Abstract
In this study, we developed two high-resolution future ocean regional projection datasets for coastal applications in Japan, in which we made use of dynamical downscaling via regional ocean models with atmospheric forcing from two climate models (i.e., MIROC5 and MRI-CGCM3) participating in Coupled Model Intercomparison Project Phase 5 (CMIP5) under historical, representative concentration pathway (RCP) 2.6, and RCP8.5 scenarios. The first dataset was an eddy-resolving 10-km resolution product covering the North Pacific Ocean area and ranging continuously from 1981 to 2100, in which the Kuroshio current and mesoscale structures were reasonably resolved. The second dataset was a 2-km resolution product covering the regional domain surrounding Japan and comprising 10–15-year time slices, in which the coastal geometry and current structure were resolved even more realistically. An important feature of these datasets was the availability of reference datasets based on atmospheric and oceanic reanalysis data for cross-validation during the historical run period. Using these reference datasets, biases of regional surface thermal properties and the Kuroshio states during the historical run period were evaluated, which constitute important information for users of the datasets. In these downscaled datasets, the future surface thermal responses were generally consistent with those of their original data. Utilizing the high-resolution property of the downscaled data, possible future impact analyses regarding coastal phenomena such as strait throughflows, coastal sea level variability, and the Kuroshio intrusion phenomenon into bays (“Kyucho” phenomenon) were demonstrated and the important role of the Kuroshio state representation was indicated, which had proved difficult to analyze using the low-resolution projection data. Given these properties, the present datasets would be useful in climate change adaptation studies regarding the Japanese coastal region.
Highlights
The North Pacific Ocean (NPO) will experience a significant transition by the end of the twenty-first century, in accordance with the current trend of global climate change, including substantial oceanographic warming (IPCC 2013, 2014; Alexander et al 2018)
3.1 Representation of the general ocean state and future projections around Japan Figure 3 displays the 10-year (1996–2005) mean maps of the ocean heat content (OHC) around Japan calculated from the NPO model with a 10-km horizontal resolution (NP10) and JPN02 historical results (JRA-55, MIROC5, and MRI-CGCM3), compared with the ocean reanalysis data FORA-WNP30 (Fig. 3a) and the original Coupled Model Intercomparison Project Phase 5 (CMIP5) ocean products (Fig. 3b, c)
On the basis of this relationship, the Kuroshio and Oyashio and Tsushima and Tsugaru coastal currents around Japan are represented by the OHC frontal structures in the OHC map, as observed in the 10-km ocean reanalysis data (Fig. 3a)
Summary
The North Pacific Ocean (NPO) will experience a significant transition by the end of the twenty-first century, in accordance with the current trend of global climate change, including substantial oceanographic warming (IPCC 2013, 2014; Alexander et al 2018). Ocean downscaling has been performed using climate model projection products (i.e., CMIP) that focus on the western North Pacific region (e.g., Sato et al 2006; Liu et al 2016) Their resolutions were eddy-permitting (about 30 km) and insufficient for assessing the future climate change impacts on the Kuroshio current or coastal problems in the NPO. High-resolution ocean downscaling using CMIP products for regional/coastal applications has been achieved for world ocean regions, such as the Northwest Atlantic Ocean (e.g., Alexander et al 2020; see their introduction for further references), the northwest European shelf regions (e.g., Hermans et al 2020; see their introduction for additional references), and several other coastal regions (e.g., Sun et al 2012; Hermann et al 2016; Toste et al 2017; Xiu et al 2018) Such a highresolution dynamical downscaling approach seems to have rarely been used for the NPO
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