Role of Horizontal Advection of Seasonal-Mean Moisture in the Madden–Julian Oscillation: A Theoretical Model Analysis

Abstract

Abstract The impact of horizontal advection of seasonal-mean moisture (SMM) on Madden–Julian oscillation (MJO) dynamics is investigated here using a theoretical model that includes moisture advection processes. The zonal advection of SMM with an eastward gradient is found to produce planetary-scale instability and promote slow eastward propagation corresponding to an intraseasonal periodicity. This is because the SMM advection by an anomalous easterly of the Kelvin waves generates a moisture source to the east of precipitation, which favors eastward propagation and unstable growth. On the other hand, the advection of SMM with a westward gradient results in a westward-propagating unstable mode. For a realistic SMM distribution, the simulated eastward propagation is enhanced over the Indo-Pacific warm pool, while the westward propagation prevails over the central-eastern Pacific. In contrast to the zonal advection of SMM, the meridional advection of SMM only affects short waves and leaves planetary waves nearly unaffected. The effect of zonal advection of SMM suggests an important mechanism for explaining the eastward propagation and growth of the MJO over the Indo-Pacific warm pool when the SMM increases eastward. However, this mechanism alone produces unrealistic Kelvin wave–like structure and strong westward propagation in the central-eastern Pacific; both disagree with observations. These caveats, however, can be remitted if the planetary boundary layer (PBL) moisture convergence feedback is included, which couples the Kelvin wave and the Rossby wave via precipitation heating, producing a realistic horizontal structure and also substantially suppressing the unrealistically growing, westward-propagating mode in the central-eastern Pacific.