Excessive Cold-tongue and Weak ENSO Asymmetry: Are These Two Common Biases in Climate Models Linked?

Abstract:

Among the biases in the tropical Pacific that are common in the climate models, two stand out. One is the excessive cold-tongue in the mean state---the pool of the cold water that is normally in the eastern tropical Pacific extends too far to the west. The other is the underestimate of the asymmetry of El Nino-Southern Oscillation—the fact that El Nino and La Nina are more or less a mirror image of each other in the models while they are not so in the observations. Results from an analysis from CMIP5 models, forced ocean GCM experiments, as well as an analytical model are presented to suggest that these two common biases in our state-of-the-art models are linked. Specifically, an excessive cold-tongue in the mean climatological state makes the ENSO system more stable and thus leads to a more symmetric ENSO, while a more symmetric ENSO in turn results in less nonlinear heating from the ENSO events to the cental equatorial Pacific which in turn contributes to the development of an excessive cold-tongue. The finding underscores that errors in the mean state and ENSO tend to reinforce each other and thus explains why it has been difficult to simulate the tropical Pacific climate. Further comparison with observations suggests that these two biases are the symptoms of a single structural inadequacy in the models: a weak dynamical coupling between the atmosphere and ocean which puts the ENSO system in the models to a different dynamic regime than the obsereved. Measures that may help push the models closer to the observations are suggested.