Changes in tropical cyclone intensity with translation speed and mixed-layer depth: idealized WRF-ROMS coupled model simulations

In this study, two components of a coupled Ocean-Atmosphere-Wave-Sediment Transport modelling system, the atmosphere model Weather Research Forecasting (WRF) and the ocean model Regional Ocean Modeling System (ROMS), are used to investigate the relationships between tropical cyclone (TC) intensity changes and ocean mixed-layer depth (MLD) as well as translation speed through a series of idealized experiments. The coupled WRF-ROMS model can well simulate TC intensity changes and sea-surface temperature (SST) cooling induced by the TCs under different ocean MLD and background flow conditions. Surface enthalpy flux is reduced due to the SST decrease, which subsequently inhibits TC development. A smaller MLD and a slower speed produce a more symmetric cold wake under the TC inner core, which can effectively weaken the TC. The SST has a nonlinear decrease with increasing translation speed under the same MLD condition. A heat budget analysis supports that upwelling plays a more important role in changing the temperature within the upper ocean when the TC moves slowly (e.g. 1 m s(-1)). However, upwelling only dominates the subsurface temperature changes when the translation speed increases so that a smaller SST cooling occurs. A translation speed of 3 m s(-1) is found to be the optimal speed for TC development when the MLD is thick enough (50 and 100 m in this study). A larger basic flow (>3 m s(-1)) produces a stronger asymmetry in the vertical velocity and results in a weaker secondary circulation, and hence inhibits TC intensification. For a shallow MLD of 20 m, a 5 m s(-1) translation speed can mitigate more cooling effect and make it more favourable for TC maintenance. This result suggests the competing effects of atmospheric and oceanic conditions in determining TC intensity.