|
|
| World Environment Day 2005 : Articles |
How likely is flood this year?
The El Nino-Southern Oscillation (ENSO) and seasonal climate
connection
Dr. Md. Rashed Chowdhury
THERE is evidence of teleconnections between the strength of El Nino
and climate anomalies in Bangladesh. Although the El Nino-Southern
Oscillation (ENSO) affects one-quarter of the globe to a significant
extent, the scientific research in Bangladesh relating to ENSO is
just beginning. The prime objective of this article is to provide an
improved description of the ENSO-related flooding picture for the
coming monsoon in Bangladesh.
Previous observations in 1982/83, 1987/88, and 1997/98, revealed
that Bangladesh rainfall maintains strong relation when SOI
(Southern Oscillation Index) extremes, and indicated negative values
of SOI (-0.8 or lower) to dry extremes (less rainfall and basin-wide
dry condition) and positive value of SOI (+0.8 and higher) to wet
ones (more rainfall and basin-wide wet condition). Major El Nino
years like 1951, 1958, 1972 recorded significant deficits in monsoon
seasonal-average (Jul-Aug-Sep) rainfall: -38 per cent in 1951, -48
per cent in 1958, and -10 per cent in 1972. Similarly, all the major
(strong) La Nina years (1964, 1973, 1988 and 1998) recorded
excessive rainfall -- the increase was: +4 per cent in 1964, +8 per
cent in 1973, +30 per cent in 1988, and +10 per cent in 1998.
In case of moderate anomaly in the SOI (-0.4 to -0.8 and +0.4 to
+0.8), the index-climate relation appeared to be contradictory --
any definitive conclusion on climate forecast is difficult from this
moderate index values. For example, while the moderate La Nina years
(1956, 1970, 1971, 1984, and 1999) recorded excessive seasonal
rainfall: +1.7 per cent in 1956, +11 per cent in 1970, +5 per cent
in 1971, +18 per cent in 1984, and +12 per cent in 1999, the
moderate El Nino years (1963, 1965, 1969, 1974, and 1987), instead
of rainfall deficit, displayed a tendency of excessive
seasonal-average (Jul-Aug-Sep) rainfall:+18 per cent in 1965, +16
per cent in 1969, +7 per cent in 1974, and +34 per cent in 1987. In
particular, Bangladesh is always found to be wetter than normal
during the moderate La Nina years; however, Bangladesh is not always
drier than average during the moderate El Nino years (for example,
1987). Instead, moderate El Nino years provided a wetter climate
during the onset of El Nino event (in June-July-August) and a drier
climate during the full maturity cycle of El Nino event (in October
to June of the following year).
It has to be remembered that we need to monitor the full climate
cycle of ENSO (usually 1-year) not the period of onset (usually
3-months only). Just for information to the readers, ENSO usually
starts to develop in summer (May-June-July), reaches its peak phase
in the following winter (October-November-December), and gradually
loses its punch through the spring (April-May-June) of the next
year. Therefore, to observe the impact of ENSO, it is essential to
monitor the ENSO phenomena from the year of onset of event to the
full maturity cycle of ENSO (usually the monitoring process starts
in July and continues up to June of the following year i.e.,
July/2004 to June/2005).
Consistent with the previous findings, Bangladesh displayed a wetter
climate (more rainfall and basin-wide flooding) during the summer of
2004. However, as the year advanced, drier climate (less rainfall
than the long-term mean average) became very distinct. Because of
negative SOI from November to February [the SOI was exceptionally
negative (-4.1) in February] particularly the months from December
2004 to February in 2005 was extremely dry. One study of the same
author found that the Ganges basin is more sensitive to ENSO signal
and, therefore, the north to north-western part of Bangladesh was
severely affected by drought.
ENSO and seasonal flooding: Causal connection
When SOI is negative (i.e. strong El Nino years), the whole Ganges-Brahmaputra-Meghna
(GBM) basin experiences less rainfall. The deficiency of rainfall
causes Bangladesh rivers to be drying because of low-flow and, as a
result, the country faces severe drought. On the other hand, when
SOI is positive (both in strong and moderate La Nina years) there is
significant increase of rainfall along the greater GBM basins
causing flooding along the whole catchments. This, in turn, severely
floods Bangladesh, as it is the lowest riparian country in these
basins. However, in case of moderate SOI-rainfall relationship
(moderate El Nino years), the basin-wide rainfall picture in
downstream Bangladesh is relatively different from upstream India.
During the onset (Jul-Aug-Sep) of moderate El Nino years (1963,
1965, 1969, 1974, and 1987), previous findings show that, other than
slight rainfall deficit in the Meghna basin (-4%), Bangladesh
experiences higher than average rainfall in the Ganges (+13.6%) and
Brahmaputra (+20.4%) basins. Observation also revealed that, in most
of the moderate El Nino years, the basin-wide upstream rainfall
(Jul-Aug-Sep) is less dominant (marginally above/or below from the
long-term mean average) for flooding in the downstream Bangladesh;
it is the exceptionally high and prolonged local rainfall mainly
contributing to flooding in the downstream Bangladesh.
ENSO-2004/05 and Bangladesh flood
According to Climate Prediction Center (CPC), based on the recent
evolution of SST anomalies and on a majority of the statistical and
coupled model forecasts, it seemed most likely that weak warm
episode (El Niño) conditions would continue to weaken during
April-June and that ENSO-neutral conditions prevail during the
summer of 2005 (web page at http://www.cpc.ncep.noaa.gov). Some
lingering effects of the weak warm episode, such as
drier-than-average conditions might continue to be experienced for a
month or two. According to the International Research Institute for
Climate Prediction (web page at: http://iri.columbia.edu/climate/ENSO),
sea surface temperatures in the east-central equatorial Pacific have
returned to near-normal or ENSO-neutral conditions, although they
remain above-average throughout much of the equatorial Pacific.
Based on the latest observations and forecasts, it is 70 per cent
likely that neutral conditions will prevail over the May-Jun-Jul
2005 season. Out of a large set of dynamical and statistical
forecast models, approximately 30 per cent predict sea surface
temperatures at weak El Niño levels for the May to July period in
the central Pacific. Overall, based on model forecasts and current
observations of the ocean surface and subsurface, the chances for
returning to El Nino conditions are approximately 30 per cent
through July 2005, increasing very slightly thereafter. The
likelihood of maintaining neutral conditions is approximately 70 per
cent through July 2005, and neutrality remains the most likely
scenario through the end of 2005. The probability of a La Nina
developing between now and July 2005 is near zero, rising to 5 per
cent thereafter. (Only models that produce a new ENSO forecast every
month are included in the above statement).
So, based on these above findings, any unusual and exceptionally
high rainfall is unlikely to occur in the greater GBM basin system
during the month of June-July-August of 2005. Bangladesh is,
therefore, unlikely to face any severe flooding this year. This is a
probabilistic forecast that is based on monitoring of the ocean and
knowledge of how the atmosphere has responded in the past to similar
SSTs in Bangladesh, with a variety of lag times.
Dr. Md Rashed Chowdhury, a former official of the Flood Forecasting
and Warning Center, Bangladesh, is presently working as Research
Scientist of the Pacific ENSO Applications Center (PEAC) in the
University of Hawaii, USA (email: rashed@hawaii.edu)
The Daily Star
Sun. June 05, 2005
|
|
