Environmental controls at multiple scales for the western Pacific: An Okinawan case study

Abstract

In this study, the principal controls on the coastal environment around the Pacific island of Okinawa, typical of the many mountainous islands of the sub-tropical and tropical northwest Pacific in the arc from Melanesia to the Ryukyu Islands, are investigated. Two principal satellite remote sensing data products were used to observe properties at regional-scale and in the Okinawan coastal zone: (1) AVHRR sea surface temperature (SST) for 1985–2006 and (2) a SeaWiFS ocean colour variable to estimate combined sediment and chlorophyll-a loadings for 1998–2006, along with direct observations (e.g. climatological data, river discharge, suspended sediment and typhoon tracks). Analysis suggested that coastal SST is strongly influenced by the outer ocean SST, indicating a regional control. However, the local winds, with associated summer rainfall and copious sediment-laden run-off, had a significant secondary impact. Typhoons are also an important factor leading to ocean colour variation. Along the western coastal zone of Okinawa, the winter East Asian Monsoon causes significant upwelling and seafloor sediment removal, while terrestrial impact is a significant secondary control in summer on ocean colour variability via sediment-laden river run-off. Previously noted changes in the magnitude of extreme rainfall events in the area suggest that sediment delivery to the coastal zones of islands in the region has been increasing in recent decades, providing a combined climate and terrestrial impact on the coastal environment. Although typhoons and heavy rainfall are episodic events, appropriate land management on the many hilly and mountainous islands of the sub-tropical northwestern Pacific is needed if such events are not to lead to deterioration, and decrease of resilience, of coastal ecosystems.