Interactions between the Madden Julian Oscillation and High-Frequency Waves

Abstract

The Madden Julian Oscillation (MJO), characterized by the pattern of a large-scale convective envelope, a planetary-scale circulation and a slow eastward phase speed of 5 m/s, is the dominant intraseasonal mode in the tropics. Although numerous theories have been proposed to explain its eastward propagation, the physical mechanisms behind the phenomenon remain open. Many state-of-the-art climate models have difficulty in capturing the realistic MJO eastward propagation, which suggests that some critical process involved is yet unrevealed. The observed multi-scale structure of the MJO convective envelope implies that the interactions between MJO and high-frequency waves (HFW) might play an important role in promoting the eastward propagation. Both the diagnosis of observational and model data and idealized model simulations are utilized to address the MJO-HFW interaction problem. Several major findings are discussed in this paper. (1) Observed HFW activity is enhanced over and to the west of the MJO convective center and weakened to the east of the MJO convective center, due to the impact of MJO-scale vertical zonal wind shear and specific humidity. (2) The weakened HFW activity to the east of the MJO convective center moistens the lower troposphere through reduced dry air mixing from extratropics, which promotes the eastward propagation of MJO convection through the development of the shallow and congestus clouds preceding the MJO convection. (3) The increased (decreased) HFW activity west (east) of the MJO convective center contributes to a positive (negative) eddy zonal momentum flux, which favors the eastward propagation of MJO flows by inducing a positive intraseasonal zonal wind tendency. (4) The aforementioned observed phase relationship between the MJO and HFW is only found in a “good” model group, which is defined as models that capture the observed eastward propagation. (5) Unrealistic distribution of HFW activity in the “poor” model group leads to unrealistic nonlinearly rectified condensational heating and eddy momentum flux divergence distributions, which promote a westward-moving tendency. (6) Whereas mixed Rossby-gravity (MRG) waves are enhanced (weakened) to the west (east) of the MJO convection due to the modulation of MJO-scale vertical wind shear, inertio-gravity waves are strengthened only over the MJO convection region due to their strong dependence on MJO-scale specific humidity. Kelvin waves, on the other hand, are modulated by both the vertical wind shear and specific humidity. It is anticipated that these findings may shed light on improving our current understanding of MJO and the representation of realistic MJO structure and eastward propagation in climate models.