Tide-Induced Eddies and Upwelling in a Semi-enclosed Basin: Nan Wan

Nan Wan is a 20 km wide tidally-dominated embayment situated between two headlands on the south coast of Taiwan. During spring tides, sudden sea-surface temperature drops occur twice each tidal cycle in the western and central regions of the bay, but only once in the eastern region. Shipboard ADCP surveys, moored measurements and numerical modelling results demonstrate that the headlands on either side of the bay generate strong tidally-induced eddies within the bay on each phase of the tide. The geometry of the region leads to considerable difference in size between the flood and ebb eddies. The flood eddy fills the entire basin, while the ebb eddy fills the western and central region only. The strong (relative vorticity ~10-16 f) cyclostrophic eddies are only weakly affected by Earthʹs rotation, and thus upwelling occurs within each eddy, causing two temperature drops per tidal cycle in the western and central region, while only one drop in the eastern region.