Sea level variations at tropical Pacific islands since 1950

Abstract

The western tropical Pacific is usually considered as one of the most vulnerable regions of the world under present-day and future global warming. It is often reported that some islands of the region already suffer significant sea level rise. To clarify the latter concern, in the present study we estimate sea level rise and variability since 1950 in the western tropical Pacific region (20°S–15°N; 120°E–135°W). We estimate the total rate of sea level change at selected individual islands, as a result of climate variability and change, plus vertical ground motion where available. For that purpose, we reconstruct a global sea level field from 1950 to 2009, combining long (over 1950–2009) good quality tide gauge records with 50-year-long (1958–2007) gridded sea surface heights from the Ocean General Circulation Model DRAKKAR. The results confirm that El Niño-Southern Oscillation (ENSO) events have a strong modulating effect on the interannual sea level variability of the western tropical Pacific, with lower/higher-than-average sea level during El Niño/La Niña events, of the order of ± 20–30 cm. Besides this sub-decadal ENSO signature, sea level of the studied region also shows low-frequency (multi decadal) variability which superimposes to, thus in some areas amplifies current global mean sea level rise due to ocean warming and land ice loss. We use GPS precise positioning records whenever possible to estimate the vertical ground motion component that is locally superimposed to the climate-related sea level components. Superposition of global mean sea level rise, low-frequency regional variability and vertical ground motion shows that some islands of the region suffered significant ‘total’ sea level rise (i.e., that felt by the population) during the past 60 years. This is especially the case for the Funafuti Island (Tuvalu) where the “total” rate of rise is found to be about 3 times larger than the global mean sea level rise over 1950–2009.