Abstract
This study addresses the projected 21st century changes in austral summer precipitation and future precipitation anomalies related to El Nino 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 the regional 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 Nino-like precipitation pattern. This trend could be a local manifestation of the large-scale drift of the tropical Pacific Ocean towards an El Nino-like spatial structure. This drift is indeed verified in the SST fields. Then, superimposed upon this slowly evolving mean state, future El Nino events would cause positive precipitation anomalies, whereas future La Nina events would cause negative precipitation anomalies. However, in the last period of RCP8.5, future El Nino events do not seem to affect precipitation anymore, suggesting a damping of the warm events, probably due to a warmer mean state.
Highlights
People living in Pacific islands are already experiencing changes in their climate, such as higher temperatures, shifts in precipitation regimes, changing frequencies of extreme events and rising sea levels (Stocker et al 2013)
Sea level rise is a major concern for Pacific islands (Becker et al 2012), the focus is on the atmospheric component, (1) for practical data availability reasons — precipitation gauges provide observed datasets long enough to conduct climate change studies — and (2) because the atmospheric component holds another crucial aspect for local populations: future water resources
Homogenized monthly precipitation amounts from 9 meteorological stations over Tahiti and Moorea were made available by the meteorological service of French Polynesia
Summary
People living in Pacific islands are already experiencing changes in their climate, such as higher temperatures, shifts in precipitation regimes, changing frequencies of extreme events and rising sea levels (Stocker et al 2013). Global climate models (GCMs) are helpful tools for climate change study, because they provide global climate scenarios, they still fail to simulate realistic spatial patterns of precipitation in the South Pacific (Brown et al 2013). They are unable to adequately resolve precipitation signals at the island scale. This relationship is applied to 21st century greenhouse gas (GHG) emission scenario runs (RCP4.5 and RCP8.5) to derive Tahiti’s precipitation projections up to 2100
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