Can the response to global warming be La Nia-like?

by Tsubasa Kohyama

Institution: University of Washington
Year: 2018
Keywords: Climate Change; Climate Dynamics; ENSO; Ocean-atmosphere interaction; Climate change; Atmospheric sciences; Physical oceanography; Atmospheric sciences
Posted: 02/01/2018
Record ID: 2212824
Full text PDF: http://hdl.handle.net/1773/40833


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 Nia-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 Nio 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 Nia-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 Nios, 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 Nios, 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 Nios dissipate but La Nias remain almost unchanged, causing a La Nia-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 responseAdvisors/Committee Members: Hartmann, Dennis L (advisor).