Dynamics and water mass structure of a summertime anticyclonic eddy in Funka Bay, Hokkaido, Japan

Abstract

The hydrography and flow structure of an anticyclonic eddy are investigated synoptically using conductivity, temperature and depth (CTD) and Acoustic Doppler Current Profiler (ADCP) data for Funka Bay on June 17 and July 23, 1997. In June, the anticyclonic eddy was characterized by a deep concave structure of isopycnal surfaces, with elliptical horizontal shape. The tangential speed of the eddy at 10 m depth approached a maximum of 30 cm s −1 at a radius of about 10 km. In July, the eddy was characterized by a sharp pycnocline with two density fronts around the outer edge of the eddy at a depth of 10 m. The horizontal shape of the eddy became circular, and the maximum tangential speed at 10 m depth decreased to 25 cm s −1 at a radius of about 15 km. The core of the eddy rotated as a rigid body through one cycle approximately once every 2.3 days in June, and every 4.8 days in July. Analysis of the tangential currents at 10 m depth using the gradient flow relationship revealed that the eddy was hydrodynamically balanced between three forces: Coriolis force, centrifugal force and a force due to the horizontal pressure gradient. The vertical and horizontal scales of the eddy changed dramatically between June and July, with the vertical scale of the eddy becoming shallower as the deep concave structure of the isopycnal surfaces was destroyed due to the intrusion of water originating from Tsugaru warm water (TW). The horizontal scale of the eddy also decreased from June to July, related to the formation of two density fronts arising due to coastal upwelling associated with south-southeasterly winds and due to contact of the eddy with water originating from TW in the upper layer. The coastal upwelling not only changed the horizontal structure of the eddy, but also transported nutrients to the surface layers from deeper layers. These nutrients would promote primary production in summer in Funka Bay. The collection of scallop larvae by aquacultural netting at fixed coastal sites would therefore be strongly affected by the eddy, suggesting that the eddy plays an important role in the ecosystem of Funka Bay.