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 he adlands on the south coast of Taiwan. During spring tides, sudden sea-surfa ce temperature drops occur twice each tidal cycle in the western and centra l regions of the bay, but only once in the eastern region. Shipboard ADCP s urveys, moored measurements and numerical modelling results demonstrate tha t the headlands on either side of the bay generate strong tidally-induced e ddies 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 approximate to 10- 16f) cyclostrophic eddies are only weakly affected by Earth's 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 e astern region. (C) 1999 Academic Press.