• Arakawa, A., 1972: Design of the UCLA general circulation model. Numerical simulation of weather and climate. Tech. Rep. 7, Dept. of Meteorology, University of California, 116 pp.

    • Search Google Scholar
    • Export Citation
  • Barnett, T. P., M. Latif, M. Kirk, and E. Roeckner, 1991: On ENSO physics. J. Climate, 4 , 487514.

  • Battisti, D. S., 1988: Dynamics and thermodynamics of a warming event in a coupled tropical atmosphere–ocean model. J. Atmos. Sci, 45 , 28892919.

    • Search Google Scholar
    • Export Citation
  • Baturin, N. G., and P. P. Niiler, 1997: Effects of instability waves in the mixed layer of the equatorial Pacific. J. Geophys. Res, 102 , 27 77127 793.

    • Search Google Scholar
    • Export Citation
  • Bentamy, A., Y. Quilfen, F. Gohin, N. Grima, M. Lenaour, and J. Servain, 1996: Determination and validation of average wind fields from ERS-1 scatterometer measurements. Global Atmos. Ocean Syst, 4 , 129.

    • Search Google Scholar
    • Export Citation
  • Blanke, B., and P. Delecluse:, 1993,: Variability of the tropical Atlantic Ocean simulated by a general circulation model with two different mixed layer physics. J. Phys. Oceanogr, 23 , 13631388.

    • Search Google Scholar
    • Export Citation
  • Boulanger, J-P., 2000: The Trident Pacific model. Part 1: The oceanic dynamical model during the TOPEX/Poseidon period. Climate Dyn., in press.

    • Search Google Scholar
    • Export Citation
  • ——, and Menkes, C., 1999: Long equatorial wave reflection in the Pacific Ocean from the TOPEX/Poseidon data running the 1992–98 period. Climate Dyn, 15 , 1520515225.

    • Search Google Scholar
    • Export Citation
  • ——, and ——,. 2000: The Trident Pacific model. Part 2: The thermodynamical model and the role of long equatorial wave reflections during the TOPEX/Poseidon period. Climate Dyn., in press.

    • Search Google Scholar
    • Export Citation
  • Bryden, H. L., and E. C. Brady, 1985: Diagnostic model of the three-dimensional circulation in the upper equatorial Pacific Ocean. J. Phys. Oceanogr, 15 , 12551273.

    • Search Google Scholar
    • Export Citation
  • ——, and ——,. 1989: Eddy momentum and heat fluxes and their effects on the circulation of the equatorial Pacific Ocean. J. Mar. Res, 47 , 5579.

    • Search Google Scholar
    • Export Citation
  • DeWitt, D. G., and E. K. Schneider, 1999: The processes determining the annual cycle of equatorial sea surface temperature: A coupled general circulation model perspective. Mon. Wea. Rev, 127 , 381395.

    • Search Google Scholar
    • Export Citation
  • Enfield, D. B., 1986: Zonal and seasonal variations in the near surface heat balance of the equatorial ocean. J. Phys. Oceanogr, 16 , 10381054.

    • Search Google Scholar
    • Export Citation
  • Flament, P., S. C. Kennan, R. Knox, P. P. Niiler, and R. Bernstein, 1996: The three-dimensional structure of an upper vortex in the tropical Pacific. Nature, 382 , 610613.

    • Search Google Scholar
    • Export Citation
  • Frankignoul, C., F. Bonjean, and G. Reverdin, 1996: Interannual variability of surface currents in the tropical Pacific during 1987–93. J. Geophys. Res, 101 , 36293647.

    • Search Google Scholar
    • Export Citation
  • Galanti, E., and E. Tziperman, 2000: On ENSO's phase locking to the seasonal cycle in the fast SST, fast wave and mixed mode regimes. J. Atmos. Phys, 57 , 29362950.

    • Search Google Scholar
    • Export Citation
  • Gibson, R., P. Kallberg, S. Uppala, A. Hernandez, A. Nomura, and E. Serrano, 1997: ERA descripton. ECMWF Re-Analysis Project Rep. Ser., No. 1,. ECMWF, Reading, United Kingdom, 71 pp.

    • Search Google Scholar
    • Export Citation
  • Glantz, M. H., 1996: Currents of Change: E1 Niño's Impact on Climate and Society. Cambridge University Press, 194 pp.

  • Gordon, C., and R. A. Corry, 1991: A model simulation of the seasonal cycle in the tropical Pacific Ocean using climatological and modelled surface forcing. J. Geophys. Res, 96 , 847864.

