Convectively coupled Kelvin waves contribution to hazardous weather in Sumatra.

Abstract:

The island of Sumatra is characterized by extremely high average precipitation accumulation. As a result, the island often suffers from precipitation driven natural hazards, which account for more than 50% of all natural disasters. Such hazardous events inflict great socio-economic loss and damage.

This study addresses employs a synergistic methodology of meteorological data analysis in the context of three independent datasets of floods. Two flood databases were derived from crowd-sourcing: Twitter and local papers. Additionally, the database from Indonesian agency BNPB was used. All three datasets were analyzed independently for the 2014-2018 period.

On a subseasonal time scale, the amount of rainfall over the island strongly modulated by eastward propagating modes of organized convection: the Madden-Julian Oscillations and convectively coupled Kelvin waves. This study investigates the relationship between those two modes of organized convection and flooding in Sumatra. It is shown that CCKWs constitute a critical dynamical predictor for flood onset.

We find that only about 27% of floods in Sumatra were immediately preceded by favorable MJO conditions and all of them were in fact also associated with a CCKW embedded within an envelope of enhanced MJO convection. This was the case during the flood in Padang on 31 May, 2017, when the MJO was active over the Indian Ocean but its enhanced precipitation had not yet reached Sumatra. Instead, a strong CCKW, which initiated over eastern Africa, brought anomalous precipitation exceeding 10 mm/day to Padang and triggered a flood.

Nearly half of CCKW events were associated with floods in Sumatra. From a different perspective, nearly all of floods in Sumatra were preceded by anomalous precipitation associated with a CCKW. This percentage is substantially higher than for favorable MJO conditions, indicating stronger interaction of local convection with CCKWs. Even though CCKW activity is modulated by the MJO itself and more CCKWs are found when the MJO is active in the Indian Ocean, during analyzed 2014-2018 period nearly 30% were associated with CCKW alone. Therefore, this study shows that CCKWs are important contributor to extreme weather in Sumatra and constitute a potential source of predictability of such hazardous events.