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Structure and dynamics of the Benguela low-level coastal jet

Abstract

Generations of coupled atmosphere–ocean general circulation models have been plagued by persistent warm sea surface temperature (SST) biases in the southeastern tropical Atlantic. The SST biases are most severe in the eastern boundary coastal upwelling region and are sensitive to surface wind stress and wind stress curl associated with the Benguela low-level coastal jet (BLLCJ), a southerly jet parallel to the Angola-Namibia coast. However, little has been documented about this atmospheric source of oceanic bias. Here we investigate the characteristics and dynamics of the BLLCJ using observations, reanalyses, and atmospheric model simulations. Satellite wind products and high-resolution reanalyses and models represent the BLLCJ with two near-shore maxima, one near the Angola-Benguela front (ABF) at 17.5°S, and the other near 25–27.5°S, whereas coarse resolution reanalyses and models represent the BLLCJ poorly with a single, broad, more offshore maximum. Model experiments indicate that convex coastal geometry near the ABF supports the preferred location of the BLLCJ northern maximum by supporting conditions for a hydraulic expansion fan. Intraseasonal variability of the BLLCJ is associated with large-scale variability in intensity and location of the South Atlantic subtropical high through modulation of the low-level zonal pressure gradient.

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Acknowledgements

This research was supported by U.S. National Science Foundation Grant OCE-1334707 and used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant ACI-1053575. High-performance computing resources provided by the Texas Advanced Computing Center (TACC) at The University of Texas at Austin and by the Texas A&M Supercomputing Facility. SCOW, NCEP CFSR, JRA-55, ECMWF-Interim, NCEP Reanalysis, and NCEP-II Reanalysis available at the Research Data Archive at the National Center for Atmospheric Research, Computational and Information Systems Laboratory, Boulder, CO. CCMP and GHRSST MUR SST available at ftp://podaac.jpl.nasa.gov. MERRA available from NASA’s Global Modeling and Assimilation Office (GMAO) and the Goddard Earth Sciences Data and Information Services Center (GES DISC). CORE-II available at http://data1.gfdl.noaa.gov/nomads/forms/core/COREv2.html. NOAA_OI_SST_V2 data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, at http://www.esrl.noaa.gov/psd. Figures created using the Grid Analysis and Display System (GrADS), GrADS functions by Chihiro Kodama, and http://colorbrewer2.org/. The authors thank J. Kurian, R. Saravanan, R. J. Small, and P. Zuidema for helpful discussions and three anonymous reviewers for their comments that have helped improve the paper.

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Correspondence to Christina M. Patricola.

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Patricola, C.M., Chang, P. Structure and dynamics of the Benguela low-level coastal jet. Clim Dyn 49, 2765–2788 (2017). https://doi.org/10.1007/s00382-016-3479-7

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Keywords

  • Low-level coastal jet
  • Benguela coastal upwelling system
  • SST bias
  • Hydraulic expansion fan