Abrupt ending of the Madden–Julian Oscillation by convectively coupled Kelvin wave precipitation leaves a swath of flooding across Indonesia

Convectively coupled Kelvin waves (CCKWs) are eastward-propagating weather systems that organise convection locally and are linked to precipitation extremes across the Maritime Continent (MC). They are often embedded in active Madden–Julian Oscillation (MJO) phases. The MJO also propagates eastwards, but influences convection in the MC over longer time-scales and larger areas. This article examines a case study during July 2021 of multiple CCKWs and westward-propagating inertio-gravity waves (WIGs) embedded within an active MJO. The final CCKW traversed the western MC, causing precipitation extremes across equatorial Indonesia and East Malaysia that led to numerous reports of flooding and landslides, with western Borneo the worst-affected region. The MJO event was terminated abruptly following the passage of this CCKW. Analysis of the total column water budget reveals that the precipitation rate exceeded the vertically integrated moisture-flux convergence provided by the CCKW, drying out the atmosphere and suppressing further convection. The performance of the UK Met Office prediction model was evaluated for this case study; parameterised convection configurations generally performed as well as or better than explicit convection models. This is possibly because they represented better the location and timing of the convective systems that developed because of interactions between CCKWs and WIGs. This research highlights how CCKWs should be viewed, not simply as convective systems that affect weather locally but, as having the potential to deliver larger-scale impacts over the entire equatorial MC, as part of a complex multiscale interaction. Such interactions can involve the MJO influencing CCKWs downscale by providing enhanced convection. Conversely, the suppressed phase of CCKWs can dampen the MJO convective signal and terminate MJO propagation. Whilst weather prediction models may forecast rainfall associated with individual equatorial modes accurately, capturing their combined effect remains a challenge.