Summary
The rainfall from January to March leading up to the 2015
Yala season was slightly lower than normal; the rain picked up in April and May
exceeding normal values. Even though the rainfall remained below normal for the
rest of the season through August, the water availability was suited for
irrigated cultivation such as for rice. North-central region of the country
mostly received above average rainfall while south western regions which
usually receive high rainfall mostly received below average rainfall. During
September the entire country received above average rainfall. Heavy rainfall
was observed during last two weeks of September which caused floods in several
areas of the country. An El Nino event which had developed to a borderline
state for many months became a fully-fledged event by July 2015. Usually during
an El Nino, the rainfall is deficient from January to March and June to August.
Rainfall in October to December is above normal and in May too is usually above
normal. In addition, a positive Indian Ocean Dipole event took place – what
this means is that the Arabian sea is warmer than normal in relation to the Bay
of Bengal seas surfaces near Sumatra. In positive dipole events, usually there
is a weak drop in rainfall from June to August and a significant rise from September
to November. So this years, rainfall is
following close to the historical averages for a combined El Nino and positive
Indian Ocean Dipole event. In addition to this the amplitude of the Madden
Julian Oscillation (MJO) in 2015 has been a mixture of very high amplitude
events and weak events. When the amplitude is higher than 1 the rainfall is
enhanced or suppressed based on the location of the MJO wave. When it is less
than 1 MJO is considered to be weak and there shall not be an impact on
rainfall. During March- April and June- July the MJO was strong. During
February, May, August- September the MJO was mostly weak. Severe drought
conditions were not observed during the early Yala season because most parts of
the country observed high rainfall in April and May. Significant below average
rainfall was only observed in July 2015.
Data
We
use ground observations and satellite derived estimates. Ground observations although
more accurate are not immediately available and are expensive – thus we rely
largely on satellite estimated data. We
have found that satellite derived data approximately follow the ground
observations in the past with a systematic under-estimation of about 10-20%
particularly in the hill country. This
small systematic deviation is due to reasons such as double cloud cover
(affecting satellite readings), wind conditions, topographical features of the
region, time of measurement and possible measurement errors of ground data. Until
ground readings are collated, quality controlled and made available affordably,
we can use satellite data with some confidence.
Drought
Conditions across Equatorial South Asia
This
year severe drought conditions were not observed in the region. Particularly
Sri Lanka received above-average rainfall during the first 3 quarters of 2015.
In the first quarter of the year (Jan- Mar) the south-western regions of Tamil
Nadu and Andhra Pradesh as well as the sea around south India and Sri Lanka
received slightly below average rainfall. This deficit of rainfall increased in
the sea towards the equator. In the 2nd Quarter south India and Sri
Lanka received above average rainfall. Only the northern region of Karnataka
and Telangana received below average rainfall. In the 3rd Quarter
the entire south Indian region received significantly less rainfall than the
historical average. The deficit in the rainfall in the western region of
Maharashtra was very large compared to any other place in the equatorial South
Asia.
Figure 1: A representation of peninsular
India that includes the subdivisions of Coastal Andhra Pradesh (22), Rayalseema
(24), South Interior Karnataka (28), Coastal Karnataka (26), Kerala (29), and
Tamil Nadu (25).
Island-Wide Rainfall over the last Five Years
In
2015 rainfall within Sri Lanka was high at the start of the Yala season (Apr-
May). Rainfall decreased at the beginning of June 2015; and low rainfall was
observed during the following 3 months. Rainfall received during 2015 Yala was
higher than that of 2014 but was lower compared to previous years. (Figure 2)
Recent Rainfall Surpluses/Shortfalls
The severity of drought
could be expressed in terms of rainfall-deficits and its duration. The monthly
rainfall surpluses and deficits for the last three years with respect to the average
for 2002 to 2013 period is shown in figure 3. In Sri Lanka above average
rainfall was observed during April and May and thereafter below average
rainfall was observed until September.
Comparison of 2015 with Past
In 2015, although below
average rainfall was observed during January. March, June, July and August high
above-average rainfall in February, April and May modulated the rainfall
deficit. In 2014 severe drought conditions were observed during Yala and very
high above average rainfall was observed from October to the end of that year.
In 2013 below average rainfall was observed during the early Yala season
(March- April) and the deficit was compensated by above average rainfall
received in May- July period. In 2012 high above average rainfall was observed
during April drought conditions persisted rest of the Yala season.
Oct- Dec 2014
Jan-Mar 2015
Apr- Jun 2015
July-Sep 2015
Figure 2: Quarterly seasonal rainfall anomalies for
Sri Lanka for 2015. Rainfall anomalies for January-March (late
Maha), and the first (April-June) and second (July-September) half of Yala are
shown. The average rainfall is calculated for January 1979-September 2015
Figure
3: Multi-year decadal (10-day)
precipitation comparison. The average
rainfall for each dekad (roughly 10d days) over Sri Lanka estimated from
satellites and ground observations is
shown for the last 6 years as a line in a separate colour over a common January
– December axis with 2015 in bold black.
Figure 4: Dekadal precipitation and
2001-2015 average. The smoother curve shows the average over
2001-2015 – this annual cycle is reproduced for each year in the above figure.
The departures from this average are shown in blue when wetter and brown when
dryer for each month for the last three years.
A dekad refers roughly to 10 days or more accurately as each month
divided into three.
Figure 5: Decadal Cumulative
Rainfall Graph . Cumulative decadal satellite derived estimates are
shown in solid black line and the cumulative recent short term average
precipitation is show in grey dotted line for the most recent 12- months period
in the selected region. The blue bars are indicative of estimates that are
above the short-term average.
Monthly Rainfall by District
Anomalies
– departures from the average for each month and district – are shown in Figure
5. The
average rainfall has been calculated for the base period 2001-2015. Wetter than
normal is shown in green and dryer than normal in brown.
Figure 5: Monthly precipitation anomalies for 2015 by district
January
|
February
|
March
|
Yala
|
||
April
|
May
|
June
|
July
|
August
|
September
|
In January the entire country received below
average rainfall. During February the rainfall increased in the entire country.
During April to September season the mean rainfall in the entire country was
above average. This is mostly due to
heavy rainfall observed in April May and September.
Further Information
Technical
details regards the Yala climate are provided in a series of research papers
published in the International Journals cited below and available via www.climate.lk. Our seasonal and weekly updates
are available at http://fectsl.blogspot.com
References
Technical
details are provided in a series of research papers published in the
International Journals
·
Lyon,
B. Zubair, L., V. Ralapanawe and Z. Yahiya, 2009
Fine scale evaluation of drought hazard for tropical climates, Journal of
Applied Meteorology and Climatology, 48 (1): 77-88.
·
Chandimala, J. and L. Zubair, 2007, Predictability of Streamflow and
Rainfall for Water Resources
Management in Sri
Lanka , Journal of Hydrology, 335
(3-4), 303-312.
·
Zubair, L. and J. Chandimala, 2006, Epochal Changes in ENSO-Streamflow
relations in Sri Lanka, Journal of Hydrometeorology, 7 (6):1237-1246.
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