Changes to ENSO under CO2 Doubling in a Multimodel Ensemble

William J. Merryfield Canadian Centre for Climate Modelling and Analysis, Meteorological Service of Canada, Victoria, British Columbia, Canada

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Abstract

An EOF analysis is used to intercompare the response of ENSO-like variability to CO2 doubling in results from 15 coupled climate models assembled for the Intergovernmental Panel on Climate Change Fourth Assessment Report. Under preindustrial conditions, 12 of the 15 models exhibit ENSO amplitudes comparable to or exceeding that observed in the second half of the twentieth century. Under CO2 doubling, three of the models exhibit statistically significant (p < 0.1) increases in ENSO amplitude, and five exhibit significant decreases. The overall amplitude changes are not strongly related to the magnitude or pattern of surface warming. It is, however, found that ENSO amplitude decreases (increases) in models having a narrow (wide) ENSO zonal wind stress response and ENSO amplitude comparable to or greater than observed. The models exhibit a mean fractional decrease in ENSO period of about 5%. Although many factors can influence the ENSO period, it is suggested that this may be related to a comparable increase in equatorial wave speed through an associated speedup of delayed-oscillator feedback. Changes in leading EOF, characterized in many of the models by a relative increase in the amplitude of SST variations in the central Pacific, are in most cases consistent with effects of anomalous zonal and vertical advection resulting from warming-induced changes in SST.

Corresponding author address: Dr. William Merryfield, Canadian Centre for Climate Modelling and Analysis, University of Victoria, P.O. Box 1700, Victoria BC V8W 2Y2, Canada. Email: bill.merryfield@ec.gc.ca

Abstract

An EOF analysis is used to intercompare the response of ENSO-like variability to CO2 doubling in results from 15 coupled climate models assembled for the Intergovernmental Panel on Climate Change Fourth Assessment Report. Under preindustrial conditions, 12 of the 15 models exhibit ENSO amplitudes comparable to or exceeding that observed in the second half of the twentieth century. Under CO2 doubling, three of the models exhibit statistically significant (p < 0.1) increases in ENSO amplitude, and five exhibit significant decreases. The overall amplitude changes are not strongly related to the magnitude or pattern of surface warming. It is, however, found that ENSO amplitude decreases (increases) in models having a narrow (wide) ENSO zonal wind stress response and ENSO amplitude comparable to or greater than observed. The models exhibit a mean fractional decrease in ENSO period of about 5%. Although many factors can influence the ENSO period, it is suggested that this may be related to a comparable increase in equatorial wave speed through an associated speedup of delayed-oscillator feedback. Changes in leading EOF, characterized in many of the models by a relative increase in the amplitude of SST variations in the central Pacific, are in most cases consistent with effects of anomalous zonal and vertical advection resulting from warming-induced changes in SST.

Corresponding author address: Dr. William Merryfield, Canadian Centre for Climate Modelling and Analysis, University of Victoria, P.O. Box 1700, Victoria BC V8W 2Y2, Canada. Email: bill.merryfield@ec.gc.ca

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  • Battisti, D. S. and A. C. Hirst, 1989: Interannual variability in a tropical atmosphere–ocean model: Influence of the basic state, ocean geometry and nonlinearity. J. Atmos. Sci., 46:16871712.

    • Search Google Scholar
    • Export Citation
  • Burgers, G. and G. J. van Oldenborgh, 2003: On the impact of local feedbacks in the central Pacific on the ENSO cycle. J. Climate, 16:23962407.

    • Search Google Scholar
    • Export Citation
  • Capotondi, A., A. Wittenberg, and S. Masina, 2006: Spatial and temporal structure of Tropical Pacific interannual variability in 20th century coupled simulations. Ocean Modell., in press.

    • Search Google Scholar
    • Export Citation
  • Chelton, D. B., R. A. deSzoeke, M. G. Schlax, K. El Naggar, and N. Siwertz, 1998: Geographical variability of the first-baroclinic Rossby radius of deformation. J. Phys. Oceanogr., 28:433460.

    • Search Google Scholar
    • Export Citation
  • Collins, M., 2000a: The El Niño–Southern Oscillation in the Second Hadley Centre Coupled Model and its response to greenhouse warming. J. Climate, 13:12991312.

    • Search Google Scholar
    • Export Citation
  • Collins, M., 2000b: Understanding uncertainties in the response of ENSO to greenhouse warming. Geophys. Res. Lett., 27:35093512.

  • Collins, M., the CMIP Modelling Groups 2005: El Niño- or La Niña-like climate change? Climate Dyn., 24:89104.

  • Dijkstra, H. A. and G. Burgers, 2002: Fluid mechanics of El Niño variability. Annu. Rev. Fluid Mech., 34:531558.

  • Eccles, F. and E. Tziperman, 2004: Nonlinear effects on ENSO’s period. J. Atmos. Sci., 61:474482.

  • Guilyardi, E., 2006: El Niño–mean state–seasonal cycle interactions in a multi-model ensemble. Climate Dyn., 26:329348.

  • Jin, F-F., 1997: An equatorial ocean recharge paradigm for ENSO. Part I: Conceptual model. J. Atmos. Sci., 54:811829.

