Editorial Type:
Article
Article Type:
Research Article

Deep Equatorial Ocean Circulation Induced by a Forced–Dissipated Yanai Beam

François Ascani Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii

Search for other papers by François Ascani in
Current site
Google Scholar
PubMed Close
,
Eric Firing Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii

Search for other papers by Eric Firing in
Current site
Google Scholar
PubMed Close
,
Pierre Dutrieux Physical Science Division, British Antarctic Survey, Cambridge, United Kingdom

Search for other papers by Pierre Dutrieux in
Current site
Google Scholar
PubMed Close
,
Julian P. McCreary Department of Oceanography, and International Pacific Research Center, University of Hawaii at Manoa, Honolulu, Hawaii

Search for other papers by Julian P. McCreary in
Current site
Google Scholar
PubMed Close
, and
Akio Ishida Research Institute for Global Change, and Institute of Observational Research for Global Change, JAMSTEC, Yokosuka, Japan

Search for other papers by Akio Ishida in
Current site
Google Scholar
PubMed Close
Print Publication:
01 May 2010
DOI:
https://doi.org/10.1175/2010JPO4356.1
Page(s):
1118–1142
Restricted access

Abstract

A complex pattern of zonal currents below the thermocline has been observed in the equatorial Pacific and Atlantic Oceans. The currents have typical speeds from 10 to 15 cm s−1 and extend as deep as 2500 m. Their structure can be divided into two overlapping parts: the equatorial deep jets (EDJs), centered on the equator and alternating in the vertical with a wavelength of several hundred meters, and the Equatorial Intermediate Current system (EICS), composed of currents with large vertical scale and alternating with latitude over several degrees on either side of the equator. The strongest EICS current is a westward flow on the equator flanked by eastward currents at 2°N and 2°S.

In the present study, the authors use idealized numerical simulations and analytical solutions to demonstrate that the EICS currents within 2.5° from the equator could result from the self-advection with dissipation of a downward-propagating beam of monthly periodic Yanai (Rossby gravity) waves. The zonally restricted beam is generated in the eastern part of the basin by instabilities of the swift near-surface equatorial currents. For a weak Yanai wave amplitude and no dissipation, mean Eulerian currents resembling the three strongest EICS currents are obtained but only within the beam; in this case, the Eulerian flow is balanced by the wave-induced Stokes drift, yielding a zero-mean Lagrangian flow, and the water parcels conserve their potential vorticity (PV) and are stationary over a wave cycle. For larger amplitudes, the Yanai waves break, losing their energy to small vertical scales where it is dissipated. This dissipation changes the mean (wave averaged) PV of a water parcel within the beam, allowing the parcel to have a persistent equatorward drift across PV contours. This can be viewed as a wave-induced Sverdrup transport; by continuity and by virtue of the westward group velocity of long Rossby waves, this Lagrangian-mean meridional flow requires a Lagrangian-mean zonal flow within and to the west of the beam, with a meridional structure consistent with the three strongest EICS currents. This mechanism of EICS formation is active in some ocean general circulation models; its importance in the ocean remains to be evaluated.

Corresponding author address: François Ascani, Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822. Email: fascani@hawaii.edu

Abstract

A complex pattern of zonal currents below the thermocline has been observed in the equatorial Pacific and Atlantic Oceans. The currents have typical speeds from 10 to 15 cm s−1 and extend as deep as 2500 m. Their structure can be divided into two overlapping parts: the equatorial deep jets (EDJs), centered on the equator and alternating in the vertical with a wavelength of several hundred meters, and the Equatorial Intermediate Current system (EICS), composed of currents with large vertical scale and alternating with latitude over several degrees on either side of the equator. The strongest EICS current is a westward flow on the equator flanked by eastward currents at 2°N and 2°S.

In the present study, the authors use idealized numerical simulations and analytical solutions to demonstrate that the EICS currents within 2.5° from the equator could result from the self-advection with dissipation of a downward-propagating beam of monthly periodic Yanai (Rossby gravity) waves. The zonally restricted beam is generated in the eastern part of the basin by instabilities of the swift near-surface equatorial currents. For a weak Yanai wave amplitude and no dissipation, mean Eulerian currents resembling the three strongest EICS currents are obtained but only within the beam; in this case, the Eulerian flow is balanced by the wave-induced Stokes drift, yielding a zero-mean Lagrangian flow, and the water parcels conserve their potential vorticity (PV) and are stationary over a wave cycle. For larger amplitudes, the Yanai waves break, losing their energy to small vertical scales where it is dissipated. This dissipation changes the mean (wave averaged) PV of a water parcel within the beam, allowing the parcel to have a persistent equatorward drift across PV contours. This can be viewed as a wave-induced Sverdrup transport; by continuity and by virtue of the westward group velocity of long Rossby waves, this Lagrangian-mean meridional flow requires a Lagrangian-mean zonal flow within and to the west of the beam, with a meridional structure consistent with the three strongest EICS currents. This mechanism of EICS formation is active in some ocean general circulation models; its importance in the ocean remains to be evaluated.