    • Search Google Scholar
    • Export Citation
  • Grima, N., A. Bentamy, K. Katsaros, Y. Quilfen, P. Delecluse, and C. Levy, 1999: Sensitivity of an oceanic general circulation model forced by satellite wind stress fields. J. Geophys. Res, 104 , 79677989.

    • Search Google Scholar
    • Export Citation
  • Hansen, D., and C. Paul, 1984: Genesis and effect of long waves in the equatorial Pacific. J. Geophys. Res, 89 , 10 34110 440.

  • Harrison, D. E., B. S. Giese, and E. S. Sarachik, 1990: Mechanisms of SST change in the equatorial waveguide during the 1982–83 ENSO. J. Climate, 3 , 173188.

    • Search Google Scholar
    • Export Citation
  • Hayes, S. P., P. Chang, and M. J. McPhaden, 1991: Variability of the sea surface temperature in the eastern equatorial Pacific during 1986–88. J. Geophys. Res, 96 , 10 55310 566.

    • Search Google Scholar
    • Export Citation
  • Huang, B., and E. K. Schneider, 1995: The response of an ocean general circulation model to surface wind stress produced by an atmospheric general circulation model. Mon. Wea. Rev, 123 , 30593085.

    • Search Google Scholar
    • Export Citation
  • Jackett, D. R., and T. J. McDougall, 1995: Minimal adjustment of hydrographic data to achieve static stability. J. Atmos. Oceanic Technol, 12 , 381389.

    • Search Google Scholar
    • Export Citation
  • Jin, F. F., D. Neelin, and M. Ghil, 1994: ENSO on devil's staircase. Science, 264 , 7072.

  • Kennan, S. C., and P. Flament, 2000: Observations of a tropical instability vortex. J. Phys. Oceanogr, 30 , 22772301.

  • Lagerloef, G. S. E., G. Mitchum, R. Lukas, and P. Niiler, 1999: Tropical Pacific surface currents with altimeter, wind, and drifter data. J. Geophys. Res, 104 , 23 31323 326.

    • Search Google Scholar
    • Export Citation
  • Lazar, A., G. Madec, and P. Delecluse, 1999: The deep interior downwelling, the Veronis effect, and mesoscale tracer transport parameterizations in an OGCM. J. Phys. Oceanogr, 29 , 29452961.

    • Search Google Scholar
    • Export Citation
  • Legeckis, R., 1977: Long waves in the eastern equatorial Pacific Ocean: A view from a geostationary satellite. Science, 197 , 11791181.

    • Search Google Scholar
    • Export Citation
  • Levitus, S., 1982: Climatological Atlas of the World Ocean. NOAA Prof. Paper No. 13, U.S. Govt. Printing Office, 173 pp. and 17 microfiche.

    • Search Google Scholar
    • Export Citation
  • Liu, W. T., K. B. Katsaros, and J. A. Businger, 1979: Bulk parameterization of air–sea exchanges of heat and water vapor including the molecular constraints at the interface. J. Atmos. Sci, 36 , 17221735.

    • Search Google Scholar
    • Export Citation
  • Madec, G., P. Delecluse, M. Imbard, and C. Levy, 1999: OPA 8.1 Ocean General Circulation Model reference manual. Notes du pôle de modélisation, Vol. 11, Institut Pierre Simon Laplace (IPSL), 91 pp.

    • Search Google Scholar
    • Export Citation
  • Maes, C., G. Madec, and P. Delecluse, 1997: Sensitivity of an equatorial pacific OGCM to the lateral diffusion. Mon. Wea. Rev, 125 , 958971.

    • Search Google Scholar
    • Export Citation
  • McCarty, M. E., and M. J. McPhaden, 1993: Mean seasonal cycles and interannual variations at 0,165E during 1986–1992. NOAA Tech. Memo. ERL PMEL-98, 64 pp.

    • Search Google Scholar
    • Export Citation
  • McCreary, J. P., and Z. Yu, 1992: Equatorial dynamics in a 2.5-layer model. Progress in Oceanography, Vol. 29, Pergamon, 61–132.

  • McPhaden, M. J., 1999: Genesis and evolution of the 1997–98 El Niño. Science, 283 , 950954.

  • ——, and Taft, B. A., 1988: On the dynamics of seasonal and intraseasonal variability in the eastern equatorial Pacific. J. Phys. Oceanogr, 18 , 17131732.

