Climate change assessment for a small island: a Tahiti downscaling experiment

International audienceThis study addresses the projected 21st century changes in austral summer precipitation and future precipitation anomalies related to El Niño Southern Oscillation (ENSO) for a South Pacific island: Tahiti, French Polynesia. An approach involving 2 successive downscaling steps is proposed here as a first attempt to simulate precipitation over a small orographic island.First, the 50 km mesh global atmospheric model ARPEGE is forced by bias-corrected sea surface temperatures (SSTs) from a CMIP5 scenario. This model drives the limited area model ALADIN. The final 12 km mesh regional simulation coarsely captures the island of Tahiti. Historical and scenario runs (RCP4.5 an d RCP8.5) are therefore available at the island scale. Linking station data and historical model outputs using quantile-quantile plots allows a correction of the biases of theregional simulation and an assessment of precipitation changes over the 21st century. For both scenarios, mean austral summer precipitation tend to increase over the century on the south-east side of Tahiti, following the present El Niño-like precipitation pattern. This trend could be a local manifestation of the large-scale drift of the tropical Pacific Ocean towards an El Niño-like spatial structure. This drift is indeed verified in the SST fields. Then, superimposed upon this slowly evolving mean state, future El Niño events would cause positive precipitation anomalies, whereas future La Niña events would cause negative precipitation anomalies. However, in the last period of RCP8.5, future El Niño events do not seem to affect precipitation anymore, suggesting a damping of the warm events, probably due to a warmer mean state