Corresponding author address: François Ascani, Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822. Email: fascani@hawaii.edu

Save
Cover Journal of Physical Oceanography
Journal of Physical Oceanography

  • Andrews, D. G., and M. E. McIntyre, 1976: Planetary waves in horizontal and vertical shear: The generalized Eliassen-Palm relation and the mean zonal acceleration. J. Atmos. Sci., 33 , 2031–2048.

    • Search Google Scholar
    • Export Citation

  • Andrews, D. G., and M. E. McIntyre, 1978: An exact theory of nonlinear waves on a Lagrangian-mean flow. J. Fluid Mech., 89 , 609–646.

  • Ascani, F., 2008: Wave-induced deep equatorial ocean circulation. Ph.D. thesis, University of Hawaii at Manoa, 158 pp.

  • Baldwin, M. P., and Coauthors, 2001: The quasi-biennial oscillation. Rev. Geophys., 39 , 179–229.

  • Bergé, P., Y. Pomeau, and C. Vidal, 1984: Order within Chaos. John Wiley & Sons, 329 pp.

  • Bourlès, B., M. d’Orgeville, G. Eldin, Y. Gouriou, R. Chuchla, Y. du Penhoat, and S. Arnault, 2002: On the evolution of the thermocline and subthermocline eastward currents in the equatorial Atlantic. Geophys. Res. Lett., 29 , 1785. doi:10.1029/2002GL015098.

    • Search Google Scholar
    • Export Citation

  • Bourlès, B., and Coauthors, 2003: The deep currents in the eastern equatorial Atlantic Ocean. Geophys. Res. Lett., 30 , 8002. doi:10.1029/2002GL015095.

    • Search Google Scholar
    • Export Citation

  • Boyd, J. P., 1976: The noninteraction of waves with the zonally averaged flow on a spherical Earth and the interrelationships of eddy fluxes of energy, heat and momentum. J. Atmos. Sci., 33 , 2285–2291.

    • Search Google Scholar
    • Export Citation

  • Brandt, P., V. Hormann, B. Bourlès, J. Fischer, F. A. Schott, L. Stramma, and M. Dengler, 2008: Oxygen tongues and zonal currents in the equatorial Atlantic. J. Geophys. Res., 113 , C04012. doi:10.1029/2007JC004435.

    • Search Google Scholar
    • Export Citation

  • Bühler, O., 2009: Waves and Mean Flows. Cambridge University Press, 370 pp.

  • Bunge, L., C. Provost, J. M. Lilly, M. d’Orgeville, A. Kartavtseff, and J-L. Melice, 2006: Variability of the horizontal velocity structure in the upper 1600 m of the water column on the equator at 10°W. J. Phys. Oceanogr., 36 , 1287–1304.

    • Search Google Scholar
    • Export Citation

  • Bunge, L., C. Provost, B. L. Hua, and A. Kartavtseff, 2008: Variability at intermediate depths at the equator in the Atlantic Ocean in 2000–06: Annual cycle, equatorial deep jets, and intraseasonal meridional velocity fluctuations. J. Phys. Oceanogr., 38 , 1794–1806.

    • Search Google Scholar
    • Export Citation

  • Cox, M. D., 1980: Generation and propagation of 30-day waves in a numerical model of the Pacific. J. Phys. Oceanogr., 10 , 1168–1186.

  • d’Orgeville, M., B. L. Hua, and H. Sasaki, 2007: Equatorial Deep Jets triggered by a large vertical scale variability within the western boundary layer. J. Mar. Res., 65 , 1–25.

    • Search Google Scholar
    • Export Citation

  • Dunkerton, T., 1980: A Lagrangian mean theory of wave, mean-flow interaction with applications to nonacceleration and its breakdown. Rev. Geophys. Space Phys., 18 , 387–400.

