Editorial Type:
Article
Article Type:
Research Article

Generation and Growth Mechanism of the Natal Pulse

Motohiko Tsugawa Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan

Search for other papers by Motohiko Tsugawa in
Current site
Google Scholar
PubMed Close
and
Hiroyasu Hasumi Atmosphere and Ocean Research Institute, University of Tokyo, Chiba, Japan

Search for other papers by Hiroyasu Hasumi in
Current site
Google Scholar
PubMed Close
Print Publication:
01 Jul 2010
DOI:
https://doi.org/10.1175/2010JPO4347.1
Page(s):
1597–1612
Restricted access

Abstract

The Natal pulses, solitary cyclonic meanders in the Agulhas Current, are reproduced in an ocean general circulation model. The model covers the region around the Agulhas Current with a grid fine enough to reproduce major eddies. The features of the reproduced Natal pulses are consistent with observational evidences in the following respects: they are generated at the Natal Bight when anticyclonic eddies come, move downstream along the Agulhas Current at speeds about 20 km day−1, and grow in its horizontal size as they move. The present simulation shows that the generation and growth of the Natal pulse occurs because of the interaction between the mean flow of the Agulhas Current and an anticyclonic eddy. A supplemental simulation, where the topography of the Natal Bight is modified, indicates that the topography of the Natal Bight does not cause the generation of the Natal pulses, contrary to a previous suggestion.

Corresponding author address: Motohiko Tsugawa, 3173-25, Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan. Email: tsugawa@jamstec.go.jp

Abstract

The Natal pulses, solitary cyclonic meanders in the Agulhas Current, are reproduced in an ocean general circulation model. The model covers the region around the Agulhas Current with a grid fine enough to reproduce major eddies. The features of the reproduced Natal pulses are consistent with observational evidences in the following respects: they are generated at the Natal Bight when anticyclonic eddies come, move downstream along the Agulhas Current at speeds about 20 km day−1, and grow in its horizontal size as they move. The present simulation shows that the generation and growth of the Natal pulse occurs because of the interaction between the mean flow of the Agulhas Current and an anticyclonic eddy. A supplemental simulation, where the topography of the Natal Bight is modified, indicates that the topography of the Natal Bight does not cause the generation of the Natal pulses, contrary to a previous suggestion.

Corresponding author address: Motohiko Tsugawa, 3173-25, Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan. Email: tsugawa@jamstec.go.jp

Save
Cover Journal of Physical Oceanography
Journal of Physical Oceanography

  • Adcroft, A., C. Hill, and J. Marshall, 1997: Representation of topography by shaved cells in a height coordinate ocean model. Mon. Wea. Rev., 125 , 2293–2315.

    • Search Google Scholar
    • Export Citation

  • Antonov, J., S. Levitus, T. Boyer, M. Conkright, T. O’Brien, and C. Stephens, 1998: Temperature of the Atlantic Ocean. Vol. 1, World Ocean Atlas 1998, NOAA Atlas NESDIS 27, 166 pp.

    • Search Google Scholar
    • Export Citation

  • Arakawa, A., and V. R. Lamb, 1977: Computational design of the basic dynamical processes of the UCLA general circulation model. Methods Comput. Phys., 17 , 173–265.

    • Search Google Scholar
    • Export Citation

  • Beal, L. M., and H. L. Bryden, 1999: The velocity and vorticity structure of the Agulhas Current at 32°S. J. Geophys. Res., 104 , 5151–5176.

    • Search Google Scholar
    • Export Citation

  • Biastoch, A., and W. Krauss, 1999: The role of mesoscale eddies in the source regions of the Agulhas Current. J. Phys. Oceanogr., 29 , 2303–2317.

    • Search Google Scholar
    • Export Citation

  • Biastoch, A., J. R. E. Lutjeharms, C. W. Böning, and M. Scheinert, 2008: Mesoscale perturbations control inter-ocean exchange south of Africa. Geophys. Res. Lett., 35 , L20602. doi:10.1029/2008GL035132.

    • Search Google Scholar
    • Export Citation

  • Biastoch, A., L. Beal, J. Lutjeharms, and T. Casal, 2009: Variability and coherence of the Agulhas Undercurrent in a high-resolution ocean general circulation model. J. Phys. Oceanogr., 39 , 2417–2435.

    • Search Google Scholar
    • Export Citation

  • Bryan, K., 1969: A numerical method for the study of the circulation of the World Ocean. J. Comput. Phys., 4 , 347–376.

  • Bryden, H. L., L. M. Beal, and L. M. Duncan, 2005: Structure and transport of the Agulhas Current and its temporal variability. J. Oceanogr., 61 , 479–492.

    • Search Google Scholar
    • Export Citation

  • Cox, M. D., 1984: A primitive equation, 3-dimensional model of the ocean. Geophysical Fluid Dynamics Laboratory Ocean Group Tech. Rep. 1, 250 pp.

    • Search Google Scholar
    • Export Citation

  • de Ruijter, W. P. M., P. J. van Leeuwen, and J. R. E. Lutjeharms, 1999: Generation and evolution of Natal pulses: Solitary meanders in the Agulhas Current. J. Phys. Oceanogr., 29 , 3043–3055.

