How Does Global Warming Impact the El Niño-Southern Oscillation?
Collins, M., An, S.-I., Cai, W., Ganachaud, A., Guilyardi, E., Jin, F.-F., Jochum, M., Lengaigne, M., Power, S., Timmermann, A., Vecchi, G. and Wittenberg, A. 2010. The impact of global warming on the tropical Pacific Ocean and El Niño. Nature Geoscience 3: 391-397.
We find an answer to this burning question in a paper recently published in Nature Geoscience by a team of twelve researchers hailing from six different countries (Australia, France, India, South Korea, the United Kingdom and the United States), wherein they review the findings of what they describe as "a hierarchy of mathematical models [that] have been used to explain the dynamics, energetics, linear stability and nonlinearity of ENSO," while noting that "complex coupled global circulation models have become powerful tools for examining ENSO dynamics and the interactions between global warming and ENSO." So what have those powerful tools revealed to us?
Among other things, Collins et al. write that "the tropical easterly trade winds are expected to weaken; surface ocean temperatures are expected to warm fastest near the equator and more slowly farther away; the equatorial thermocline that marks the transition between the wind-mixed upper ocean and deeper layers is expected to shoal; and the temperature gradients across the thermocline are expected to become steeper [italics added]." However, they state that "it is not yet possible to say whether ENSO activity will be enhanced or damped, or if the frequency of events will change." Or, we might add, if their several expectations will ever come to pass.
Interestingly, students of the subject a decade ago (including two of the authors of the Collins et al. paper) seemed to actually "know" much more than those of today do; and they were likewise much more sure of themselves. Timmermann et al. (1999), for example, developed a global climate model that indicated, in their words, that when "forced by a realistic future scenario of increasing greenhouse-gas concentrations, more frequent El-Niño-like conditions and stronger cold events in the tropical Pacific Ocean result," a scenario that was also suggested by Collins (2000a,b) and Cubasch et al. (2001). As more has been learned, however, and as models have been improved and more paleoclimate data have been acquired, it has become clear that we currently "know" much less about the global warming/ENSO connection than we thought we did a decade ago. And perhaps a decade from now we will discover we actually know much less than we believe we know now, although we obviously hope for the opposite. For the present, however, we are left with the conclusion of Collins et al., i.e., that "it is not clear at this stage which way ENSO variability will tip ... As far as we know, it could intensify, weaken, or even undergo little change depending on the balance of changes in the underlying processes."
Collins, M. 2000a. Understanding uncertainties in the response of ENSO to greenhouse warming. Geophysical Research Letters 27: 3509-3513.
Collins, M. 2000b. The El Niño Southern Oscillation in the second Hadley center coupled model and its response to greenhouse warming. Journal of Climate 13: 1299-1312.
Cubasch, U., Meehl, G.A., Boer, G.J., Stouffer, R.J., Dix, M., Noda, A., Senior, C.A., Raper, S. and Yap, K.S. 2001. Projections of future climate change. In: Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P., Dai, X., Maskell, K. and Johnson, C.I. (Eds.). Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the 3rd Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK, pp. 525-582.
Timmermann, A., Oberhuber, J., Bacher, A., Esch, M., Latif, M. and Roeckner, E. 1999. Increased El Niño frequency in a climate model forced by future greenhouse warming. Nature 398: 694-696.