Increased El Nin o frequency in a climate model forced by future greenhouse warming
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eastern Pacific. This `dynamical thermostat' will lead to overall conditions resembling
those observed during La Nin as, eventually retarding global warming.
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9. Fraser, P. et al. Lifetime and emission estimates of 1,1,2-trichlorotri¯uorethane (CFC-113) from daily global background observations June 1982±June 1994. J. Geophys. Res. 101, 12585±12599 (1996). 10. Simmonds, P. G. et al. Global trends and emission estimates of CCl4 from in situ background observations from July 1978 to June 1996. J. Geophys. Res. 103, 16017±16028 (1998). 11. Prinn, R. G. et al. Atmospheric trends and lifetime of CH3CCl3 and global OH concentrations. Science 269, 187±192 (1995). 12. Butler, J. H. et al. Growth and distribution of halons in the atmosphere. J. Geophys. Res. 103, 1503± 1511 (1998). 13. Prather, M. J. & Watson, R. T. Stratospheric ozone depletion and future levels of atmospheric chlorine and bromine. Nature 344, 729±734 (1990). 14. Daniel, J. S., Solomon, S. & Albritton, D. L. On the evaluation of halocarbon radiative forcing and global warming potentials. J. Geophys. Res. 100, 1271±1285 (1995). 15. Jackman, C. H. et al. Past, present, and future modeled ozone trends with comparisons to observed trends. J. Geophys. Res. 101, 28753±28767 (1996). 16. Portmann, R. W. et al. Role of aerosol variations in anthropogenic ozone depletion in the polar regions. J. Geophys. Res. 101, 22991±23006 (1996). 17. Solomon, S. et al. The role of aerosol variations in anthropogenic ozone depletion at northern midlatitudes. J. Geophys. Res. 101, 6713±6727 (1996). 18. Shindell, D. T., Rind, D. & Lonergan, P. Increased polar stratospheric ozone loss and delayed eventual recovery owing to increasing greenhouse-gas concentrations. Nature 392, 589±592 (1998). 19. McCulloch, A. Global production and emissions of bromochlorodi¯uoromethane and bromotri¯uoromethane (halons 1211 and 1301). Atmos. Environ. A 26, 1325±1329 (1992). 20. UNEP Technology and Economic Assessment Panel April 1998 Report (United Nations Environment Programme, Nairobi, Kenya, 1998); also available at hhttp://www.teap.org/html/teap_reports.htmli. 21. Prather, M. J. et al. in Climate Change 1995, the Science of Climate Change (eds Houghton, J. T. et al.) Ch. 33 (Cambridge Univ. Press, 1996). 22. Report of the Halon Fire Extinguishing Agents Technical Options Committee (United Nations Environment Programme, Nairobi, Kenya, 1994). 23. Volk, C. M. et al. Evaluation of source gas lifetimes from stratospheric observations. J. Geophys. Res. 102, 25543±25564 (1997). 24. Burkholder, J. B. et al. Atmospheric fate of CF3Br, CF2Br2, CF2ClBr, and CF2BrCF2Br. J. Geophys. Res. 96, 5025±5043 (1991). 25. Butler, J. H. et al. A decrease in the growth rates of atmospheric halon concentrations. Nature 359, 403±405 (1992). 26. Montzka, S. A. et al. Global tropospheric distribution and calibration scale of HCFC-22. Geophys. Res. Lett. 20, 703±706 (1993). 27. Montzka, S. A. et al. Early trends in the global tropospheric abundance of hydrochloro¯uorocarbon141b and -142b. Geophys. Res. Lett. 21, 2483±2486 (1994). 28. Trenberth, K. E. & Guillemot, C. J. The total mass of the atmosphere. J. Geophys. Res. 99, 23079±23088 (1994). 29. Fraser, P. J. et al. Southern hemispheric halon trends (1978±1998) and global halon emissions. J. Geophys. Res. (in the press). Acknowledgements. We thank all personnel involved in collecting ¯ask samples at the NOAA/CMDL Observatories and at cooperative sampling sites. S.A.M. appreciates discussions and suggestions from S. Solomon and P. Midgley, and we appreciate the past technical assistance of R. C. Myers and T. H. Swanson. This work was supported in part by NOAA's Radiatively Important Trace Species research and by the Atmospheric Chemistry project of the NOAA Climate and Global Change Program. Correspondence and requests for materials should be addressed to S.A.M. (e-mail: smontzka@ cmdl.noaa.gov). Data used in this Letter are available at hhttp://www.cmdl.noaa.gov/ftpdata.htmli or hftp://ftp.cmdl.noaa.gov/noahi.