    • Search Google Scholar
    • Export Citation

  • de Ruijter, W. P. M., H. Ridderinkhof, J. R. E. Lutjeharms, M. W. Schouten, and C. Veth, 2002: Observations of the flow in the Mozambique Channel. Geophys. Res. Lett., 29 , 140–141.

    • Search Google Scholar
    • Export Citation

  • Gründlingh, M., 1979: Observation of a large meander in the Agulhas Current. J. Geophys. Res., 84 , (C7). 3776–3778.

  • Haney, R. L., 1971: Surface thermal boundary condition for ocean circulation models. J. Phys. Oceanogr., 1 , 241–248.

  • Harris, T., R. Legeckis, and D. van Forest, 1978: Satellite infrared images in the Agulhas Current system. Deep-Sea Res., 25 , 543–548.

    • Search Google Scholar
    • Export Citation

  • Lutjeharms, J. R. E., 2006: Three decades of research on the greater Agulhas Current. Ocean Sci., 3 , 939–995.

  • Lutjeharms, J. R. E., and H. R. Roberts, 1988: The Natal pulse: An extreme transient on the Agulhas Current. J. Geophys. Res., 93 , 631–645.

    • Search Google Scholar
    • Export Citation

  • Lutjeharms, J. R. E., and W. P. M. de Ruijter, 1996: The influence of the Agulhas Current on the adjacent coastal ocean: Possible impacts of climate change. J. Mar. Syst., 7 , (2–4). 321–336.

    • Search Google Scholar
    • Export Citation

  • Lutjeharms, J. R. E., O. Boebel, P. C. F. van der Vaart, W. P. M. de Ruijter, T. Rossby, and H. L. Bryden, 2001: Evidence that the Natal pulse involves the Agulhas Current to its full depth. Geophys. Res. Lett., 28 , 3449–3452.

    • Search Google Scholar
    • Export Citation

  • Murray, R., 1996: Explicit generation of orthogonal grids for ocean models. J. Comput. Phys., 126 , 251–273.

  • Pacanowski, R., and S. Philander, 1981: Parameterization of vertical mixing in numerical models of tropical oceans. J. Phys. Oceanogr., 11 , 1443–1451.

    • Search Google Scholar
    • Export Citation

  • Pacanowski, R., and A. Gnanadesikan, 1998: Transient response in a Z-level ocean model that resolves topography with partial cells. Mon. Wea. Rev., 126 , 3248–3270.

    • Search Google Scholar
    • Export Citation

  • Pearce, A. F., 1977: Some features of the upper 500 m of the Agulhas Current. J. Mar. Res., 35 , 731–753.

  • Research Data Archive, 2001: NGDC, ETOPO2 global 2’ elevations. National Center for Atmospheric Research Computational and Information Systems Laboratory. [Available online at http://dss.ucar.edu/datasets/ds759.3/].

    • Search Google Scholar
    • Export Citation

  • Röske, F., 2001: An atlas of surface fluxes based on the ECMWF Re-Analysis—A climatological dataset to force global ocean general circulation models. Max-Planck-Institut für Meteorologie Rep. 323, 31 pp.

    • Search Google Scholar
    • Export Citation

  • Schouten, M. W., W. P. M. de Ruijter, and P. J. van Leeuwen, 2002: Upstream control of Agulhas Ring shedding. J. Geophys. Res., 107 , 3109. doi:10.1029/2001JC000804.

    • Search Google Scholar
    • Export Citation

  • Semtner Jr., A. J., 1974: An oceanic general circulation model with bottom topography, numerical simulations of weather and climate. University of California, Los Angeles Tech. Rep. 9, 99 pp.

    • Search Google Scholar
    • Export Citation

  • Siedler, G., M. Rouault, A. Biastoch, B. Backeberg, C. J. C. Reason, and J. R. E. Lutjeharms, 2009: Modes of the southern extension of the East Madagascar Current. J. Geophys. Res., 114 , C01005. doi:10.1029/2008JC004921.

    • Search Google Scholar
    • Export Citation

  • Speich, S., J. R. E. Lutjeharms, P. Penven, and B. Blanke, 2006: Role of bathymetry in Agulhas Current configuration and behaviour. Geophys. Res. Lett., 33 , L23611. doi:10.1029/2006GL027157.

    • Search Google Scholar
    • Export Citation

  • Tsugawa, M., Y. Tanaka, and T. Matsuno, 2008: An ocean general circulation model on a quasi-homogeneous cubic grid. Ocean Modell., 22 , 66–86.

    • Search Google Scholar
    • Export Citation

  • Tsujino, H., N. Usui, and H. Nakano, 2006: Dynamics of Kuroshio path variations in a high-resolution general circulation model. J. Geophys. Res., 111 , C11001. doi:10.1029/2005JC003118.

    • Search Google Scholar
    • Export Citation

  • van der Vaart, P. C. F., and W. P. M. de Ruijter, 2001: Stability of western boundary currents with an application to pulselike behavior of the Agulhas Current. J. Phys. Oceanogr., 31 , 2625–2644.

    • Search Google Scholar
    • Export Citation

  • van Leeuwen, P. J., W. P. M. de Ruijter, and J. R. E. Lutjeharms, 2000: Natal pulses and the formation of Agulhas rings. J. Geophys. Res., 105 , 6425–6436.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 416 168 28
PDF Downloads 251 80 10