Factors that Affect The Amplitude of El Niño in Global Coupled
Climate Models
Gerald A. Meehl, Peter R. Gent, Julie M. Arblaster, Bette L.
Otto-Bliesner, Esther C. Brady, and Anthony Craig
National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307
Historically, El Niño-like events simulated in global coupled climate
models have had reduced amplitude compared to observations. Here, El
Niño-like phenomena are compared in ten experiments using two recent global
coupled models. These models have various combinations of horizontal and
vertical ocean resolution, ocean physics, and atmospheric model resolution. It
is demonstrated that the lower the value of the ocean background vertical
diffusivity, the greater the amplitude of El Niño variability and sharper
the equatorial thermocline. Among models with low background vertical
diffusivity, stronger equatorial zonal wind stress produces relatively higher
amplitude El Niño variability along with more realistic east-west sea
surface temperature (SST) gradient along the equator, with ocean model
resolution a secondary factor. The SST seasonal cycle in the eastern tropical
Pacific has too much of a semiannual component with a double Intertropical
Convergence Zone (ITCZ) in all experiments, and thus does not affect, nor is it
affected by, the amplitude of El Niño variability. Systematic errors
affecting the spatial variability of El Niño in the experiments are
characterized by the eastern equatorial Pacific cold tongue regime extending too
far westward into the warm pool. However, the cold tongue regime in the
experiments with weaker equatorial zonal wind stress does not extend as far west
as in the experiments with stronger wind stress. the timescales of El Niño
variability show peaks in the 3-4 year band in the model experiments, which is
near the short end of the observed range of about 3-7 years.
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Hongjun Zhang:
zhangho@ucar.edu