Response of a tropical cyclone to a subsurface ocean eddy and the role of boundary layer dynamics

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. © 2021 The Authors. Quarterly Journal of the Royal Meteorological Society published by John Wiley Sons Ltd on behalf of the Royal Meteorological Society