Can the response to global warming be La Niña-like?
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The majority of the models that participated in the Coupled Model Intercomparison Project Phase 5 global warming experiments warm faster in the eastern equatorial Pacific Ocean than in the west. GFDL-ESM2M is an exception among the state-of-the-art global climate models in that the equatorial Pacific sea surface temperature (SST) in the west warms faster than in the east, and the Walker circulation strengthens in response to warming. This dissertation shows that this ``La Niña-like" response simulated by GFDL-ESM2M could be a physically consistent response to warming, and that the forced response may be detectable during this century. To highlight the uniqueness of GFDL-ESM2M, two other models are also examined: GFDL-ESM2G, which differs from GFDL-ESM2M only in the oceanic components, warms without a clear change in the zonal SST gradient in the tropical Pacific; HadGEM2-CC exhibits a warming pattern that resembles the multi-model mean, with more warming in the eastern than western Pacific. A fundamental observed constraint between the amplitude of the El Niño Southern Oscillation (ENSO) and the mean-state zonal SST gradient is reproduced well by GFDL-ESM2M, but not by the other two models, which display substantially weaker ENSO nonlinearity than is observed. Under this constraint, the weakening nonlinear ENSO amplitude in GFDL-ESM2M rectifies the mean state to be La Niña-like. GFDL-ESM2M exhibits more realistic equatorial thermal stratification than GFDL-ESM2G, which appears to be the most important difference for the ENSO nonlinearity and the warming response. On longer time scales, the weaker polar amplification in GFDL-ESM2M may also explain the origin of the colder equatorial upwelling water, which could in turn weaken the ENSO amplitude. Using an idealized model, we further explore the cause of this exceptional response and propose a new mechanism, the Nonlinear ENSO Warming Suppression (NEWS), where the transient heating rate difference between the atmospheric and oceanic reservoirs annihilates extreme El Niños, causing a suppression of mean-state warming in the east. Heat budget analyses of GFDL-ESM2M robustly show that nonlinear dynamical heating, which is necessary for extremely warm El Niños, becomes negligible under warming. An idealized nonlinear recharge oscillator model suggests that, if the temperature difference between the atmospheric and oceanic reservoirs becomes larger than some threshold value, the upwelling becomes too efficient for ENSO to retain its nonlinearity. Therefore, extreme El Niños dissipate but La Niñas remain almost unchanged, causing a La Niña-like mean-state warming. NEWS is consistent with observations and GFDL-ESM2M but not with the majority of state-of-the-art models, which lack realistic ENSO nonlinearity. NEWS and its opposite response to atmospheric cooling, the Nonlinear ENSO Cooling Suppression (NECS), might contribute to the Pacific multi-decadal natural variability and global warming hiatuses. Then, to explore necessary conditions of NEWS, the ENSO amplitude response to global warming is examined in two global climate models with realistic nonlinearity of the El Niño Southern Oscillation (ENSO). GFDL-ESM2M and MIROC5 are the two models that exhibit realistic ENSO nonlinearity. With quadrupled atmospheric carbon dioxide, the ENSO amplitude of GFDL-ESM2M decreases by about 40%, whereas that of MIROC5 remains almost constant. Because GFDL-ESM2M exhibits stronger climatological thermal stratification than MIROC5, greenhouse gas forcing increases the upper ocean stability and causes the thermocline to be less sensitive to wind perturbations. The stiffer thermocline inhibits the large, nonlinear variations of subsurface temperature anomalies so that the ENSO amplitude substantially weakens. Idealized nonlinear recharge oscillator model experiments further support climatological thermal stratification as a determinant of the warming response. Observations exhibit stronger thermal stratification than both models, so the real world may terminate strong, nonlinear El Niños sooner than model-based projections. Based on the NEWS mechanism, this physical explanation for the termination of extreme El Niños supports the notion that the response to global warming could be La Niña-like.
- Atmospheric sciences