Modeled changes in extreme wave climates in the Pacific Ocean during the 21st century and implications for low-lying U.S. and U.S.-affiliated atoll islands

Abstract:

Waves are the dominant influence on the coastal morphology and ecosystem structure of tropical Pacific islands. 21^st century wave heights, periods, and directions were forecast using output from four IPCC’s CMIP5 coupled atmosphere-ocean global climate models (GCMs), for representative concentration pathways (RCP) 4.5 and 8.5. Wind fields from the GCMs were used to drive the global WAVEWATCH-III wave model to generate hourly time-series of bulk wave parameters for 25 islands in the mid to western tropical Pacific Ocean. Extreme significant wave heights decreased by the end of the 21^st century under both climate scenarios compared to historical runs (1976-2010), except during the June – August season. Trends under both scenarios were similar, with the higher-emission 8.5 scenario displaying a greater decrease in significant wave heights compared to the lower-emission 4.5 scenario. The islands in the central Equatorial Pacific displayed the most change from historical values. Extreme significant wave heights within the region decreased by more than 30 cm during the December – February season, whereas in June – August, extreme wave heights increased by more than 20 cm and associated wave directions often rotated more than 30° clockwise. The largest directional changes occurred under RCP 4.5, possibly indicating a weakening of the trade winds’ influence on extreme wave directions during June – August and increasing influence of forcing from the Southern Ocean. Though the June – August season historically demonstrated smaller significant wave heights, the forecasted increase, coupled with a dramatic change in the direction of these extreme events, could result in changes to the morphology of these small coral islands. Large changes in wave direction would result in modifications to alongshore sediment transport gradients and, in turn, new accretional and erosional patterns, potentially damaging infrastructure and impacting the limited freshwater lenses on atoll islets. As most atoll islets accrete during large wave events, decreasing wave heights during other seasons may inhibit atoll islet accretion such that the low-lying islets may not be able to keep up with projected sea-level rise.