Basin-scale gradients in typhoon-generated wave disturbance to coral reefs

Cyclones (also called typhoons or hurricanes) are ecologically instantaneous terawatt events and the most acute form of disturbance to coral reefs likely to be observed in a human lifetime. Still, they can occur over twice a year on some western Pacific reefs. Early empirical studies of cyclone effects assisted in shifting the focus from equilibrium-based models of community structure to those incorporating stochastic events. Conclusions from most later studies though, have been speculative or, when qualitatively robust, rather obvious. This history has had two results: 1) continued ignorance about how cyclones affect variation in community structure at all but the smallest spatial and temporal scales and 2) a widely held belief that cyclones are unimportant ecologically because they are infrequently observed and their effects are locally unpredictable. However, the few large-scale observations to date suggest increasing predictability with increasing scale. Moreover, a considerable body of theory from fluid mechanics exists to guide hypothesis testing. We are developing a mean-field approach based on linear wave theory for studying the ecological effects of cyclone waves at local to global scales. So far, we have found an unanticipated wave climate that may be useful for isolating the long-term effects of cyclones on the community structure and geomorphology of coral reefs: The cyclonic windfields and stereotypical recurving path of typhoons in the western Pacific interact to produce a circum-island pattern of wave forces that is dependent on an island’s geographic position: The figure below shows hindcast mean significant breaking wave height (Hb) in meters of typhoon-generated waves in the western North Pacific Ocean from 1990 to 2000. A) Hb on north-facing coastlines. B) Hb on south-facing coastlines. Hb based on shore with bottom slope of 0.2.