Wind Induced Current and Its Impact on Tidally Forced Water Circulation Pattern in a Coral Reef Area of Southwest Japan

Abstract

Present study was conducted in the coastal region of Ishigaki Island, southwest Japan to analyze possible driving forces of water circulation which is crucial to the understanding of coastal ecosystems. Data were collected through the deployment of oceanographic instruments in and outside the reef. The time series of current velocities indicates that Shiraho coral reef is characterized by an almost regular and controlled water circulation under calm wind condition. Tidally forced oceanic water entered the reef through prominent channel and finally drained out through the southern part of the study area. In the offshore area, tidal currents during flood tide and ebb tide were nearly parallel to the north-south stretch of the reef. Data from Ishigaki Meteorological Observatory indicated that local wind can be an important factor in driving currents both in and outside the reef area. Though the wind field of the region was characterized by both north and south wind, the current field was more strongly influenced by north wind than the south wind. North wind developed a dominant south current inside the reef with a significant temperature drop during winter. During summer, northerly wind contributed to the development of south current in the offshore area as north-south component of wind and current showed the existence of low frequency density peaks. As dominant force came from north wind, no strong peak was seen in the east-west (cross-shore) component of wind and current.