Dynamical Downscaling of Near-Term Internal Climate Variability and Change for the Main Hawaiian Islands Using WRF Ensemble Simulations

As climate models improve, the demand from resource managers and decision-makers for more accurate climate projections is increasing. However, natural climate variability poses a limit to the confidence in regional climate change projections, in particular for near-term projections into the mid-21st century. The unique geographic location of the Hawaiian Islands and its regional climate provides a challenging opportunity for climate modelers. In this project, natural climate variability is examined for its impacts on near-term climate change projections. The Pacific Decadal Oscillation (PDO) and El Niño–Southern Oscillation (ENSO) are two examples of internal climate variability that effect precipitation in the Hawaiian Islands region. To study their effects, the Community Earth System Model (CESM) Large Ensemble Project (Kay et al. 2015) is used in connection with the WRF model for dynamical downscaling. Of primary interest is the question how strong the anthropogenically forced changes are compared to natural internal variability. Impacts of the phase of the PDO are assessed by comparing a 20-member ensemble of 10-yr long simulations. CESM simulations are selected according to PDO phase for the historical decade (1996-2005) and future decade (2026-2035) to supply the boundary conditions for the WRF model. Results from the WRF model output rainfall analysis indicate that the projected wet season rainfall totals for the near-term are greater across the Hawaiian Islands during positive PDO conditions than during negative PDO conditions . It is also projected that wet season rainfall totals during positive PDO conditions will increase on the windward sides of Maui and Big Island and decrease or experience no change elsewhere. Overall, wet season rainfall totals for the near-term are projected to increase across much of the Hawaiian Islands, with the exception of Oahu and parts of Kauai, where wet season rainfall totals are projected to decrease. These results suggest that the roles of anthropogenic forcing and internal variability on rainfall projections are still uncertain. By examining the roles of different types of variability on a regional scale, such as the Hawaiian Islands, it is hoped that future climate projections can be better understood and communicated to the public on a global scale as well.