CHAOS IN THE PACIFICS CORAL-REEF BLEACHING CYCLE

There is no simple explanation for the unusual increase in coral reef bleaching events that have been occurring on a global scale over the l ast 2 decades. Recent studies focusing on this problem reveal that mas s bleaching events have a strong periodic component, arising every sim ilar to 3-4 yr in step with the El Nino climatic phenomenon. To explor e this possibility further, we examine a simple oceanographic-ecologic al model designed to simulate the warm and cool phases of the Pacific Ocean cycle and gauge its effect on local coral reefs. This allows us to identify causes for localized ''hot spots'' in the ocean, whose hig h sea surface temperatures have disastrous consequences for corals. Th e underlying wave dynamics of the model lead to chaotic oscillations ( every similar to 3-4 yr), which help explain the coexistence of both o rder and irregularity in the dynamics of mass bleaching. The model mak es use of a temperature threshold mechanism-a bleaching event is trigg ered whenever temperature anomalies exceed a critical level. In a vari able environment, the threshold mechanism is sensitive to background f luctuations, and their effects are studied by making use of a ''stocha stic resonance'' formulation. Global climate change and other trends i n external background environmental conditions are all shown to strong ly influence the distribution of mass coral bleaching events.