    • Search Google Scholar
    • Export Citation
  • ——, and Yu, X., 1999: Equatorial waves and the 1997–98 El Niño. Geophys. Res. Lett, 26 , 29612964.

  • ——, Busalacchi, A., and J. Picaut, 1988: Observations and wind-forced simulations of the mean seasonal cycle in the tropical Pacific sea surface topography. J. Geophys. Res, 93 , 81318146.

    • Search Google Scholar
    • Export Citation
  • ——, and Coauthors, 1998: The Tropical Ocean–Global Atmosphere observing system: A decade of progress. J. Geophys. Res, 103 , 14 16914 240.

    • Search Google Scholar
    • Export Citation
  • Menkes, C., J-P. Boulanger, A. J. Busalacchi, J. Vialard, P. Delecluse, M. J. McPhaden, E. Hackert, and N. Grima, 1998: Impact of TAO vs. ERS wind stresses onto simulations of the tropical Pacific Ocean during the 1993–1998 period by the OPA OGCM. Euroclivar Workshop Rep. 13,. , 4648.

    • Search Google Scholar
    • Export Citation
  • Miller, A. J., J. M. Oberhuber, N. E. Graham, and T. P. Barnett, 1992: Tropical Pacific Ocean response to observed winds in a layered general circulation model. J. Geophys. Res, 97 , 73177340.

    • Search Google Scholar
    • Export Citation
  • ——, Barnett, T. P., and N. E. Graham, 1993: A comparison of some tropical ocean models: Hindcast skill and El Niño evolution. J. Phys. Oceanogr, 23 , 15671591.

    • Search Google Scholar
    • Export Citation
  • Neelin, J. D., 1991: The slow SST mode and the fast wave limit: Analytic theory for tropical interannual oscillations and experiments in an hybrid coupled model. J. Atmos. Sci, 48 , 584605.

    • Search Google Scholar
    • Export Citation
  • Niiler, P. P., and E. B. Kraus, 1977: One-dimensional models of the upper ocean. Modelling and Prediction of the Upper Layers of the Ocean, E. B. Kraus, Ed., Pergamon, 143–172.

    • Search Google Scholar
    • Export Citation
  • Oberhüber, J. M., 1988: An atlas based on the COADS data set: The budgets of heat, buoyancy and turbulent kinetic energy at the surface of the global ocean. Report 15, Max-Planck-Institut für Meteorologie, 20 pp.

    • Search Google Scholar
    • Export Citation
  • Philander, S. G. H., 1976: Instabilities of zonal equatorial currents, Part 1. J. Geophys. Res, 81 , 37253735.

  • ——, and Seigel, A. D., 1985: Simulation of El Niño of 1982–1983. Coupled Ocean–Atmosphere Models, J. C. J. Nihoul, Ed., Elsevier, 517–541.

    • Search Google Scholar
    • Export Citation
  • Picaut, J., M. Ioualalen, T. Delcroix, M. J. McPhaden, and C. Menkes, 1996: Mechanism of the zonal displacements of the Pacific warm pool: Implications for ENSO. Science, 274 , 14861489.

    • Search Google Scholar
    • Export Citation
  • ——, Masia, F., and Y. du Penhoat, 1997: An advective–reflective conceptual model for the oscillatory nature of the ENSO. Science, 277 , 663666.

    • Search Google Scholar
    • Export Citation
  • ——, Ioualalen, M., T. Delcroix, F. Masia, R. Murtugudde, and J. Vialard, 2001: Displacements of an oceanic zone of convergence on the eastern edge of the Pacific warm pool: Consequences for ENSO and biogeochemical phenomena. J. Geophys. Res, 106 , 23632386.

    • Search Google Scholar
    • Export Citation
  • Pontaud, M., and O. Thual, 1998: Coupled processes for equatorial Pacific interannual variability. Quart. J. Roy. Meteor. Soc, 124 , 527555.

    • Search Google Scholar
    • Export Citation
  • Proehl, J. A., 1996: Linear stability of equatorial zonal flows. J. Phys. Oceanogr, 26 , 601621.

  • Rasmunsson, E. M., and T. H. Carpenter, 1982: Variations in tropical sea surface temperature and surface wind fields associated with the Southern Oscillation/El Niño. Mon. Wea. Rev, 110 , 354384.

    • Search Google Scholar
    • Export Citation
  • Reverdin, G., C. Frankignoul, E. Kestenare, and M. J. McPhaden, 1994: Seasonal variability in the surface currents of the equatorial Pacific. J. Geophys. Res, 99 , 20 32320 344.

