range of climates and climate model configurations. The study develops a theory to link seed frequency to large-scale environmental factors. 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 (“seeds”). This can be connected to large-scale changes in vorticity and ascent – and can be understood in terms of atmospheric energy flux convergence. This study connects observed and modeled changes in TC frequency to large-scale climatic parameters using first principles, such as conservation of energy, mass and momentum. The goal is to build a theoretical constraint on tropical cyclone frequency and its response to climate perturbations—for example, the greenhouse-induced warming of the planet.
TC rapid intensification (or RI) is a phenomenon by which a TC’s wind intensity will increase by a substantial amount (approximately 35 knots) in less than 24 hours. RI remains difficult to predict and results in TCs that have a great potential to produce damage because of their intensity. Understanding RI is consequently a topic of growing scientific and societal interest. It has been observed that the fraction of TCs undergoing RI has increased substantially over recent decades (Bhatia et al., 2019). The likelihood of RI is projected to increase over the current century in response to greenhouseinduced warming (Bhatia et al. 2018). The Vecchi group found that the recent increase in RI was fueled in part by recent surface warming and was unlikely to have occurred due to random climate fluctuations (Bhatia et al., 2022). Researchers are currently building on these results to better understand whether the recent (1980s-present) increase in RI proportion was driven by greenhouse-induced warming or other climate forcing agents, such as atmospheric aerosols, to better constrain predictions of future RI.
Another area of study for the Vecchi group researchers this year was an analysis of the notable eastward shift that North Atlantic hurricanes have exhibited in their tracks between 1971 and 2020. They worked to understand whether this track shift was driven by climate factors or by random atmospheric fluctuations (Kortum et al., 2023). They found that, unlike the changes in RI (Bhatia et al., 2022), the multi-decadal shift in North Atlantic hurricane tracks includes a substantial component arising from weather-scale fluctuations, which can be thought of as effectively random. This implies that predicting multi-decadal changes in hurricane tracks will remain challenging and require a probabilistic framework.
Finally, the researchers explored the characteristics of hurricane-induced flooding in the Carolinas, by looking at recent, observed TCs (Liu et al., 2022). They found that in the Carolinas TCs are a dominant driver of floods (Figure 7.1); the top 10 TCs account for 2/3 of the record floods in the Carolinas – which highlights the impact of these rare extreme events in flooding. These results highlight local vulnerability to warming climates in the Carolinas, since the peak rainfall of TCs is expected to increase considerably in response to a warming climate (Liu et al., 2019).
Bhatia, K. et al, 2022. A potential explanation for the global increase in tropical cyclone rapid intensification. Nature Communications 13:6626. (https://doi.org/10.1038/s41467-022- 34321-6).
Bhatia, K., G. Vecchi, H. Murakami, S. Underwood, and J. Kossin, 2018. Projected response of tropical cyclone intensity and intensification in a global climate model. Journal of Climate 31(20):8281-8303. (https://doi.org/10.1175/JCLI-D-17-0898.1).
Bhatia, K., G.A. Vecchi, T. Knutson, H. Murakami, J. Kossin, K.W. Dixon, and C.E. Whitlock, 2019. Recent increases in tropical cyclone intensification rates. Nature Communications 10,635. (https://doi.org/10.1038/s41467-019-08471-z).
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 55:3177–3196. (https://doi.org/10.1007/s00382-020-05446-5).
Hsieh, T.L., W. Yang, M. Zhao, and G.A. Vecchi, 2022. Model spread in the tropical cyclone frequency and seed propensity index across global warming and ENSO-like perturbations. Geophysical Research Letters 49(7): e2021GL097157. (https://doi.org/10.1029/2021GL097157).
Hsieh, T.L. et al, 2023. The influence of large-scale radiation anomalies on tropical cyclone frequency.
Kortum, G., G. Vecchi, T.L. Hsieh, and W. Yang, 2023. Influence of weather and climate on multidecadal trends in Atlantic hurricane genesis and track.
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).
Liu, M., G.A. Vecchi, J. Smith, and T. Knutson, 2019. Causes of large projected increases in hurricane precipitation rates with global warming. Npj Climate and Atmospheric Science 2:38. (https://doi.org/10.1038/s41612-019-0095-3)
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).