Because of the climatic impact ENSO has on Northern America and the globe assessing and understanding future changes of ENSO in response to global warming is essential to addressing issues relating to the interaction of climate change and natural climate variability. The objective of the proposed research is to identify and assess systematic changes ENSO undergoes in response to greenhouse warming. We propose to undertake a critical examination of simulations previously conducted with a number of coupled GCMs and CMIP5 experiments, focusing on the following questions: as anthropogenic greenhouse warming progresses:

1. How do the changes in the climate background state and in the ocean-atmosphere feedbacks alter ENSO characteristics, such as pattern, amplitude, and period?
2. Are these changes gradual or abrupt?
3. Are current state-of-the-art climate models capable of capturing the different “flavors” of ENSO, as well as their transitions?

We will also conduct a 2-parameter space exploration of the ENSO regimes using state-of-art climate system models. This exploration will enable us to identify the existence of co-dimension-2 ENSO bifurcations and associated transitions in the qualitative behavior of ENSO. Two key parameters: greenhouse gas forcing and an important sensitivity parameter for ENSO in the cumulus convection scheme, will be systematically changed. We will then assess the nature of ENSO regime transition in this parameter space captured by different models, and the likelihood of future ENSO regime changes using these coupled models. The proposed research falls in line with the activities outlined by the program announcement: activities that focus on the identification, evaluation, and understanding of low frequency modes, (e.g., ENSO) and how these will change in a changing climate. The project will ultimately support the BER climate science activity’s Long-Term Measure (LTM) by improving assessment of the potential response of the Earth's climate to increased greenhouse gas levels.