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.
On a beta-plane with no background flow, the model cyclone moves towards the northwest. With ocean coupling, it leaves a cold wake behind it, mostly to the right of its track. The cooling reduces the heat flux from the ocean and thereby the moist static energy in the boundary layer. As a result, the cyclone is less intense in the mature stage than in the case without cooling.
The magnitude and distribution of the cooling depends strongly on the method for representing entrainment. The method based on the surface friction velocity is more effective in reducing the heat flux from the ocean to the storm under the eyewall region and leads to a greater reduction of the tropical cyclone intensity.
With ocean coupling,the surface heat flux and moist static energy in the boundary layer are reduced mainly in the rear-right quadrant of the cyclone core. The region of convergence in the lower troposphere in the coupled experiments rotates counterclockwise from the rear quadrant of inner core to the east of the core, presumably in response to the change in the distribution of moist static energy in the boundary layer. In addition, the region of maximum upward motion in the core region shifts from the rear-right quadrant to the front-right quadrant. These changes are associated mainly with changes in the divergence pattern in the lower troposphere rather than in the boundary layer. |
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