How Important Are Horizontal Eddy Moisture Transports for the MJO's Eastward Propagation?

The eastward propagation of the Madden-Julian oscillation (MJO) is known to hinge crucially on the effects of horizontal moisture advection, which involve two main types of circulation anomalies. The first are those of the MJO itself, while the second are those of embedded Rossby-type eddies, which tend to be most active to the west of the MJO's convective center. To quantify the relative importance of the eddies, a novel approach is taken in which their formal definition is given by the residual of a least-squares fit to an observed bivariate MJO index. Results show that the eddies, when defined in this way, are generally of leading importance for fostering the MJO's eastward propagation in terms of column-integrated moisture. The picture is seen to be reversed, however, when using a traditional filter-based method to define the eddies, which are then strictly high-frequency in nature.

Automated summary

The eastward propagation of the MJO depends on the effects of horizontal moisture advection, with the importance of two types of circulation anomalies, namely those of the MJO and those of embedded Rossby-type eddies. A novel approach quantifies the relative importance of the eddies by defining them as the residual of a least-squares fit to an observed bivariate MJO index. The results show that the eddies play a leading role in promoting the MJO's eastward propagation in terms of moisture.