    • Search Google Scholar
    • Export Citation

  • Eriksen, C. C., 1985: Moored observations of deep low-frequency motions in the central Pacific Ocean: Vertical structure and interpretation as equatorial waves. J. Phys. Oceanogr., 15 , 1085–1113.

    • Search Google Scholar
    • Export Citation

  • Eriksen, C. C., and J. G. Richman, 1988: An estimate of equatorial wave energy flux at 9-and 90-day periods in the central Pacific. J. Geophys. Res., 93 , 15455–15466.

    • Search Google Scholar
    • Export Citation

  • Firing, E., 1987: Deep zonal currents in the central equatorial Pacific. J. Mar. Res., 45 , 791–812.

  • Firing, E., S. Wijffels, and P. Hacker, 1998: Equatorial subthermocline currents across the Pacific. J. Geophys. Res., 103 , 21413–21423.

    • Search Google Scholar
    • Export Citation

  • Gill, A. E., 1974: The stability of planetary waves on an infinite beta-plane. Geophys. Fluid Dyn., 6 , 29–47.

  • Gill, A. E., 1982: Atmosphere–Ocean Dynamics. Academic Press, 662 pp.

  • Gouriou, Y., B. Bourlès, H. Mercier, and R. Chuchla, 1999: Deep jets in the equatorial Atlantic Ocean. J. Geophys. Res., 104 , 21217–21226.

    • Search Google Scholar
    • Export Citation

  • Gouriou, Y., and Coauthors, 2001: Deep circulation in the equatorial Atlantic Ocean. Geophys. Res. Lett., 28 , 819–822.

  • Harrison, D., 1989: On climatological monthly mean wind stress and wind stress curl fields over the World Ocean. J. Climate, 2 , 57–70.

    • Search Google Scholar
    • Export Citation

  • Harvey, R. R., and W. C. Patzert, 1976: Deep current measurements suggest long waves in the eastern equatorial Pacific. Science, 193 , 883–885.

    • Search Google Scholar
    • Export Citation

  • Holton, J. R., and R. S. Lindzen, 1972: An updated theory for the quasi-biennial cycle of the tropical stratosphere. J. Atmos. Sci., 29 , 1076–1080.

    • Search Google Scholar
    • Export Citation

  • Hua, B. L., M. d’Orgeville, M. D. Fruman, C. Ménesguen, R. Schopp, P. Klein, and H. Sasaki, 2008: Destabilization of mixed Rossby gravity waves and the formation of equatorial zonal jets. J. Fluid Mech., 610 , 311–341.

    • Search Google Scholar
    • Export Citation

  • Ishida, A., Y. Kashino, H. Mitsudera, N. Yosioka, and T. Kadokura, 1998: Preliminary results of a global high-resolution GCM experiment. J. Fac. Sci. Hokkaido Univ., 11 , 441–460.

    • Search Google Scholar
    • Export Citation

  • Kasahara, A., 1974: Various vertical coordinate systems used for numerical weather prediction. Mon. Wea. Rev., 102 , 509–522.

  • Lebedev, K. V., H. Yoshinari, N. A. Maximenko, and P. W. Hacker, 2007: YoMaHa’07: Velocity data assessed from trajectories of Argo floats at parking level and at the sea surface. IPRC Tech. Note 4, 16 pp.

    • Search Google Scholar
    • Export Citation

  • Li, X., and P. Chang, 1999: Mixing induced by the Atlantic equatorial wave activity in an eddy-resolved OGCM. J. Geophys. Res., 104 , 13303–13315.

    • Search Google Scholar
    • Export Citation

  • Lyman, J. M., G. C. Johnson, and W. S. Kessler, 2007: Distinct 17- and 33-day tropical instability waves in subsurface observations. J. Phys. Oceanogr., 37 , 855–872.

    • Search Google Scholar
    • Export Citation

  • Maltrud, M. E., and J. L. McClean, 2005: An eddy resolving global 1/10° ocean simulation. Ocean Modell., 8 , 31–54.

  • Masina, S., and S. G. H. Philander, 1999: An analysis of tropical instability waves in a numerical model of the Pacific Ocean. 1. Spatial variability of the waves. J. Geophys. Res., 104 , 29613–29635.

    • Search Google Scholar
    • Export Citation

  • Masumoto, Y., and Coauthors, 2004: A fifty-year eddy-resolving simulation of the world ocean–preliminary outcomes of OFES (OGCM for the Earth Simulator). J. Earth Simul., 1 , 35–56.

    • Search Google Scholar
    • Export Citation

  • McCreary, J. P., 1984: Equatorial beams. J. Mar. Res., 42 , 395–430.

