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Kumar, Arjun U.; Brüggemann, Nils; Smith, Roger K. and Marotzke, Jochem (2021): Response of a tropical cyclone to a subsurface ocean eddy and the role of boundary layer dynamics. In: Quarterly Journal of the Royal Meteorological Society, Vol. 148, No. 742: pp. 378-402

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We analyse a tropical cyclone simulated for a realistic ocean-eddy field using the global, nonhydrostatic, fully coupled atmosphere-ocean ICOsahedral Nonhydrostatic (ICON) model. After intensifying rapidly, the tropical cyclone decays following its interaction with a cold wake and subsequently reintensifies as it encounters a subsurface, warm-core eddy. To understand the change in the azimuthal-mean structure and intensity of the tropical cyclone, we invoke a conceptual framework, which recognises the importance of both boundary-layer dynamics and air-sea interactions. Crucially, the framework recognises that the change in the mean radius of updraught at the boundary-layer top is regulated by the expanding outer tangential wind field through boundary-layer dynamics. The decrease in the average equivalent potential temperature of the boundary-layer updraught during the early decay phase is related to an increase in the mean radius of the updraught rather than air-sea interactions. However, later in the decay phase, air-sea interactions contribute to the decrease, which is accompanied by a decrease in the vertical mass flux in the eyewall updraught and, ultimately, a more pronounced spin-down of the tropical cyclone. Air-sea interactions are also important during reintensification, where the tendencies are reversed, that is, the mean radius of the boundary-layer updraught decreases along with an increase in its average equivalent potential temperature and vertical mass flux. The importance of boundary-layer dynamics to the change in the azimuthal-mean structure is underscored by the ability of a steady-state slab boundary-layer model to predict an increasing and, to a lesser extent, decreasing radius of forced ascent for periods of decay and reintensification, respectively. Finally, our simulation highlights the importance of the ocean-eddy field for tropical cyclone intensity forecasts, since the simulated warm-core eddy does not display any sea-surface temperature (SST) signal until it is encountered by the tropical cyclone.

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