Properties of the El Niño-Southern Oscillation (ENSO) undergo decadal variations, but the causes of these variations are not yet understood. This study explores a self-modulation mechanism that suggests extreme ENSO events should occur on decadal timescales. The mechanism depends on a two-way interaction between ENSO activity and the mean state of the tropical Pacific, in which the spatial asymmetry between El Niño and La Niña is responsible for transitions between weak and strong ENSO states. Analyses of observational data and numerical experiments using coupled general circulation models (CGCM), an intermediate coupled model, and an ocean only model, will be carried out to examine this mechanism and to understand decadal ENSO variability and its interaction with the tropical Pacific climate.
Investigations will focus on the following issues: (1) How do ENSO and Pacific climate interact under various types of ENSO dynamics? (2) What is the cause of the spatial asymmetry between El Niño and La Niña and why does it reverse between a strong ENSO state and a weak ENSO state? (3) What roles do the Indian Ocean and monsoons play in the decadal ENSO variability? Does the hypothesized ENSO-Pacific climate interaction mechanism need to be modified to include the effects of the Indian Ocean and monsoons? (4) Are there any linkages between decadal ENSO variability, the proposed mechanism, and the 1976/77 rapid climate shift?
This study will make use of the unprecedented collection of high-quality CGCM integrations that were produced for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. The variety of ENSO behaviors simulated by these models implies that they have different ENSO dynamics and so comprise a test bed for examining ENSO-Pacific climate interaction under different ENSO dynamics and different greenhouse gas forcings. Numerical experiments that limit ocean-atmosphere coupling to individual ocean basins or constrain the Pacific basic state will be performed with the two CGCMs and an intermediate coupled model. Model results will be verified using observational data and simulations with an ocean model.
Broader impacts of this study include the potential to improve long-term predictions of extreme ENSO events and to improve the simulation of ENSO in CGCMs. Ph.D. students will be trained, and undergraduate Earth System Science majors will be involved in the research.
