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 15-19th of May
2003, a tropical storm made its way across the Bay of Bengal. 900 km away there
was heavy rainfall over the South-Western corner of Sri Lanka leading to the
loss of perhaps 300 lives and displacement of 200,000. This cyclonic storm first
formed 700 km to the West of Sri Lanka on May 11 and then made its way
North by about 500 km and it stalled there for a week for 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. This cyclonic storm eventually
reached Burma on the 23rd of May and dissipated thereafter. It was extremely
rare that a cyclonic storm whose centre was 700-1500 km away and which was
relatively weak cyclone could cause such damage. 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.
Overall, what occured was a rare combination of three factors:
1. South-Easterly Winds in the Bay of Bengal that stalled
the cyclone near the Andaman Islands for
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 hover over Sri Lanka.
All of this resulted in high winds near the South-Western Coast of Sri
Lanka. 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 week in May was that the winds were
extremely high and it also took place during a point in which there are cloud
bands located over Sri Lanka ready to precipitate.
The rainfall
during the two weeks were very high. Here is the spatial pattern of rainfall for
the early period of the storm.
The map above 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 to 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 be called a cyclone.
The
Intense South-Easterly Winds that stalled the Cyclone for a week.
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