Thesis or Dissertation Dynamical downscaling of future sea-level change in the western North Pacific using ROMS

劉, 昭君

The future regional sea-level (RSL) rise in the western North Pacific is investigated by dynamical downscaling with the Regional Ocean Modeling System (ROMS) with an eddy-permitting 0.25° resolution based on three Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models— MIROC-ESM, CSIRO-Mk3.6.0, and GFDL-CM3— under the highest greenhouse-gas emission Representative Concentration Pathway (RCP) 8.5 scenario. The dynamical downscaling is performed for two epochs, historical run (1950–2000) and future run (2051–2100). The historical run, ROMS-Hist, is forced with the air-sea fluxes calculated from Coordinated Ocean Reference Experiment version 2 (COREv2). Three future runs— ROMS-MIROC, ROMS-CSIRO, and ROMS-GFDL— are forced with an atmospheric field constructed by adding the difference between the climate model parameters for the 21st and 20th century to the forcing fields in ROMS-Hist. The downscaled future RSLs commonly exhibit strong, bullseye-like RSL rise maxima centered on 41°N, 142°E to the east of the Tsugaru Strait, and three zonally aligned maxima along 37°N between 140°E and 160°E. In all ROMS downscaling, the RSL rise along the eastern coast of Japan is generally one-third or less of the RSL rise maxima off the eastern coast of Japan. The projected regional (total) sea level rises along Honshu coast during 2081–2100 relative to 1981–2000 are 22–29 (101–108), 8–15 (73– 80), and 8–18 (80–90) cm in ROMS-MIROC, ROMS-CSIRO, and ROMS-GFDL, respectively. The largest downscaled RSL rise along the Japan coast occurs at the Sanriku coast in all models. Although the CMIP5 models substantially underestimate the maxima of the offshore RSL rise compared with the ROMS downscaling, the discrepancies of the RSL rise along the Honshu coast between the climate models and ROMS downscaling are less than 10 cm. The maxima of the RSL rise to the east of the Tsugaru Strait and those along 37°N are probably related to the enhanced northward intrusion of the Kuroshio Current along the eastern coast of Japan and to the northward shift of the Kuroshio Extension, respectively. The projected RSL change along Hokkaido coast during 2081–2100 relative to 1981–2000 is smaller than that on Honshu coast. At Okinawa Island, RSL changes are nearly 30 cm in ROMS-MIROC and ROMS-CSIRO, which is higher than those along Honshu. The difference of RSL rise at Okinawa Island between ROMS-CSIRO and CSIRO-Mk3.6.0 exceeds 10 cm. ROMS-MIROC and ROMS-GFDL simulations suggest that the RSL changes are likely induced by wind stress changes rather than heat or freshwater fluxes at the surface, whereas ROMS-CSIRO suggests that heat and freshwater fluxes play relatively larger roles than other two models. All ROMS simulations commonly show that the thermosteric components give the major features of regional dynamic height (RDH) and thus major regional sea-level changes, while halosteric components contribute to yield overall meridional gradients in regional sea-level changes.
Hokkaido University(北海道大学). 博士(理学)

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