    • Search Google Scholar
    • Export Citation
  • Reynolds, R. W., and T. M. Smith, 1994: Improved global sea surface temperature analyses. J. Climate, 7 , 929948.

  • Seager, R., 1989: Modeling tropical Pacific sea surface temperature: 1970–87. J. Phys. Oceanogr, 19 , 419434.

  • Smith, S. D., 1988: Coefficients for sea surface wind stress, heat flux and wind profiles as a function of wind speed and temperature. J. Geophys. Res, 93 , 15 46715 472.

    • Search Google Scholar
    • Export Citation
  • Stevenson, J. W., and P. P. Niiler, 1983: Upper ocean heat budget during the Hawaii-to-Tahiti shuttle experiment. J. Phys. Oceanogr, 13 , 18941907.

    • Search Google Scholar
    • Export Citation
  • Swenson, M. S., and D. V. Hansen, 1999: Tropical Pacific Ocean mixed layer heat budget: The pacific cold tongue. J. Phys. Oceanogr, 29 , 8391.

    • Search Google Scholar
    • Export Citation
  • Tziperman, E. L., M. Cane, and H. Jarosh, 1994: El Niño chaos: Overlapping of resonances between the seasonal cycle and the Pacific Ocean–atmosphere oscillator. Science, 264 , 7274.

    • Search Google Scholar
    • Export Citation
  • Vialard, J., 1997: Influence de la salinité sur les interactions couplées océan–atmosphère dans le Pacifique Ouest. Ph.D. thesis, l'Université Pierre et Marie Curie, Paris, France, 218 pp.

    • Search Google Scholar
    • Export Citation
  • ——, and Delecluse, P., 1998a: An OGCM study for the TOGA decade. Part I: Role of salinity in the physics of the western Pacific fresh pool. J. Phys. Oceanogr, 28 , 10711088.

    • Search Google Scholar
    • Export Citation
  • ——, and ——,. 1998b: An OGCM study for the TOGA decade. Part II: Barrier layer formation and variability. J. Phys. Oceanogr, 28 , 10891106.

    • Search Google Scholar
    • Export Citation
  • Wang, C., and R. H. Weisberg, 1994: On the “slow mode” mechanism in ENSO-related coupled ocean–atmosphere models. J. Climate, 7 , 16571667.

    • Search Google Scholar
    • Export Citation
  • ——, and ——,. 2000: The 1997–98 evolution relative to previous El Niño events. J. Climate, 13 , 488501.

  • Wang, W., and M. J. McPhaden, 1999: The surface layer heat balance in the equatorial Pacific Ocean. Part I: Mean seasonal cycle. J. Phys. Oceanogr, 29 , 18121831.

    • Search Google Scholar
    • Export Citation
  • Wyrtki, K., 1981: An estimate of equatorial upwelling in the Pacific. J. Phys. Oceanogr, 11 , 12051214.

  • Yu, X., and M. J. McPhaden, 1999: Seasonal variability in the equatorial Pacific. J. Phys. Oceanogr, 29 , 925947.

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A Model Study of Oceanic Mechanisms Affecting Equatorial Pacific Sea Surface Temperature during the 1997–98 El Niño

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  • 1 Laboratoire d'Océanographie Dynamique et de Climatologie, Paris, France
  • | 2 Pacific Marine Environmental Laboratory, Seattle, Washington
  • | 3 Laboratoire d'Océanographie Dynamique et de Climatologie, Paris, France
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Abstract

In this study, the processes affecting sea surface temperature variability over the 1992–98 period, encompassing the very strong 1997–98 El Niño event, are analyzed. A tropical Pacific Ocean general circulation model, forced by a combination of weekly ERS1–2 and TAO wind stresses, and climatological heat and freshwater fluxes, is first validated against observations. The model reproduces the main features of the tropical Pacific mean state, despite a weaker than observed thermal stratification, a 0.1 m s−1 too strong (weak) South Equatorial Current (North Equatorial Countercurrent), and a slight underestimate of the Equatorial Undercurrent. Good agreement is found between the model dynamic height and TOPEX/Poseidon sea level variability, with correlation/rms differences of 0.80/4.7 cm on average in the 10°N–10°S band. The model sea surface temperature variability is a bit weak, but reproduces the main features of interannual variability during the 1992–98 period. The model compares well with the TAO current variability at the equator, with correlation/rms differences of 0.81/0.23 m s−1 for surface currents. The model therefore reproduces well the observed interannual variability, with wind stress as the only interannually varying forcing.

