Ocean effects on tropical cyclone intensification and inner-core asymmetries

The interaction between a tropical cyclone and the ocean is investigated using a minimal three-dimensional tropical cyclone model coupled with a two-layer ocean model. Two representations for entrainment into the ocean mixed layer are compared: one based on the assumption that the velocity scale for entrainment is the surface friction velocity, the other on the assumption that this scale is the magnitude of the mean velocity difference across the base of the mixed layer. It is shown that the magnitude and distribution of the ocean cooling depends strongly on the method for representing entrainment velocity. The method based on the surface friction velocity is more effective in reducing the heat flux from the ocean to the storm in the inner-core region and leads to a greater reduction of the tropical cyclone intensity. With ocean coupling, the surface heat flux is reduced in the inner core, mainly in the rear-right quadrant relative to the track, which is toward the northwest. As a result, the potential temperature distribution in the core region is more asymmetric in the coupled model, with a higher value in the northern sector than in the southern sector. The region of convergence in the lower troposphere in the coupled experiments is rotated counterclockwise from the rear (or southeastern) sector of the inner core to the eastern part, apparently in response to the change in the temperature distribution in the middle troposphere. In addition, the region of strong upward motion in the middle troposphere shifts also counterclockwise from the rear-right quadrant to the front-right quadrant. These changes are accompanied with changes in the divergence pattern that are mainly in the lower troposphere rather than in the boundary layer. The presence of ocean coupling has little influence on the track of the model cyclones, unlike the case in some previous studies.