Formation of the summertime anticyclonic eddy in Funka Bay, Hokkaido, Japan

Abstract

The anticyclonic eddy emerging every summer in Funka Bay, Hokkaido, Japan, was investigated through mooring and hydrographic observations in 2000 in order to elucidate the mechanism of its formation. The eddy is characterized by stable flows under geostrophic balance in the uppermost 20 m of the water column over a strong concave pynocline. Dramatic changes in water temperature, salinity and current structure were detected immediately prior to the formation of the eddy, highlighted by a considerable increase in temperature and decrease in salinity at 20 m in the central part of the bay. This period was dominated by northwestward flow in the upper layer, with a weaker underlying southeastward with a node at 30 m. The wind field over Funka Bay was found to be strongly influenced by surrounding topography, characterized by southeast winds over the main body of the bay and east–northeast winds near the mouth. This wind field, with a negative wind stress curl, drove the upwelling of cool deeper water into the upper layer of the coastal regions, particularly in the west and south, giving rise to the horizontal convergence of surface water toward the central part of the bay. The northwestward flow was closely correlated with the wind field, where the surface northwestward flow was driven by the east–northeast winds under the influence of the Earth's rotation near the mouth of Funka Bay, contributing to the convergence flow. This northwestward flow transported warmer, less saline and less dense surface water to the central part of Funka Bay, where the warm water accumulated. The thickening of the upper layer in the center of the bay resulted in the increase in water temperature and decrease in salinity at 20 m, indicative of the formation of concave isopycnal surfaces. The central water mass rotates over this density structure by geostrophic adjustment, giving rise to the observed anticyclonic rotation of the upper water layer of Funka Bay.