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What controls the extreme flow through the Kerama Gap: a global HYbrid Coordinate Ocean Model reanalysis point of view

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Abstract

The temporal variability of volume transport from the North Pacific Ocean to the East China Sea (ECS) through the Kerama Gap (between Okinawa Island and Miyakojima Island—a part of Ryukyu Island Arc) is investigated using a 20-year global HYbrid Coordinate Ocean Model reanalysis with the Navy Coupled Ocean Data Assimilation from 1993 to 2012. We study the causes of extreme flow events defined as the volume transport through the Kerama Gap exceeding one standard deviation (4.50 Sv) above (inflow, from the North Pacific into the ECS) or below (outflow, from the ECS to the North Pacific) the mean volume transport (1.95 Sv). In our results, the impinging mesoscale eddies from the Pacific Ocean are the most important factor in controlling the extreme flow occurrence, explaining 70% (68%) of the mean extreme inflow (outflow)-mean volume transport anomaly. Extreme inflow (outflow) through the Kerama Gap is associated with the impinging anticyclonic (cyclonic) eddies along the eastern side of Okinawa and/or cyclonic (anticyclonic) eddies to the southwest of the gap. The Kuroshio frontal meanders cause the mean Kuroshio axis to shift away from (toward) the Kerama Gap during the extreme inflow (outflow)-mean events and play a secondary role in determining the extreme flow occurrence, representing 15% (14%) of the mean extreme inflow (outflow)-mean volume transport anomaly. The interannual variability of the flow through the gap also plays a subsidiary role, explaining another 13% (14%) of the mean extreme inflow (outflow)-mean volume transport anomaly.

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Data availability

The numerical output used for this paper can be found on the http://www.hycom.org data server under the “HYCOM + NCODA Global 1/12° Reanalysis” link.

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Acknowledgments

The authors thank the anonymous reviewers for their input to improve the original manuscript. Computer time was provided by the Department of Defense (DoD) High Performance Computing Modernization Program and the simulations were performed on IBM Power 6 (daVinci) and IBM iDataPlex (Kilrain) at the Navy DoD Supercomputing Resources Center, Stennis Space Center, MS. This is NRL contribution NRL/JA/7320-18-4062. It has been approved for public release and distribution is unlimited. We wish to thank Dr. Jim Richman at Florida State University for his valuable suggestions.

Funding

This effort was funded by the “6.1 Kuroshio and Ryukyu Current Dynamics” project sponsored by the Office of Naval Research under program element 0601135N. Z. Y. was supported by a Post-Doctoral Fellowship from the American Society for Engineering Education, with funding provided by the Naval Research Laboratory, Stennis Space Center, MS.

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Authors and Affiliations

  1. Oceanography Division, Naval Research Laboratory, Stennis Space Center, Hancock County, MS, 39529, USA

    Zhitao Yu & Edward J. Metzger

  2. Florida State University, Tallahassee, FL, USA

    Harley E. Hurlburt

  3. Perspecta, Stennis Space Center, Hancock County, MS, USA

    Ole Martin Smedstad

Authors
  1. Zhitao Yu
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  2. Edward J. Metzger
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  3. Harley E. Hurlburt
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  4. Ole Martin Smedstad
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Correspondence to Zhitao Yu.

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Responsible Editor: Pierre De Mey-Frémaux

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Yu, Z., Metzger, E.J., Hurlburt, H.E. et al. What controls the extreme flow through the Kerama Gap: a global HYbrid Coordinate Ocean Model reanalysis point of view. Ocean Dynamics 69, 899–911 (2019). https://doi.org/10.1007/s10236-019-01284-0

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