Nishikawa et al. Progress in Earth and Planetary Science (2020) 7:29 Progress in Earth and https://doi.org/10.1186/s40645-020-00342-2 Planetary Science RESEARCH ARTICLE Open Access Detection of the Oyashio and Kuroshio fronts under the projected climate change in the 21st century Haruka Nishikawa1* , Shiro Nishikawa1, Hiroshi Ishizaki1, Tsuyoshi Wakamatsu2,3 and Yoichi Ishikawa1 Abstract We evaluate the Oyashio and Kuroshio fronts latitudinal transition under the projected climate change scenarios using eddy resolving regional ocean climate projection products from 1981 to 2100. The regional ocean climate projections are produced based on dynamical downscaling of four CMIP5 models and two RCP experiments RCP2.6 and RCP8.5. Two approaches of the fronts detection methods are compared. One is conventional approach based on subsurface specific isotherm and another is newly proposed approach based on the Oyashio and Kuroshio water temperature and salinity (TS) profiles that may change as the global warming progresses. It is found that the Oyashio TS profile rapidly shift to be high both in temperature and salinity in RCP8.5 cases while the Kuroshio TS profile shows small change toward the end of the twenty-first century in all cases. Northward shift of the Oyashio and Kuroshio fronts reaches 2° northward by the late twenty-first century in extreme case of RCP8.5 cases from the temperature-based definition. On the other hand, the northward shifts of fronts are less than 1° when we use the TS-based definition. The comparison of both fronts positions with the surface velocity distributions suggest that the TS-based definition captures transitions of the fronts better than the temperature-based definition. Keywords: Global warming, Climate projection, Kuroshio, Oyashio Introduction extension, Kuroshio Extension (KE), a strong westward Oyashio is the western boundary current and its extension boundary current of the North Pacific Ocean subtropical of the western subarctic gyre in the North Pacific Ocean circulation. Area between the Oyashio front and the Kuro- (Fig. 1). A part of East Kamchatka Current (EKC) flows shio front, the Kuroshio-Oyashio Transition Zone into the Okhotsk Sea in the northern Kuril Straits and is (KOTZ), is known to be highly productive fishing then mixed with Okhotsk Sea Mode Waters (OSMW). grounds. Nutrient-rich Oyashio waters sustain large stocks This is the origin of the Oyashio water. Oyashio flows of small pelagic fish (e.g., Pacific saury, Japanese sardine) southward along Japanese coast and its south western while the warm Kuroshio waters provide them winter edge reaches off Tohoku region. Turning eastward off the spawning grounds. The small pelagic fish migrate season- Tohoku region, Oyashio current follows the Sub-Arctic ally between the Kuroshio waters for spawning and the Front (SAF) characterized by the temperature front and Oyashio waters for growing in KOTZ. In autumn, fishing forms the Oyashio extension or the Subarctic Current vessels in Tohoku region can catch fat fish that is migrat- (Zhang and Hanawa 1993). South of the Oyashio front lies ing from the Oyashio waters to the Kuroshio waters in the Kuroshio front associated with Kuroshio and its their neighborhood ocean. KOTZ is also important for fish stock itself. Recruitment of the Pacific saury varies with the spring productivity in this region through the lar- * Correspondence: [email protected] vae survival rate (Ichii et al. 2018). 1Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan Full list of author information is available at the end of the article © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Nishikawa et al. Progress in Earth and Planetary Science (2020) 7:29 Page 2 of 12 Fig. 1 Schematic diagram of the Kuroshio and Oyashio. Oyashio is a mixture of the East Kamchatka Current (EKC) and Okhotsk Sea Mode Water (OSMW) that outflows to the Pacific through the Kuril Straits. The black triangle represents the area where we picked up waters as the pure Oyashio water and the shaded area below the Kuroshio (gray arrow) represent the area where we picked up waters as the pure Kuroshio water Since KOTZ is defined as an area that lies between the under the climate change by using high-resolution future Oyashio and Kuroshio fronts, evaluation of its transition ocean regional projections based on the fifth phase of under the climate change requires concurrent analysis of the Climate Model Intercomparison Project (CMIP5) both fronts. If the both Oyashio and Kuroshio fronts products. The new regional ocean projections have 10 shift