Western boundary currents regulated by interaction between ocean eddies and the atmosphere

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Abstract

Current climate models systematically underestimate the strength of oceanic fronts associated with strong western boundary currents, such as the Kuroshio and Gulf Stream Extensions, and have difficulty simulating their positions at the mid-latitude ocean’s western boundaries1. Even with an enhanced grid resolution to resolve ocean mesoscale eddies—energetic circulations with horizontal scales of about a hundred kilometres that strongly interact with the fronts and currents—the bias problem can still persist2; to improve climate models we need a better understanding of the dynamics governing these oceanic frontal regimes. Yet prevailing theories about the western boundary fronts are based on ocean internal dynamics without taking into consideration the intense air–sea feedbacks in these oceanic frontal regions. Here, by focusing on the Kuroshio Extension Jet east of Japan as the direct continuation of the Kuroshio, we show that feedback between ocean mesoscale eddies and the atmosphere (OME-A) is fundamental to the dynamics and control of these energetic currents. Suppressing OME-A feedback in eddy-resolving coupled climate model simulations results in a 20–40 per cent weakening in the Kuroshio Extension Jet. This is because OME-A feedback dominates eddy potential energy destruction, which dissipates more than 70 per cent of the eddy potential energy extracted from the Kuroshio Extension Jet. The absence of OME-A feedback inevitably leads to a reduction in eddy potential energy production in order to balance the energy budget, which results in a weakened mean current. The finding has important implications for improving climate models’ representation of major oceanic fronts, which are essential components in the simulation and prediction of extratropical storms and other extreme events3,4,5,6, as well as in the projection of the effect on these events of climate change.

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Figure 1: Mesoscale surface THF and SST relationship.
Figure 2: Kuroshio response to OME-A feedback simulated by eddy-resolving global and regional climate models.
Figure 3: Simulated energy and enstrophy spectra in the Kuroshio Extension region.
Figure 4: Schematic of OME-A feedback regulating the KEJ.

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Acknowledgements

This research is a collaboration between TAMU and OUC led by P.C. at TAMU, and is supported by US National Science Foundation grants AGS-1462127 and AGS-1067937, and National Oceanic and Atmospheric Administration grant NA11OAR4310154, as well as by China’s National Basic Research Priorities Programme (2013CB956204 and 2014CB745000) and the Natural Science Foundation of China (41490644 and U1406401). P.C. was partially supported by the Excellence Cluster, Future Ocean, Kiel and the SFB754 and by the 2015 Francis Bretherton Visitorship during his sabbatical leave at GEOMAR and NCAR, respectively. The Texas Advanced Computing Center at The University of Texas at Austin and the Texas A&M High Performance Research Computing provided the computing resources we needed to perform our CRCM simulations.

Author information

X.M. conducted the CRCM simulations and performed most of the analyses. Z.J. carried out the eddy energetic analysis. P.C. (lead investigator) conceived the project and wrote the manuscript. X.L. assisted with data analyses. R.M. developed CRCM and its parallel Loess filter and provided assistance during model setup and use. R.J.S. and F.O.B. performed the global CESM simulations. R.J.G. and P.B. were involved in interpreting the results. D.W, X.L. and L.W. participated in the design of the study. All authors contributed to improving the manuscript.

Correspondence to Ping Chang.

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The authors declare no competing financial interests.

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Reviewer Information

Nature thanks K. Kelly and M. Petersen for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Long-term mean Kuroshio response to OME-A feedback simulated by CESM.

Composite of winter-season (ONDJFM) mean U velocity (colour scale and contours in metres per second) zonally averaged between 145° E and 160° E for the whole of the 11-year CESM simulations in CTRL (a), MEFS (b) and MEFS minus CTRL (c). The composite was taken according to the axis of the winter mean KEJ defined by the latitude of maximum U velocity. The x axis is the distance from the KEJ axis.

Extended Data Figure 2 Large-scale wind stress and curl change simulated by CESM.

Annual mean large-scale wind stresses (vectors) and wind stress curl (colour scale) in CTRL (a) and MEFS minus CTRL (b) averaged over the 11-year CESM simulations. The large-scale wind stresses are obtained using a 10° (longitude) × 10° (latitude) boxcar filter. The wind stress curl differences, significant with P value less than 0.05, based on monthly mean data using a Student’s t-test are marked by red crosses in b.

Extended Data Figure 3 EPE budget in the Kuroshio Extension region.

a, Vertical profile of winter season (ONDJFM) mean EPE budget averaged over 145° E–160° E, 30° N–42° N for the CRCM CTRL ensemble (solid) and CRCM MEFS ensemble (dashed). b, As for a, except for MEFS minus CTRL. The inset tables summarize the vertically integrated EPE budget in the upper 250 m for CTRL and MEFS in a and for MEFS minus CTRL in b. Each of the EPE budget terms is indicated using a different colour, and solid and dashed lines in a represent CTRL and MEFS, respectively.

Extended Data Figure 4 Temperature gradient response in the Kuroshio Extension region simulated by CESM and CRCM.

ac, Composite of winter-season mean meridional temperature gradient (colour scale and contours in degrees Celsius per degree latitude) averaged between 145° E and 160° E across the Kuroshio Extension Front in CTRL (a), MEFS (b) and MEFS minus CTRL (c) based on 11-year CESM simulations. The x axis is the distance from the KEJ axis at 0 defined by the latitude of winter mean maximum U and the y axis is depth. df, As for ac, but for the CRCM twin ensembles.

Extended Data Figure 5 Mean flow and eddy response to OME-A feedback in the Gulf Stream simulated by CESM.

ac, Composite of winter season mean U velocity (colour scale and contours in metres per second) along 60° W based on 11-year CESM simulations. The composite was taken according to the axis of the winter mean Gulf Stream current along 60° W defined by the latitude of maximum U velocity. The x axis is the distance from the Gulf Stream current. d, Ratio of EKE spectra between MEFS and CTRL in the Gulf Stream Extension region (35° N–42° N, 70° W–55° W) based on 11-year CESM simulations. e, As for d, but for eddy enstrophy (ENS) spectra. The grey shading indicates the geometric standard deviation of EKE and enstrophy ratio in individual years of the CESM simulations (d and e).

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Ma, X., Jing, Z., Chang, P. et al. Western boundary currents regulated by interaction between ocean eddies and the atmosphere. Nature 535, 533–537 (2016) doi:10.1038/nature18640

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