The weather leading to the recent floods
and landslides in Sri Lanka
Lareef Zubair
Sri Lanka Meteorology, Oceanography and Hydrology Network and
International Research Institute for Climate Prediction, Earth Institute
at Columbia University, New York, USA.
lareef@iri.columbia.edu
From the 11th to the 19th of May 2003, a tropical storm made its way across
the Bay of Bengal causing perhaps 300 deaths, the displacement of 200,000
persons, and heavy damage to the infrastructure, economy and livelihoods
of the South-Western Sri Lanka. This storm first formed 700 km to
the West of Sri Lanka on May 11 and then made its way North by about
500 km. It stalled there for a week. On the 14th of May, the wind
speeds were high enough for it to be classified as a cyclone for a day,
there after the wind speeds declined. 900 km away frmom the eye of the cyclone,
there was heavy rainfall over the South-Western corner of Sri Lanka. This
cyclonic storm eventually reached Burma on the 23rd of May and dissipated
thereafter.
May is the month when the heavy Yala season rains start and the South-West
of Country is expected to become wet. Indeed, there have been heavy rainfall
of 600 mm in Sri Lanka during the May from 1920 to 1940 but not thereafter.
Of course, there have historically been no record of a cyclone bearing
on Sri Lanka during May. It was also rare that a cyclonic storm whose centre
was 700-1500 km away and which was relatively weak cyclone could cause such
damage.
Overall, the disaster resulted from a rare combination of three factors:
1. Intense South-Easterly Winds in the Bay of Bengal that
stalled the cyclone in the middle of the Bay of Bengal and also contributed
to the high winds in the South-West of Sri Lanka
2. The cyclone being stalled in approximately the right location
that woudl induce high wind speeds over Sri Lanka
3. This event taking place in the middle week of May when
the Tropical Convergence Zone band of clouds hovers over Sri Lanka.
All of this resulted in high winds near the South-Western coast. The component
of these winds that is directed towards the North-East gets obstructed by
the topographic features such as the Adam's Peak and Gongala mountains.
As is usually the case, with mountain induced rainfall, the maximum rain
fell to the windward side of the mountain. This obstructions lead to
high rainfall in the South-Western corner of Sri Lanka every year during
this period. What was different that fateful week in May was that the winds
were much more intense than is usual. It also took place during a point in
which there are cloud bands located over Sri Lanka ready to precipitate.
The map below shows the rainfall for the week leading into the worst damage.
Thus already the Western Plains region was wet. The rains in the following
weeks were twice as intense and located pretty much in the same area. The
storm abated by the 21st as it made landfall in Burma.
In a historical sense, the monthly rainfall for May is high (600 mm) but
is not the highest on record in the South-Western region as shown in the
graph below. But the distribution of sustained high rainfall in
just two weeks led to the heavy flooding and landslips.
There were floods and cyclones leading to much of the damage.
Cyclones usually are steered towards Sri Lanka only during the North-East
Monsoon period and we only expect Cyclones to make landfall during this
period. So the forecasters were perhaps caught off-guard. Indeed, this cyclone
did not make landfall in Sri Lanka.
This was really a cyclone that tracked its way across the Bay of Bengal
at least 750 miles away from Sri Lanka.
There was only a single day when the winds reached high enough speeds (May
14, 2003) so that it could the storm could be called a cyclone.
The map below shows the intense South-Easterly Winds that stalled the Cyclone
for a week and also the cyclonic pattern in the Bay of Bengal. .
The wind vectors in the Bay of Bengal were in a South-Easterly direction
and as a result the eye of the cyclone howered around th e same place for
a week. The particular location of Sri Lanka led to an intensification of
wind near Sri Lanka.
These high winds during the particular weeks in May in which the Tropical
Convergence Zone cloud bands hover over Sri Lanka - this is likely to be
one reason why there was such high rainfall in Sri Lanka.
The component of these winds that is directed towards the North-East gets
obstructed by the topography of Sri Lanka (see below). This obstructions
lead to the orographic rainfall mechanism that year-in year out causes high
rainfall in the South-Western corner of Sri Lanka. What was different that
week in May was that the winds were extremely high.
The regional pattern of rainfall as recorded by the TRMM Satellite Observations
were:
Note that the heaviest rainfall took place in Sri Lanka from the 17th until
the 20th of May.
Mail Queries to Lareef Zubair at lareef@iri.columbia.edu