Ä Increased El Nino frequency in a climate model forced by future greenhouse warming
A. Timmermann, J. Oberhuber*, A. Bacher, M. Esch, M. Latif & E. Roeckner
È Max-Planck-Institut fur Meteorologie and * Deutsches Klimarechenzentrum, Bundesstrasse 55, D-20146 Hamburg, Germany
model with suf®cient resolution in the tropics to adequately represent the narrow equatorial upwelling and low-frequency waves. When the model is forced by a realistic future scenario of increasing greenhouse-gas concentrations, more frequent ElÄ Nino-like conditions and stronger cold events in the tropical Paci®c Ocean result. Our global climate model8,9 uses a meridional resolution of 0.58 in the tropics. The model, which is `¯ux corrected', simulates an irregular ENSO cycle similar to the observed one, and the amplitude and dynamics of the simulated ENSO cycle are consistent with those derived from observations8,9. The simulated ENSO period is too short, however, and amounts to about two years, whereas observations indicate a main period of about four years. Our model successfully predicted the onset and decline of the 1997/1998 El Ä Nino several months in advance10. Here, two experiments were performed. The ®rst experiment is a 300-year-long control integration with ®xed present-day atmospheric concentrations of greenhouse gases. The second experiment is a transient greenhouse warming simulation in which the model was forced by increasing levels of greenhouse gases as observed (1860±1990) and according to IPCC scenario IS92a11 (1990±2100). The changes in the mean state at the surface of the tropical Paci®c Ocean, as derived from the transient greenhouse warming simulation, are reminiscent of the anomalous climate state observed Ä during present-day El Nino conditions (Fig. 1). The sea-surfacetemperature (SST) trend pattern is characterized by strongest warming in the equatorial east Paci®c, accompanied by westerly near-surface wind anomalies in the equatorial region to the west of the maximum warming and strong equatorward ¯ow off the Equator (not shown). The associated trend in rainfall is rather Ä similar to that simulated during present-day El Ninos (not shown). There has been some discussion about the relative roles of different feedbacks involved in the time-mean response of the tropical Paci®c climate system to greenhouse warming. On the one hand, it has been suggested that regional differences in the cloud-albedo feedback will lead to surface warming that is strongest in the equatorial east Paci®c12. The argument is that the equatorial west Paci®c is so warm that even modest additional warming would lead to a cloud shielding effect, with high cirrus clouds, reducing incoming solar radiation at the surface and inhibiting further warming13. This `thermostat' would be less ef®cient in the eastern equatorial Paci®c, so it would warm more than the western Paci®c. This would lead to a slackening of the winds along the Equator and
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Ä The El Nino/Southern Oscillation (ENSO) phenomenon is the strongest natural interannual climate ¯uctuation1. ENSO originates in the tropical Paci®c Ocean and has large effects on the ecology of the region, but it also in¯uences the entire global climate system and affects the societies and economies of many countries2. ENSO can be understood as an irregular lowÄ frequency oscillation between a warm (El Nino) and a cold (La Ä Ä Nina) state. The strong El Ninos of 1982/1983 and 1997/1998, Ä along with the more frequent occurrences of El Ninos during the past few decades, raise the question of whether human-induced `greenhouse' warming affects, or will affect, ENSO3. Several global climate models have been applied to transient greenhouse-gasinduced warming simulations to address this question4±6, but the results have been debated owing to the inability of the models to fully simulate ENSO (because of their coarse equatorial resolution)7. Here we present results from a global climate
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Figure 1 Simulated temperature trends in equatorial waters. The linear trends in the temperatures of the upper 210 m of the Paci®c Ocean at the Equator (8C rise per 100 years) are derived from the full (240-year-long) transient greenhouse warming simulation. There is a warming trend near the surface and a cooling trend at deeper levels, leading to a stronger thermocline. The trends at the surface resemble the anomalous conditions observed during present-day El Ninos. Ä NATURE | VOL 398 | 22 APRIL 1999 | www.nature.com
© 1999 Macmillan Magazines Ltd
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