Synchronization between Gulf Stream and Kuroshio and its implications for climate in Japan

2020 Fiscal Year Final Research Report

• Project/Area Number: 19K23460
• Research Category: Grant-in-Aid for Research Activity Start-up
• Allocation Type: Multi-year Fund
• Review Section: 0204:Astronomy, earth and planetary science, and related fields
• Research Institution: Ochanomizu University
• Principal Investigator: Kohyama Tsubasa お茶の水女子大学, 基幹研究院, 助教 (40845715)
• Project Period (FY): 2019-08-30 – 2021-03-31
• Keywords: 西岸境界流 / 同期現象

Outline of Final Research Achievements

The Gulf Stream and the Kuroshio Current are separated by the North American continent, and thus, they cannot exchange heat by oceanic processes within a few years. Based on data analyses of satellite observations and global climate models (GCMs), we show that sea surface temperatures of the Gulf Stream and the Kuroshio are synchronized for the decadal time scale. This synchronization is associated with meridional migrations of the atmospheric jet stream.
Output from four global climate models are analyzed: GFDL-CM4C192, MIROC6subhires, and their counterparts with lower resolutions. Cross-spectral analysis reveals that GCMs with higher resolutions exhibit higher low-frequency coherence than those with lower resolutions. Lag-correlation analysis shows that the temperature variations are almost simultaneous between the two currents. Atmospheric GCM experiments show that SST variability of the MIROC6subhires model actively modulates the position of the westerly jet stream.

Free Research Field

気象学

Academic Significance and Societal Importance of the Research Achievements

本課題で調査した境界流同期現象は，偏西風ジェットの位置決定および猛暑や豪雨に重要な役割を果たすと考えられる。たとえば研究代表者の結果では，2018年7月の気温が境界流同期指標でよく説明された。また近年，西岸境界流は地球温暖化が速いことが指摘されているが，これを「偏西風ジェットの極側シフト」と関連づけることで，我が国の気象の支配要因を明らかにする一助となることが考えられる。
学術的には，「中緯度海洋は大気に受動的に応答する」という従来の見方か，あるいは近年の研究報告に基づき「中緯度の大気海洋結合系をなす」と解釈すべきかについて，理論構築やモデル実験等を通して物理的解釈を確立する一助となる。