Dynamical downscaling of near-term internal climate variability and change for the main Hawaiian Islands using WRF ensemble simulations

Abstract:

Regional climate change studies play an important role in the development of high-resolution data products for the implementation of sustainable climate change adaptation plans. Moreover, for near-term climate change projections, it is important to account for natural climate variability and the associated uncertainties. This project examines both natural climate variability associated with the Pacific Decadal Oscillation (PDO) and anthropogenic influence for their impacts on near-term projections for the Hawaiian Islands. Of primary interest is how strong the anthropogenic-induced changes in regional precipitation are compared to those of natural decadal variability associated with the PDO. The Community Earth System Model (CESM) Large Ensemble Project (Kay et al. 2015) is used in connection with WRF for direct dynamical downscaling (i.e., 6-hourly CESM data supply the boundary conditions for the WRF model). Decade-long runs from CESM ensemble members are grouped according to PDO phase for the present-day (1996-2005) and future (2026-2035). Influences of the PDO and anthropogenic climate change are assessed by comparing the future simulations to the present-day simulations and the PDO(+) simulations to the PDO(-) simulations. The analysis of wet season mean rainfall indicates that the windward sides of the islands will become wetter in the near-term, particularly on Big Island, Maui, and Kauai during the PDO(+) phase. Dry season mean rainfall results suggest increases across most of the islands during both PDO phases, except on the windward sides of Big Island and Maui, which may experience decreases in mean rainfall during the PDO(-) phase. The analysis of 90^th and 99^th percentile wet season rainfall shows increases in both extremes for Big Island and the windward slopes of Maui and Kauai, particularly during the PDO(-) phase. Overall, the results suggest that natural variability will continue to contribute to uncertainty in near-term precipitation projections, masking the forced signal. These results also suggest that additional downscaling efforts, ideally coordinated under the umbrella of the Coordinated Regional Downscaling Experiment (CORDEX), are needed to fully assess the roles of anthropogenic change and internal variability on rainfall projections in Hawaii.