SLS - Margaret Duffy
An energetic evaluation of the response of the Walker Circulation to warming
I will give an overview of my thesis work with an emphasis on the response of the Walker Circulation to warming, described below.
Theory suggests a weakening of the Walker circulation with warming, consistent with an overall weakening of the tropical divergent circulation. Trends in observations are mixed, with longer - term observational evide nce indicating a weakening and shorter - term observational evidence indicating a strengthening. Model projections of the future tend to simulate a weakening of the Walker Circulation with warming, but with substantial spread in the degree of weakening. Here we diagnose this spread in CMIP and AMIP model projections using a moist static energy (MSE) budget. In each model, we evaluate the contribution to changes in the Walker Circulation strength due to changes in gross moist stability (GMS), horizontal MSE ad vection, radiation, and surface fluxes. We find that the spread in the Walker Circulation response is substantial across both the CMIP and AMIP models, implying that differences in atmospheric models are important for the spread in projected Walker Circula tion strength. Further, we find that changes in GMS are strongly anticorrelated with changes in Walker Circulation strength. To evaluate the role of differing lapse rates in the spread in GMS response, we vary the entrainment rate in an idealized GCM. The idealized GCM is run with a simplified Betts - Miller convection scheme, modified to represent entrainment. The idealized GCM simulates a weakening of the Walker Circulation with warming, and this weakening is dampened by increasing entrainment. However, the spread due to differing entrainment rates does not fully account for the spread across AMIP and CMIP models.