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Contribution of sea-surface wind curl to the maintenance of the SST gradient along the upstream Kuroshio Extension in early summer

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Abstract

The seasonal cycle of the meridional sea-surface temperature (SST) gradient in the upstream Kuroshio Extension (KE) region was examined using satellite observation data and model simulations. In general, the meridional SST gradient is small in summer. However, in early summer (June and July), the SST front is sustained or intensified on the northern side of the KE near the coast of eastern Japan. This observed seasonal cycle was successfully simulated in the North Pacific Ocean model for the Earth Simulator (NP-OFES). Analysis of the forecast data revealed that the vertical profiles of temperature and salinity are shifted upward along the KE in early summer. As a result, the permanent thermocline depth is shallowest during summer, causing a relatively small SST tendency. In addition, significant cyclonic vorticity in the lower atmosphere related to the southwesterly sea-surface wind was found to the south of the KE, associated with the Baiu frontal zone (BFZ). It was inferred that the positive vorticity causes Ekman upwelling over the KE region, resulting in suppressed SST warming on the northern side of the KE. These results suggest that the BFZ contributes to maintaining or strengthening the SST front.

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Acknowledgments

We wish to express our gratitude to H. Nakamura at the University of Tokyo, S. Minobe of Hokkaido University, and R. Kawamura of Kyusyu University for encouragement and assistance in this work. We also thank the editor and the two anonymous reviewers for their encouragement and constructive comments. The altimeter products are produced by Ssalto/Duacs and distributed by Aviso, with support from Cnes (http://www.aviso.altimetry.fr/duacs/). TMI data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science MEaSUREs DISCOVER Project. QuikScat data are produced by Remote Sensing Systems and sponsored by the National Aeronautics and Space Administration (NASA) Ocean Vector Winds Science Team. TMI and QuikScat data are available at http://www.remss.com. The GSMaP data were provided by the Japan Aerospace Exploration Agency (JAXA) (http://www.eorc.jaxa.jp/). The NP-OFES simulations were conducted on the Earth Simulator with the support of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The GFD-DENNOU Library was utilized for drawing the figures. This study was supported by the Japan Ministry of Education, Culture, Sports, Science and Technology (MEXT) through a Grant-in-Aid for Scientific Research in Innovative Areas 2205. This work was also partly supported by the Global Environment Research Fund (2-1503) of the Ministry of the Environment, Japan.

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Authors and Affiliations

  1. Tokyo Gakugei University, Koganei, 184-8501, Japan

    Naoki Sato

  2. Japan Agency for Marine-Earth Science and Technology, Yokosuka, 237-0061, Japan

    Naoki Sato, Masami Nonaka, Yoshikazu Sasai, Hideharu Sasaki, Youichi Tanimoto & Ryuichi Shirooka

  3. Faculty of Environmental Earth Science, Graduate School of Environmental Science, Hokkaido University, Sapporo, 060-0808, Japan

    Youichi Tanimoto

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  1. Naoki Sato
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  2. Masami Nonaka
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  3. Yoshikazu Sasai
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Correspondence to Naoki Sato.

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Sato, N., Nonaka, M., Sasai, Y. et al. Contribution of sea-surface wind curl to the maintenance of the SST gradient along the upstream Kuroshio Extension in early summer. J Oceanogr 72, 697–705 (2016). https://doi.org/10.1007/s10872-016-0363-3

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  • DOI: https://doi.org/10.1007/s10872-016-0363-3

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