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The impact of ocean eddies on tropical cyclone intensity on a global scale and the role of boundary layer dynamics


Kumar,  Arjun Unnithan
IMPRS on Earth System Modelling, MPI for Meteorology, Max Planck Society;
Director’s Research Group OES, The Ocean in the Earth System, MPI for Meteorology, Max Planck Society;

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Kumar, A. U. (2022). The impact of ocean eddies on tropical cyclone intensity on a global scale and the role of boundary layer dynamics. PhD Thesis, Universität Hamburg, Hamburg. doi:10.17617/2.3432708.

Cite as: https://hdl.handle.net/21.11116/0000-000B-449A-B
Warm-core ocean eddies have been linked to the rapid intensification of tropical cyclones across the world’s oceans, from Hurricane Katrina in the Atlantic, to Supertyphoon Maemi in the Pacific and Cyclone Nargis in the Indian Ocean. Normally, sea-surface cooling induced by tropical cyclones acts as a brake on tropical cyclone intensity. By suppressing sea-surface cooling, warm-core eddies enable tropical cyclones to intensify further. Although coupled atmosphere-ocean forecasting models simulate sea-surface cooling, rapid intensification remains difficult to predict. A better understanding of the effect of warm-core eddies on tropical cyclone intensity is key to improving forecasts. In my thesis, I focus on the dynamics of the tropical cyclone boundary layer. Boundary layer dynamics influence the structure of the tropical cyclone circulation, which is inextricably linked to tropical cyclone intensity. While boundary layer dynamics are important for tropical cyclone intensity in atmosphere-only models, they have not been explored in coupled models. I address this gap by analysing a simulation of a tropical cyclone that decays due to sea-surface cooling and subsequently reintensifies when the sea-surface cooling is suppressed by a warm-core eddy. I find boundary layer dynamics play an important role modulating the decay and reintensification through changes in the structure of the tropical cyclone. Moreover, I find the differing rates of decay cannot be explained without boundary layer dynamics. These results underscore the importance of accurately representing boundary layer dynamics in coupled forecasting models and guide the choice of boundary layer parameterisations, which affect the ability of forecasting models to predict rapid intensification. In addition, I investigate the impact of warm-core and cold-core eddies on tropical cyclone intensity on a global scale with the first analysis of a global coupled simulation that resolves tropical cyclones and ocean eddies. I find encounters between tropical cyclones and ocean eddies are common. My analysis reveals that tropical cyclones that encounter warm-core eddies reach on average a higher peak intensity, whereas cold-core eddies do not have an effect on the average peak intensity. If these results prove to be robust, they point to a potential bias in future projections of tropical cyclone intensity, which are based on climate simulations that do not resolve ocean eddies. Disentangling the impact of ocean eddies in future eddy-resolving climate simulations will hinge on the sensitivity of tropical cyclone intensity to boundary layer schemes and our understanding of boundary layer dynamics.