The usual explanation for variability in the Walker circulation involves dynamical coupling between the atmosphere and ocean via the ‘Bjerknes feedback.’ This project will examine the extent to which dynamical coupling with the ocean influences decadal variability and anthropogenically forced changes in the Walker circulation. In particular, we will focus on an ‘Atmosphere Walker Mode’ (AWM) which is present in climate models, including those without dynamical coupling to the ocean. We will test the following hypotheses:

1. Hypothesis I: The AWM is a stochastically forced, dominant mode of internal atmospheric variability
2. Hypothesis II: The influence of ocean dynamics on the AWM is timescale dependent
3. Hypothesis III: The AWM is ‘excited’ in response to anthropogenic forcing.

These hypotheses will be tested with a number of tools: We will perform a multimodel analysis of CMIP climate models with varying degrees of coupling between the ocean and atmosphere. We will perform idealized atmosphere (CCSM4) and ocean GCM simulations with novel experimental design. We will also use model results to interpret 20th century observations with particular emphasis on identifying the signatures of mechanisms arising from internal, decadal-timescale fluctuations in the Walker circulation vs. those arising from anthropogenically forced changes. The findings will fill critical gaps in our understanding of the:

1. Mechanisms of decadal variability
2. Ability to predict decadal climate variations
3. Interaction between internal modes of variability and the climate response to anthropogenic forcing.

This work will be performed in collaboration with NCAR (C. Deser), and will contribute to the activities of the Climate Variability working group.