Role of the Mean State Moisture Gradient on the Propagation of the MJO Across the Maritime Continent
The Madden-Julian Oscillation (MJO) is the dominant mode of sub-seasonal variability in the tropics. Unfortunately, an accurate representation of the MJO has historically been a challenging task for many, if not most, global climate models, especially in and around the Maritime Continent (MC) region. The mean state distribution of atmospheric moisture has been highlighted as a key aspect affecting the simulation of MJO propagation in many recent modeling studies. When many different models are compared, however, it was difficult to isolate the role of the mean state in the multi-model studies because different models use different parameterizations of moist physics that affect both the mean state and the MJO directly. In this study, we examine the relationship between the mean state and MJO propagation in an ensemble of simulations made with a single coupled model. Each ensemble member differs only in its initial conditions and thus the parameterizations and resolution are identical. We found that MJO propagation over the MC in the model is strongly affected by the background meridional moisture gradient to the north and south of the equator, with MJO propagation being enhanced in the periods with a steeper meridional moisture gradient.
Our results demonstrate that a steeper mean meridional moisture gradient in the Maritime Continent region could lead to a stronger MJO in the MC region via enhancing moisture recharging ahead of MJO convection.
This study examines the role of the mean state on the propagation of the MJO over the MC. We use an ensemble of simulations made with a single model - the Community Earth System Model version 2– to assess the effect of the mean state that is unaffected by that of model components such as parameterization schemes. Results show that the background meridional moisture gradient is much steeper over the MC region in the periods with a stronger MJO propagation. Column water vapor budget of the MJO strongly suggests that the simulated mean state affects MJO via its impacts on moisture dynamics - a greater advection of mean moisture by MJO wind in the MC region is responsible for the anomalous MJO activity.