The Vertical Tilt of Mesoscale Eddy in the Northern South China Sea in a High-Resolution Numerical Simulation

Using a high-resolution numerical model, we investigate the influence of slope topography on the vertical tilted structure of mesoscale eddies in the northern South China Sea (NSCS). The model effectively captures key features including trajectories, intensities, and three-dimensional tilted structures of these eddies. By compositing all NSCS eddies on the slope and in the basin, a more pronounced southwestward tilt in slope eddies is found from the sea surface down to the deep, compared to basin eddies. The tilt distance of slope eddies is approximately 1.8 times that of basin eddies, with the strongest eddy tilt near the Dongsha Islands. Variations in topography gradient induce noticeable changes in both the magnitude and direction of the eddy tilt. Due to potential vorticity conservation, the eddies' lower part follows isobaths, while the upper part drifts southwestward. This rapidly increases a southward eddy tilt during the upslope phase (USP) and a gradual transition to a southwestward tilt in the downslope phase (DSP). Moreover, the lower eddy part responds more significantly and earlier (about 20 days) to topography than the upper eddy part, moving faster and tilt-developing more rapidly. The lower eddy part tilts larger during the USP, roughly 2.5 times than that of the upper eddy part. In the subsequent DSP, the tilt of lower eddy part decreases, about half of that in the upper eddy part. This study reveals the complex interactions between mesoscale eddies and steep slope topography, aiding further understanding the dynamics of eddy tilt and propagation.