Recent analyses of global warming projections simulated with global climate models (GCMs) suggest that the tropical Pacific does not become El Niño- or La Niña-like in response to increased greenhouse gases (GHGs). Rather, the physical mechanisms that drive tropical Pacific climate change depart substantially from the El Niño-Southern Oscillation (ENSO) analogy often invoked for interpreting future climate changes [e.g., Knutson and Manabe, 1995; Meehl and Washington, 1996; Cane et al., 1997; Collins et al., 2005; Meehl et al., 2007; Lu et al., 2008; Cox et al., 2004] and past climate changes [e.g. Lea et al., 2001; Koutavas et al., 2002]. This presents an opportunity for reconciling theory, models, and observations.
An ENSO analogy typically is invoked for interpreting tropical Pacific climate change because if an external forcing introduces some east-west asymmetry, this asymmetry can be amplified in the same way as interannual perturbations are, through the positive ocean-atmosphere Bjerknes feedback. This then would lead to an altered mean state of the tropical Pacific resembling El Niño or La Niña [Dijkstra and Neelin, 1995]. For instance, the model projections used for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) anticipate tropical Pacific climate change in response to increased GHGs that has been described as El Niño—like [Meehl et al., 2007]. These models project robust enhanced equatorial warming [Liu et al., 2006; DiNezio et al., 2009] and a weakening of the overturning atmosphere circulation across the tropical Pacific, i.e., the Walker circulation [Vecchi and Soden, 2007], both of which occur during El Niño events. However, these experiments also show a shoaling and sharpening of the equatorial thermocline [Vecchi and Soden, 2007; DiNezio et al., 2009] (Figure 1a). This is in contrast to El Niño events, when the thermocline response is heavily dominated by a relaxed tilt (Figure 1b).