Speaker: Scott Powell from from Colorado State University

Title:"Circumnavigation and Convective Onset of Successive Madden-Julian Oscillation Events"

Abstract: The Madden-Julian Oscillation (MJO) is a major component of sub-seasonal variability in tropical precipitation, and it also impacts extreme precipitation events in the subtropics and the mid-latitudes. This talk focuses on convective onset of the MJO over the tropical warm pool and the circumnavigation of the precursor equatorial wave that appears to influence onset of successive MJO events. This talk explores observational datasets collected during the 2011–2012 Dynamics of the MJO (DYNAMO) field campaign to demonstrate how the changing cloud population over the Indian Ocean impacted the dynamics of MJO onset. Specifically, I will show how and over what time scale the cloud population transitioned from suppressed to widespread convectively active conditions. Results confirming that cloud-scale processes were responsible for large-scale moistening of the low- to mid-troposphere will be illustrated. How a low-wavenumber equatorial Kelvin or gravity wave drove the convective moistening will be illustrated using observational datasets, reanalysis, and regional modeling simulations of two observed MJO cases. Finally, I will show evidence that circumnavigating Kelvin/gravity waves prior to successive MJO events in reanalysis are weakly coupled to convection as they propagate through the Western Hemisphere. These conclusions highlight the need of a holistic MJO theory that accommodates both convectively-coupled gravity waves and moisture waves, and the transitions of either into the other.