An Example of North Atlantic Deep-Ocean Swell Impacting Ascension and St. Helena Islands in the Central South Atlantic

Abstract

Abstract During 1999, the dataloggers of the pressure transducer–based tide gauges at Ascension and St. Helena Islands were upgraded in order to enable the monitoring of wave conditions in addition to the measurement of still water levels. Within a few months, the gauges had recorded an example of unusually large deep-ocean swell, which, from the inspection of numerical wave model output, appears to have been generated by the remains of Hurricane Irene in the North Atlantic almost 1 week earlier. This fortuitous event serves to remind us of the potential importance of swells to communities on distant, low-lying coasts, particularly if the climatology of swells is modified under future climate change, and of the importance of in situ wave recording to wave model development. It is suggested that global ocean monitoring programs should place greater emphasis than hitherto on swell monitoring and prediction, with one component of the monitoring being provided by island tide gauges.