The Role of Atmosphere-Ocean Coupling in Eastward Propagation of the MJO Convection

The convective initiation and eastward propagation of the Madden-Julian Oscillation (MJO) over the tropical Indian Ocean and Maritime Continent are not well understood, and are poorly represented by the numerical weather prediction (NWP) and climate models. Previous studies have shown that air-sea interaction is a contributing factor in producing successful MJO simulations and improving its predictability.

In this study, the role of atmosphere-ocean coupling is investigated through a set of three model experiments – an uncoupled (atmosphere only), a coupled atmosphere-ocean without ocean tides, and a fully coupled atmosphere-ocean simulations. All experiments are carried out using the Unified Wave Interface – Coupled Model (UWIN-CM). The atmosphere component of UWIN-CM is the Weather Research and Forecasting (WRF) model, with nested grids of 36-, 12-, and 4-km horizontal resolutions and with convection being resolved explicitly in the innermost domain. The ocean component is the Hybrid Coordinate Ocean Model (HYCOM) with a uniform 0.08° resolution.

A well-observed MJO event during the DYNAMO (Dynamics of the Madden-Julian Oscillation) field campaign (MJO2, November – December 2011) is used in this study. The DYNAMO observations of this MJO event are used to evaluate the model simulations. Although all experiments to some extent capture the eastward propagation of the leading edge of the MJO convective envelope and associated surface westerly winds, the coupled atmosphere-ocean simulations have the best overall eastward propagation of the MJO convection, whereas the uncoupled atmospheric model produces excessive rainfall, especially behind (west of) the leading edge of the MJO convection. The SST and upper ocean cooling in the coupled model appear to play a key role in improving the eastward propagation of the MJO by reducing unrealistic convection west of the leading edge that is present in the uncoupled model. A comprehensive analysis of the three experiments is underway and will be presented at the conference.