This good agreement with observations provides confidence in the comprehensive three-dimensional circulation and thermal structure of the model. A close examination of mixed layer heat balance is thus undertaken, contrasting the mean seasonal cycle of the 1993–96 period and the 1997–98 El Niño. In the eastern Pacific, cooling by exchanges with the subsurface (vertical advection, mixing, and entrainment), the atmospheric forcing, and the eddies (mainly the tropical instability waves) are the three main contributors to the heat budget. In the central–western Pacific, the zonal advection by low-frequency currents becomes the main contributor. Westerly wind bursts (in December 1996 and March and June 1997) were found to play a decisive role in the onset of the 1997–98 El Niño. They contributed to the early warming in the eastern Pacific because the downwelling Kelvin waves that they excited diminished subsurface cooling there. But it is mainly through eastward advection of the warm pool that they generated temperature anomalies in the central Pacific. The end of El Niño can be linked to the large-scale easterly anomalies that developed in the western Pacific and spread eastward, from the end of 1997 onward. In the far-western Pacific, because of the shallower than normal thermocline, these easterlies cooled the SST by vertical processes. In the central Pacific, easterlies pushed the warm pool back to the west. In the east, they led to a shallower thermocline, which ultimately allowed subsurface cooling to resume and to quickly cool the surface layer.

 Current affiliation: European Centre for Medium-Range Weather Forecasts, Berkshire, Reading, United Kingdom

Corresponding author address: Jérôme Vialard, ECMWF, Shinfield Park, Berkshire, Reading RG2 9AX, United Kingdom.Email: j.vialard@ecmwf.int

Abstract

In this study, the processes affecting sea surface temperature variability over the 1992–98 period, encompassing the very strong 1997–98 El Niño event, are analyzed. A tropical Pacific Ocean general circulation model, forced by a combination of weekly ERS1–2 and TAO wind stresses, and climatological heat and freshwater fluxes, is first validated against observations. The model reproduces the main features of the tropical Pacific mean state, despite a weaker than observed thermal stratification, a 0.1 m s−1 too strong (weak) South Equatorial Current (North Equatorial Countercurrent), and a slight underestimate of the Equatorial Undercurrent. Good agreement is found between the model dynamic height and TOPEX/Poseidon sea level variability, with correlation/rms differences of 0.80/4.7 cm on average in the 10°N–10°S band. The model sea surface temperature variability is a bit weak, but reproduces the main features of interannual variability during the 1992–98 period. The model compares well with the TAO current variability at the equator, with correlation/rms differences of 0.81/0.23 m s−1 for surface currents. The model therefore reproduces well the observed interannual variability, with wind stress as the only interannually varying forcing.

This good agreement with observations provides confidence in the comprehensive three-dimensional circulation and thermal structure of the model. A close examination of mixed layer heat balance is thus undertaken, contrasting the mean seasonal cycle of the 1993–96 period and the 1997–98 El Niño. In the eastern Pacific, cooling by exchanges with the subsurface (vertical advection, mixing, and entrainment), the atmospheric forcing, and the eddies (mainly the tropical instability waves) are the three main contributors to the heat budget. In the central–western Pacific, the zonal advection by low-frequency currents becomes the main contributor. Westerly wind bursts (in December 1996 and March and June 1997) were found to play a decisive role in the onset of the 1997–98 El Niño. They contributed to the early warming in the eastern Pacific because the downwelling Kelvin waves that they excited diminished subsurface cooling there. But it is mainly through eastward advection of the warm pool that they generated temperature anomalies in the central Pacific. The end of El Niño can be linked to the large-scale easterly anomalies that developed in the western Pacific and spread eastward, from the end of 1997 onward. In the far-western Pacific, because of the shallower than normal thermocline, these easterlies cooled the SST by vertical processes. In the central Pacific, easterlies pushed the warm pool back to the west. In the east, they led to a shallower thermocline, which ultimately allowed subsurface cooling to resume and to quickly cool the surface layer.

 Current affiliation: European Centre for Medium-Range Weather Forecasts, Berkshire, Reading, United Kingdom

Corresponding author address: Jérôme Vialard, ECMWF, Shinfield Park, Berkshire, Reading RG2 9AX, United Kingdom.Email: j.vialard@ecmwf.int

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