Temporal changes of the Oyashio water distribution east of Japan under the changing climate: development of an objective evaluation method and its application

IF 1.3 4区 地球科学 Q4 OCEANOGRAPHY
Yuma Kawakami, Hideyuki Nakano, L. Shogo Urakawa, Takahiro Toyoda, Kunihiro Aoki, Nariaki Hirose, Norihisa Usui
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Abstract

The Oyashio, a southern part of the western boundary current in the North Pacific subarctic gyre, carries cold and fresh seawater with abundant nutrients southward from the high-latitude, influencing regional climate in the East Asia and marine environment in the western mid-latitude North Pacific. Previously, a distribution of the Oyashio water has been evaluated by empirical temperature thresholds; for example, in spring (March–May) when the Oyashio intrudes southward into the east of Japan, the Oyashio water is defined at 100-m depth as ≤ 5 °C. However, this method is not necessarily adequate under the changing climate because upper ocean temperature may change over time due to some causes unrelated to cold water transport by the Oyashio (e.g., surface heat fluxes). In this study, we developed an objective method to evaluate the Oyashio water distribution applicable under the changing climate with a focus on a thermohaline front located at the warm- and salty-side boundary of the Oyashio water. We identified isohalines at 100-m depth best corresponding to the thermohaline front in each month and used them as the Oyashio water threshold. Using the developed method, we further investigated the springtime Oyashio water distribution east of Japan (in the North Pacific south of 43°N, 141–148°E). The area of the Oyashio water shows inter-annual variation and significant long-term decrease. It was suggested that these temporal variation and change reflect changes in a distribution of anti-cyclonic meso-scale eddies off Hokkaido, which block the southward Oyashio intrusion into the east of Japan.

Abstract Image

气候变化下日本东部大盐水分布的时间变化:客观评价方法的开发及其应用
大矢潮是北太平洋亚北极涡旋西边界流的南部部分,它携带着含有丰富营养物质的冷淡海水从高纬度南下,影响着东亚地区的气候和北太平洋中纬度西部的海洋环境。以前,人们通过经验温度阈值来评估大盐水的分布;例如,在春季(3 月至 5 月)大盐水向南侵入日本东部时,100 米深处的大盐水被定义为≤ 5 °C。然而,在气候不断变化的情况下,这种方法并不一定适用,因为海洋上层的温度可能会随着时间的推移而发生变化,其原因与大矢海水的冷水输送(如表层热通量)无关。在这项研究中,我们开发了一种客观的方法来评估适用于不断变化的气候条件下的矢志水分布,重点是位于矢志水暖咸边界的温盐锋面。我们确定了各月与温盐前线相对应的 100 米等深线,并将其作为 Oyashio 水阈值。利用所开发的方法,我们进一步调查了日本以东(北纬 43°、东经 141-148° 以南的北太平洋)的春季 Oyashio 水分布情况。Oyashio 水的面积显示出年际变化和长期显著减少。研究认为,这些时间上的变化反映了北海道近海反气旋中尺度漩涡分布的变化,这些漩涡阻挡了向南侵入日本东部的 Oyashio 水。
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来源期刊
Journal of Oceanography
Journal of Oceanography 地学-海洋学
CiteScore
3.50
自引率
13.00%
发文量
42
审稿时长
6-12 weeks
期刊介绍: The Journal of Oceanography is the official journal of the Oceanographic Society of Japan and open to all oceanographers in the world. The main aim of the journal is to promote understandings of ocean systems from various aspects including physical, chemical, biological, geological oceanography as well as paleoceanography, etc. The journal welcomes research focusing on the western North Pacific and Asian coastal waters, but the study region is not limited to the Asian Pacific. The journal publishes original articles, short contributions, reviews, and correspondence in oceanography and related fields.
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