Different flavors of horizontal moisture advection influence equatorial Rossby wave and MJO propagation over the west Pacific Ocean

One of the central processes to an eastward propagating Madden-Julian Oscillation (MJO) is moistening to the east and drying to the west of the MJO convective center via horizontal moisture advection. Since horizontal moisture advection is often dominated by advection of the background moisture field, the MJO community has placed focus on improving the mean state moisture in models. The second component of horizontal moisture advection, the atmospheric circulation, is just as important for MJO propagation, especially east of the convective center where models typically underestimate the anomalous poleward and easterly flow. The link between the MJO and the mean state moisture has recently shed light on the fact that there are boreal winter seasons when intraseasonal convection over the west Pacific Ocean is dominated by convectively coupled equatorial Rossby (ER) waves.

In this study, we scrutinize the different spatiotemporal scales of the moisture and horizontal flow fields that contribute to horizontal moisture advection and help to control interannual variability of MJO and ER wave convective propagation over the west Pacific Ocean. We quantify one intraseasonal convective propagation speed for each winter season and determine seasons with eastward propagation (MJO), westward propagation (ER waves), and an intermediate group with slow propagation speeds. We find a variety of flavors in horizontal moisture advection for each group, some of which help explain why some seasons have a mean moisture distribution favorable for strong MJO propagation (e.g., some El Niño years) but do not come to fruition because of unfavorable circulation patterns.