Convective activity within a tropical cyclone undergoing extratropical transition over a warmer ocean

Abstract. The Northeastern Atlantic basin is a region where the number of cyclones with tropical features can increase in the future due to anthropogenic climate change, a particularly important concern given the region’s vulnerability to such systems. This work analyses the influence of warmer sea surface temperatures (SSTs), expected in future climates, in the convective activity of Tropical Storm Delta. Delta, which caused strong damage over the Canary Islands (Spain) in November 2005, is representative of a tropical cyclone (TC) experiencing an extratropical transition (ET) on its path to western Europe. Two simulations of the storm were performed with the high-resolution atmospheric numerical model HARMONIE-AROME: a control simulation with initial and boundary conditions from the ERA5 reanalysis, and a warm simulation where a uniform perturbation of +2 °C was added to the SSTs surrounding the cyclone. The convective activity was analysed only in the convective cells near the cyclone's centre, employing the cloud tracking package Tobac, based on brightness temperature. Results show that increases in low-level water vapor flux, together with lower LCL and LFC levels and increased CAPE, create an environment more favourable for the development of deep moist convection in the warmer ocean simulation. These thermodynamic changes lead to more frequent intense moist updrafts and a larger number of convective cells associated with the cyclone, with greater vertical extent and higher precipitation rates. Consequently, Delta becomes a more intense and deeper TC, driven by latent heat release, reaching hurricane status. Later, Delta’s ET starts earlier and gets extended over time, while turning notably more severe too. These results may contribute to a better understanding of the behaviour of convection within cyclones with tropical characteristics affecting the Macaronesia and Western Europe under future climates.