  • McCreary, J. P., and Z. Yu, 1992: Equatorial dynamics in a 2 1/2-layer model. Prog. Oceanogr., 29 , 61–132.

  • McEwan, A. D., 1971: Degeneration of resonantly-excited standing internal gravity waves. J. Fluid Mech., 50 , 431–448.

  • McIntyre, M. E., 1980: Towards a Lagrangian-mean description of stratospheric circulations and chemical transports. Philos. Trans. Roy. Soc. London, 296A , 129–148.

    • Search Google Scholar
    • Export Citation

  • McIntyre, M. E., and T. N. Palmer, 1983: Breaking planetary waves in the stratosphere. Nature, 305 , 593–600.

  • McIntyre, M. E., and T. N. Palmer, 1984: The ‘surf zone’ in the stratosphere. J. Atmos. Terr. Phys., 46 , 825–849.

  • McIntyre, M. E., and T. N. Palmer, 1985: General concept of wave breaking for Rossby and gravity waves. Pure Appl. Geophys., 123 , 964–975.

    • Search Google Scholar
    • Export Citation

  • McIntyre, M. E., and W. A. Norton, 1990: Dissipative wave-mean interactions and the transport of vorticity or potential vorticity. J. Fluid Mech., 212 , 403–435.

    • Search Google Scholar
    • Export Citation

  • Ménesguen, C., B. L. Hua, M. D. Fruman, and R. Schopp, 2009: Dynamics of the combined extra-equatorial and equatorial deep jets in the Atlantic. J. Mar. Res., 67 , 323–346.

    • Search Google Scholar
    • Export Citation

  • Menkes, C. E., and Coauthors, 2002: A whirling ecosystem in the equatorial Atlantic. Geophys. Res. Lett., 29 , 1553. doi:10.1029/2001GL014576.

    • Search Google Scholar
    • Export Citation

  • Moore, D. W., 1970: The mass transport velocity induced by free oscillations at a single frequency. Geophys. Fluid Dyn., 1 , 237–247.

  • Muench, J. E., and E. Kunze, 2000: Internal wave interactions with equatorial deep jets. Part II: Acceleration of the jets. J. Phys. Oceanogr., 30 , 2099–2110.

    • Search Google Scholar
    • Export Citation

  • Ollitrault, M., M. Lankhorst, D. Fratantoni, P. Richardson, and W. Zenk, 2006: Zonal intermediate currents in the equatorial Atlantic Ocean. Geophys. Res. Lett., 33 , L05605. doi:10.1029/2005GL025368.

    • Search Google Scholar
    • Export Citation

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

    • Search Google Scholar
    • Export Citation

  • Schott, F. A., L. Stramma, and J. Fischer, 1995: The warm water inflow into the western tropical Atlantic boundary regime, spring 1994. J. Geophys. Res., 100 , 24745–24760.

    • Search Google Scholar
    • Export Citation

  • Schott, F. A., and Coauthors, 2003: The zonal currents and transports at 35°W in the tropical Atlantic. Geophys. Res. Lett., 30 , 1349. doi:10.1029/2002GL016849.

    • Search Google Scholar
    • Export Citation

  • Tang, T. Y., R. H. Weisberg, and D. Halpern, 1988: Vertical structure of low frequency variability in the eastern equatorial Pacific Ocean. J. Phys. Oceanogr., 18 , 1009–1019.

    • Search Google Scholar
    • Export Citation

  • von Schuckmann, K., P. Brandt, and C. Eden, 2008: Generation of tropical instability waves in the Atlantic Ocean. J. Geophys. Res., 113 , C08034. doi:10.1029/2007JC004712.

    • Search Google Scholar
    • Export Citation

  • Weisberg, R. H., and A. M. Horigan, 1981: Low-frequency variability in the equatorial Atlantic. J. Phys. Oceanogr., 11 , 913–920.

  • Weisberg, R. H., A. M. Horigan, and C. Colin, 1979: Equatorially trapped Rossby-gravity wave propagation in the Gulf of Guinea. J. Mar. Res., 37 , 67–86.

    • Search Google Scholar
    • Export Citation

  • Yu, J-Y., and W. T. Liu, 2003: A linear relationship between ENSO intensity and tropical instability wave activity in the eastern Pacific Ocean. Geophys. Res. Lett., 30 , 1735. doi:10.1029/2003GL017176.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 588 223 88
PDF Downloads 306 105 1