northward, the transition zone will shift northward, km horizontal resolution which is high enough to repre- and if the only Kuroshio front shifts north, the area of sent the Oyashio and Kuroshio fronts in realistic man- transition zone will reduce. These changes should have ner. Combinations of four models and two experiments serious impacts on migration ecology and stock level of are chosen from CMIP5 multi model ensemble datasets fish and fishing industry. Inter-annual to decadal time and this is the first model intercomparison study of the scale variation of the Oyashio and Kuroshio fronts can Oyashio and Kuroshio fronts transition under the cli- be explained mainly by linear dynamics of wind-driven mate change. ocean circulation theory (Seager et al. 2001). There are Since the Oyashio front and the Kuroshio front are both some studies on the future projection of Kuroshio front characterized by strong surface current as extensions of sub- under the climate change. Most of studies suggested that polar and subtropical western boundary currents, the best the Kuroshio front will shift north along with the Kuro- way to identify the location of both fronts are to follow the shio intensification (Sato et al. 2006; Yang et al. 2016;Li associated strong surface current axis. The most reliable et al. 2017) and one study suggested that the Kuroshio Kuroshio front location, which is reported by the Japan Coast front location will not change (Sakamoto et al. 2005). In Guard (JCG), is estimated by various observation data. They contrast, there are no studies conducted on the Oyashio see the velocity, temperature, and sea surface height (SSH) front transition under the climate change to authors’ distribution collected by national patrol boats, merchant ves- knowledge. sels, buoys, radars, and satellites and finally they decide the In general, realistic representation of the Oyashio and location by hand. However, such works take a lot of time. Kuroshio fronts and associated complex water mass For that reason, the locations of the Oyashio and Kuroshio properties in KOTZ in numerical simulation requires fronts are conventionally defined based on respective specific eddy-resolving horizontal resolution (e.g., Hurlburt et al. isotherm at reference depth because both fronts are charac- 1996). However, the resolution of most of ocean compo- terized by temperature fronts. nents in CMIP5 models are in the range of 1–2° and Under the current climate condition, the Oyashio front their horizontal resolutions are too coarse to reproduce roughly follows 5 °C isotherm at 100 m depth and the KOTZ and its associated fronts. In this study, we investi- Kuroshio front roughly follows 14–15 °C isotherm at gated how the Oyashio and Kuroshio fronts will shift 200 m depth (Kawai 1972). These definitions are the Nishikawa et al. Progress in Earth and Planetary Science (2020) 7:29 Page 3 of 12 most popular because they are more practical and robust boundaries at its west, south, and north boundaries and than other definitions (e.g., using sea surface height gra- model temperature and salinity within 2° band along dient or sea surface temperature) and the defined fronts each boundary are restored to prescribed monthly mean correspond well to the core of the surface velocity (Kida temperature and salinity data gridded on the World et al. 2015). Many studies (e.g., Masujima et al. 2003; Ocean Atlas (WOA) standard grid with 1° horizontal Kouketsu et al. 2005; Kakehi et al. 2017), and govern- resolution and 33 levels. ment offices (e.g., Japan Meteorological Agency, Fisher- The monthly mean temperature and salinity data at ies Research, and Education Agency Japan) used these the model boundaries during CMIP5 historical run definitions. Also, previous studies for the climate change period (1960–2005) are derived from World Ocean Atlas adopt these definitions (e.g., Yokouchi et al. 2006). How- 94 (WOA94) database (Levitus and Boyer 1994; Levitus ever, we expect that this conventional approach would et al. 1994). At the end of the historical run period, the not be effective for front detection in the future projec- boundary values are switched from WOA to an inter- tion products, since the specific index temperature asso- annual monthly mean data constructed by merging ciated with each front gradually changes as the climate CMIP5 ocean products (1996–2100) and the WOA change progresses. product. First, CMIP5 temperature and salinity data are We propose new front detection method that uses both regridded to the common WOA grid and 21 years cli- temperature and salinity profiles.