Vertical Structure and Physical Processes of the Madden-Julian Oscillation: A Model Evaluation Project

Abstract:

The fundamental physics of the generation, maintenance and propagation of the Madden-Julian Oscillation are intensely debated. Many theories focus on instabilities arising from interactions between components of the vertical diabatic heating from convection and the large-scale circulation. Most general circulation models (GCMs) exhibit biases in diabatic-heating profiles against observed or reanalysis products, but those products also exhibit considerable discrepancies with one another.

The Vertical structure and physical processes of the MJO project is a novel model-evaluation project, designed to assess relationships between diabatic processes in GCMs and their representations of the MJO. A key advantage of the project is the acquisition of temperature, moisture and momentum tendencies from each of the model sub-grid physics schemes. The project has three components, designed to take advantage of known links between biases in short-range forecasts and climate simulations: (1) 20-year AMIP-type simulations to assess the representation of the MJO in each GCM; (2) 2-day hindcasts of two YoTC MJO events to investigate the timestep-level behaviour of physical parameterisations; (3) 20-day hindcasts of the same events to identify links between degradations in forecast skill and the representation of diabatic processes.

Analysis of the three component has identified several over-arching conclusions. First, many process-oriented MJO diagnostics derived from past studies using one or several GCMs fail to distinguish between the GCMs in this project that simulate the MJO well and those that do so poorly, whether for initialised hindcasts or 20-year simulations. Second, there is little correlation between GCM hindcast skill for these cases and the fidelity of the MJO representation in climate simulations. Third, all three components have demonstrated that a reliable representation of the moistening profile, particularly lower- and mid-tropospheric moistening during the transition between MJO suppressed and active phases, may be critical to simulating the MJO in these GCMs. The evolution of the moistening profile during the transition phase was found to be a better determinant of MJO fidelity than the evolution of the heating profile.