A Moisture Mode Analysis of Counterintuitive MJO Events

The MJO is often considered a “moisture mode” where the MJO is driven by moisture anomalies. The importance of this is observed in previous work where continuing (i.e., propagating) events have a higher amplitude of moist entropy (ME) east of the convective center than terminating (i.e., non-propagating) events. Our previous work finds some events will propagate when they are otherwise expected to terminate (e.g., an event continues despite unfavorable conditions in the upstream environment) given the distributions of ME tendencies for propagating and non-propagating MJOs. This research presents a further analysis of these cases, which intends to reveal the presence of other processes at work or predictors for these apparent outlier events.

A climatology (1979-2021) of continuing and terminating MJO events over the Indian Ocean is created from the RMM index with an event identification algorithm used in previous work. These events are catalogued by the ME tendencies to the east of the convective envelope determined from ERA-5 data. Box and whisker plots of ME and budget terms are created for continuing and terminating events to document the variation among individual events and the climatological mean state of ME.

We define counterintuitive continuing events as those MJOs with unfavorable conditions that are in the lower quartile of total ME tendency and the upper quartile for counterintuitive terminating events. Based on the previous sorting, we analyze the counterintuitive events in a budget analysis of ME using a Lagrangian window following the convective envelope of the MJO. Results show similarities in mean horizontal ME advection and latent heat flux east of the MJO convective center for counterintuitive continuing events compared to expected terminating events. Counterintuitive terminating events meanwhile have higher values of horizontal ME advection than terminating events. Counterintuitive events are then analyzed in terms of amplitude by RMM index and total ME over the convective center. Early results show that counterintuitive continuing events have a higher mean RMM amplitude than terminating events while counterintuitive terminating events have a lower mean RMM amplitude than continuing events. This suggests that counterintuitive continuing events having a higher RMM amplitude can cause the event to continue despite unfavorable conditions and vice-versa. Finally, the presence of anomalous convectively coupled wave activity, as obtained from Wheeler-Kiladis filtering, is also examined as a potential reason to explain the counterintuitive events.