Effects of Atmosphere-Ocean Coupling on MJO Eastward Propagation: A Missing Piece of the Puzzle

The Madden-Julian Oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics. Large-scale convection fueling the MJO is initiated over the tropical Indian Ocean (IO) and propagates eastward across the Maritime Continent (MC) and into the western Pacific as a pattern of alternating phases of active and suppressed convection. The MJO’s convective initiation and eastward propagation over the tropical IO are not well understood, and are poorly represented in numerical weather prediction and climate models. Previous studies have shown that air-sea interaction is a contributing factor in improving MJO predictability and producing successful MJO simulations.

This study examines the effects of atmosphere-ocean coupling and increased model resolution on the eastward propagation of MJO using a fully coupled atmosphere-ocean model (UWIN-CM) developed at University of Miami. The atmosphere component of UWIN-CM is the Weather Research and Forecasting (WRF) model v.3.6.1 with nested grids of 36-, 12-, 4-, and 1.3-km. Convection is resolved explicitly in the 4- and 1.3-km domains. The ocean component is the Hybrid Coordinate Ocean Model (HYCOM) with a uniform resolution of 0.08°. Three model configurations are compared: an uncoupled, atmosphere only simulation with constant SST and without the 1.3 km domain; an atmosphere-ocean coupled simulation without the 1.3 km domain; and an atmosphere-ocean coupled simulation using all four domains. The event simulated is the second MJO that occurred during DYNAMO (November-December 2011), and the simulations are compared to satellite and in-situ observations available.

Results indicate that MJO-induced upper ocean cooling is essential in helping to suppress the precipitation over IO, allowing the MJO to propagate eastward and into the MC. In the uncoupled atmosphere model simulation, the large-scale convection persists over IO, which produces a stationary MJO. Increasing model resolution improves the representation of mesoscale convective systems and westerly winds and induces stronger ocean cooling that is more comparable to observations. This helps suppress convection over IO and is favorable for eastward propagation of the MJO. A comprehensive analysis of the air-sea interactive processes using the coupled model and the DYNAMO atmosphere and upper ocean data is under way and will be presented at the conference.