Impact of Madden Julian Oscillation (MJO) Phenomena on the Heavy Precipitation and Flood Disaster in May 2017 in Sri Lanka

Abstract

Sri Lanka experiences south west monsoon normally about 200-3,000 mm from May-September. In
this case, Sri Lanka experienced heavy rainfall in the last decade during the latter part of May 2017.
Over 0.5 million people were affected and over 230 people were killed. The deep depression formed
in May over the south west of Sri Lanka intensifies into a tropical storm event. During the 24 hour
period on 25th May recorded rainfall of 300-500 mm, led to heavy floods in the western part of the
country where Rathnapura received nearly 453 mm rainfall leading to flood and landslides across the
region. This study is conducted to examine the Madden Julian Oscillation (MJO) impact on recordbreaking precipitation, tropical storm, and floods. Daily rainfall data from 20th to 31st May obtained
from the NOAA IRI site. During the period April, May and June average monthly Outgoing
Longwave Radiation (OLR) shows negative anomalies and active MJO index for the May on 15th
,
16th, 27th, 28th, 29th, 30th and 31st can be observed over Sri Lanka. The convective phase of the MJO
favoured the conditions for the tropical storm. Easterly winds were observed in the southern part of
Sri Lanka, which carry the moisture air parcels from the east of Sri Lanka. Horizontal wind analysis
at different levels shows that the wind from the east of Sri Lanka and west of Sri Lanka have
converged in the south west of Sri Lanka. This converging wind pattern induces surface upwelling of
moisture over the north eastern part of the Indian Ocean and Sri Lanka. Also, over the Indian Ocean
counter clockwise formation of deep depression was encountered at that period. This deep depression
brings the cross-equatorial moisture flux over the south western part of Sri Lanka. Thus, a positive
environment for the tropical storm surge was created by the MJO. The easterly wind associated with
MJO phases 3 and westerly winds formed the deep depression over south western Sri Lanka. The
moisture mass movement over the south western part of Sri Lanka further enhance the depression
into the tropical storm surge. Because Sri Lanka is highly prone to tropical extreme climate events
natural disasters claim lives, and properties. There is an urgent need for timely weather forecast
information for better disaster preparation.

Keywords: Flood, Climate extremes, MJO, Storm surge, Disaster preparation