How does the MJO affect extreme rainfall around the Tropics?

Abstract:

The MJO is well-known to modulate mean rainfall as its convective envelope moves eastward. Fewer studies have examined its effects on extreme rain and they focused on limited regions. This study takes a broader view and examines that variability around the tropics. It uses more than 20 years of NASA IMERG precipitation to identify five-day rainfall events with two- and five-year return intervals. These extreme events are binned by MJO phase to identify how the MJO affects their probability of occurrence. The MJO’s rainfall is strongest in the deep tropics, but the effects on extreme rain extend well into the subtropics. Australia is affected most strongly with most of the continent having more than a five-fold increase in these events during MJO phases 6 and 7. However, another notable region is the northern Coast of Africa to the Middle East where these events are most common in phases 7 and 8.

These preliminary results motivate further research into the subseasonal variability of extreme rain in the tropics and subtropics. For example, a valuable model diagnostic could be the degree to which models replicate these patterns. Models can skillfully forecast the MJO index out to three weeks. Those forecasts could be leveraged with these results to make long-range extreme rainfall forecasts. The analysis could also be extended to other subseasonal tropical modes like equatorial waves. The results will prove valuable to stakeholders in these vulnerable regions by allowing them to prepare for these extreme events.