  • Kang, I-S. and S-I. An, 1998: Kelvin and Rossby wave contributions to the SST oscillation of ENSO. J. Climate, 11:24612469.

  • Kirtman, B. P., 1997: Oceanic Rossby wave dynamics and the ENSO period in a coupled model. J. Climate, 10:16901704.

  • Knutson, T. R. and S. Manabe, 1994: Impact of increased CO2 on simulated ENSO-like phenomena. Geophys. Res. Lett., 21:22952298.

  • Knutson, T. R. and S. Manabe, 1998: Model assessment of decadal variability and trends in the tropical Pacific Ocean. J. Climate, 11:22732296.

    • Search Google Scholar
    • Export Citation
  • Knutson, T. R., S. Manabe, and D. Gu, 1997: Simulated ENSO in a global coupled ocean–atmosphere model: Multidecadal amplitude modulation and CO2 sensitivity. J. Climate, 10:138161.

    • Search Google Scholar
    • Export Citation
  • Monahan, A. H., 2001: Nonlinear principal component analysis: Tropical Indo-Pacific sea surface temperature and sea level pressure. J. Climate, 14:219233.

    • Search Google Scholar
    • Export Citation
  • Monahan, A. H. and A. Dai, 2004: The spatial and temporal structure of ENSO nonlinearity. J. Climate, 17:30263036.

  • Meehl, G. A. and W. M. Washington, 1996: El Niño-like climate change in a model with increased CO2 concentrations. Nature, 382:5660.

    • Search Google Scholar
    • Export Citation
  • Meehl, G. A., G. W. Branstator, and W. M. Washington, 1993: Tropical Pacific interannual variability and CO2 climate change. J. Climate, 6:4263.

    • Search Google Scholar
    • Export Citation
  • Meehl, G. A., W. D. Collins, B. Boville, J. T. Kiehl, T. M. L. Wigley, and J. M. Arblaster, 2000: Response of the NCAR Climate System Model to increased CO2 and the role of physical processes. J. Climate, 13:18791898.

    • Search Google Scholar
    • Export Citation
  • Meehl, G. A., H. Teng, and G. W. Branstator, 2006: Future changes of El Nino in two global coupled climate models. Climate Dyn., 26:549566.

    • Search Google Scholar
    • Export Citation
  • Münnich, M., M. A. Cane, and S. E. Zebiak, 1991: A study of self-excited oscillations of the tropical ocean–atmosphere system. Part II: Nonlinear cases. J. Atmos. Sci., 48:12381248.

    • Search Google Scholar
    • Export Citation
  • Neelin, J. D., D. S. Battisti, A. C. Hirst, F-F. Jin, Y. Wakata, T. Yamagata, and S. E. Zebiak, 1998: ENSO theory. J. Geophys. Res., 103:1426114290.

    • Search Google Scholar
    • Export Citation
  • Saenko, O. A., 2006: Influence of global warming on baroclinic Rossby radius in the ocean: A model intercomparison. J. Climate, 19:13541360.

    • Search Google Scholar
    • Export Citation
  • Suarez, M. J. and P. S. Schopf, 1988: A delayed action oscillator for ENSO. J. Atmos. Sci., 45:32833287.

  • Tett, S., 1995: Simulation of El Niño–Southern Oscillation-like variability in a global AOGCM and its response to CO2 increase. J. Climate, 8:14731502.

    • Search Google Scholar
    • Export Citation
  • Timmermann, A., M. Latif, A. Bacher, J. Oberhuber, and E. Roeckner, 1999: Increased El Niño frequency in a climate model forced by future greenhouse warming. Nature, 398:694696.

    • Search Google Scholar
    • Export Citation
  • Tudhope, A. W., Coauthors 2001: Variability in the El Niño–Southern Oscillation through a glacial–interglacial cycle. Science, 291:15111517.

    • Search Google Scholar
    • Export Citation
  • van der Vaart, P. C. F., H. A. Dijkstra, and F-F. Jin, 2000: The Pacific cold tongue and the ENSO mode: A unified theory within the Zebiak–Cane model. J. Atmos. Sci., 57:967988.

    • Search Google Scholar
    • Export Citation
  • van Oldenborgh, G. J., S. Y. Philip, and M. Collins, 2005: El Niño in a changing climate: A multi-model study. Ocean Sci., 1:8195.

  • von Storch, H. and F. W. Zwiers, 1999: Statistical Analysis in Climate Research. Cambridge University Press, 528 pp.

  • Wang, B. and S-I. An, 2001: Why the properties of El Niño changed during the late 1970s. Geophys. Res. Lett., 28:37093712.

  • Yu, B. and G. J. Boer, 2002: The roles of radiation and dynamical processes in the El Niño-like response to global warming. Climate Dyn., 19:539554.

    • Search Google Scholar
    • Export Citation
  • Zelle, H., G. J. van Oldenborgh, G. Burgers, and H. Dijkstra, 2005: El Niño and greenhouse warming: Results from ensemble simulations with the NCAR CCSM. J. Climate, 18:46694683.

    • Search Google Scholar
    • Export Citation
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