Mechanisms of Low-Frequency Variability of the Atmospheric Circulation Over the 20th Century

All climate models predict a weakening of the tropical atmospheric circulation in response to anthropogenic increases in greenhouse gases. Analysis of climate model simulations from the CMIP4 archive indicate that the atmospheric circulation may weaken by as much as 25% by the end of the century (Vecchi and Soden, 2006). However, observed variability of the Walker circulation over the past few decades appears dominated by unforced internal variability (Burgman et al. 2008). This decadal variability in the atmospheric circulation also appears to be amplified by the response of low-level clouds (Clement et al. 2009), and hence these clouds may be an important component of decadal variability. Circulation changes can have significant impacts on cloud feedbacks in response to anthropogenic warming, particularly marine stratocumulus clouds, which have been identified as one of the main sources of uncertainty in global warming projections. While our prior work has identified patterns of decadal variability in the tropical circulation, the causes of these changes and their implications for climate are still not known. For example, what are the relative contributions of internally-generated variability and external forcing? What role does the ocean play in generating decadal atmospheric variability? What role do low clouds play on decadal and longer-timescales? We propose to address these questions with a modeling and diagnostic study that is focused on three separate tasks:

• Task I: Determine what aspects of low-frequency changes in the atmospheric circulation over recent decades can be attributed to unforced internal variability, or to external (anthropogenic and natural) climate forcings using both observations and CMIP5 climate model simulations under different forcing scenarios.

• Task II: Investigate the mechanisms of variability in the tropical atmospheric circulation using idealized climate model simulations

• Task III: Evaluate the impacts of atmospheric circulation changes on cloud feedback from marine stratocumulus clouds.

The proposed work will address the FY 2010 priority of ‘understanding the causes of climate variability over the observational record,’ and we will ‘attempt to quantify the roles of radiative forcing and natural climate variability for explaining the observed climate record.’