The Effect of Air-Sea Interactions on Tropical Cyclone Predictions in the West Pacific

Feedbacks between tropical cyclones and the underlying ocean can modify the intensity and track of tropical cyclones, as well as the influence the probability of secondary cyclogenesis in the same region, primarily through wake cooling of the oceanic mixed layer. Wake cooling is induced mainly by atmospheric thermodynamic forcing, but also by vertical and horizontal ocean transports. These effects are often neglected in the atmospheric models used for numerical weather prediction (NWP) of tropical cyclones, particularly at high resolution, due to computational expense. We investigate the role of atmosphere-ocean interactions in tropical-cyclone prediction for the West Pacific, using forecasts from a novel atmosphere--ocean-mixed-layer coupled configuration of the U.K. Met Office NWP model. The model represents thermodynamic air-sea feedbacks, as well as the effects of vertical Ekman transports, at a greatly reduced computational cost relative to a coupled model with full ocean dynamics. We use this model to perform case-study re-forecasts of several recent tropical cyclones, including Typhoon Nock-Ten in December 2016, in 17km global and 4km (convection-permitting) regional configurations. We compare our coupled re-forecasts to atmosphere-only re-forecasts with the same model version, in which we prescribe persisted initial SSTs to the atmosphere. We demonstrate that the mixed-layer ocean captures the majority of the wake cooling from the tropical cyclones, while reducing the tendency of the convection-permitting model to produce overly deep cyclones, via negative thermodynamic air-sea feedbacks. We propose mixed-layer ocean coupling as a means to incorporate the key physics of air-sea interactions in tropical cyclone prediction for a minor increase in the computational cost.