Flood–ebb disparity of tidally induced recirculation Eddies in a semi-enclosed basin: Nan Wan Bay

Nan Wan Bay is a semi-enclosed basin forming the southern tip of Taiwan. Around spring tides, a recirculating cyclonic eddy occupying a good portion of the basin develops daily during the ebb current. The eddy induces sudden temperature drops inside the Bay lasting for a few hours each. An anticyclonic recirculation eddy, if any, is much reduced in size and confined to the eastern side of the basin during the flood current. Mechanisms leading to the flood–ebb disparity of recirculation eddies are investigated below using a three-dimensional numerical tidal model. By systematically adding or removing topographic features in and around Nan Wan, the process of elimination points to two topographic features that are responsible for the flood–ebb disparity. One is a bottom ridge around the eastern cape of the Bay; the other is an elongated seamount along the mouth of the semi-enclosed basin. These topographic features deflect tidal currents in the anticyclonic sense. Bayward and seaward deflections of tidal currents weaken and strengthen recirculating eddies, respectively, producing the flood-ebb disparity. The two topographic features enhance the cyclonic eddy during ebb, especially when the prevailing tidal current off the Bay is strong. For a typical embayment with tidal currents parallel to the bay mouth, the cyclonic eddy is expected to dominate over the anticyclone if there is a prominent seamount blocking the middle reaches of the mouth region.