Advancing Climate Prediction for the Pacific Islands: Validation of Updates to the PICASO Forecast System

The Pacific Island Countries Advanced Seasonal Outlook (PICASO) is a climate prediction tool focused on the Pacific Islands, which was launched in 2018. PICASO currently provides 3-month seasonal precipitation forecasts using predictors derived from the APEC Climate Center Multi-Model Ensemble (APCC MME) hindcast dataset for 1983–2005. Under the UNEP CIS-Pac5 project, we have identified new seasonal and monthly predictors for both precipitation and temperature forecasts, based on the updated APCC hindcast dataset (1991–2010). Using these new predictors, we validated the performance of PICASO’s precipitation forecasts at ten observation stations across five countries (Cook Islands, Marshall Islands, Niue, Palau and Tuvalu). The validation period covered 2016–2023, and forecast performance was evaluated using the Heidke Skill Score (HSS). Overall, seasonal forecasts exhibited higher skill than monthly forecasts. When comparing forecasts produced using the old and new hindcast datasets, most stations recorded higher scores. Future work will evaluate the predictive skill of temperature forecasts. In addition, monthly forecasts will be produced on a regular basis and included in an upgraded version of PICASO to be released in the near future.