Fine-scale rainfall over New Caledonia under climate change

Global climate models projections indicate no clear future rainfall changes over the Southwestern Pacific islands in response to anthropogenic forcing. Yet, these models have low (similar to 100-200 km) spatial resolution and suffer from large systematic biases, such as the trademark "double ITCZ". Here, 4 km-resolution simulations were used with a nested regional atmospheric model, which resolves the New Caledonian mountainous topography. The resulting present-day rainfall amount, spatial structure, seasonal cycle, and extremes compare much better with observations than at 20 km resolution. We applied projected changes from global climate models under the RCP8.5 scenario at the boundaries, following an approach that corrects both their present-day and projected sea surface temperature biases. Unlike climate models, our refined projections reveal an 18% decrease in annual mean rainfall over New Caledonia by 2080-2100, especially on the leeward side of the island (up to 30%) and during the hot season (that accounts for similar to 80% of the rainfall decrease). This drying is robust without bias correction, and much stronger than at similar to 20 km resolution. It is mainly driven by circulation changes. A weather regime classification further demonstrates that similar to 80% of the hot season drying relates to a strong anti-cyclonic and air subsidence anomalies centred on the north of NC, which reduces moisture convergence over the archipelago. The frequency and intensity of extreme precipitation events is also reduced by similar to 20% by 2080-2100. This drastic projected drying would have dramatic impacts on water resources and terrestrial ecosystems, pleading for carefully-planned adaptation policies for New Caledonia