1.
Introduction
It was during the second pentad of November 2008 that the northeasterly winds
appeared to be persistent over the northern South China Sea. As many as four surges
occurred over the South China Sea through the period of November 2008. These four surges
occurred during the second, third, fifth and last pentads of November respectively. The first
two of the surge events coincided respectively with the passage of the Tropical Cyclone (TC)
Maysak and TC Noul which minimised the impacts of the surges over the Malaysian and
southern Thailand regions as well as eastern Indonesia in particular. However, the early
formation of TC Maysak over the South China Sea had triggered excessive rainfall over
western part of Philippines during the early week of November. Severe flooding caused by
heavy rain brought in by a cold front also had affected northern Philippines during the last
week of November. The TC Noul which struck central Vietnam on 17 November 2008 also
had induced heavy rain and floods as well as landslides over few provinces in this region.
2.
Weather Conditions During November 2008
Anomalously warm or near-normal temperatures persisted during September to first
half of November in most parts of the Asian regions. The ENSO-neutral conditions also
persist as the equatorial sea surface temperatures over the central and western tropical
Pacific Ocean are near-average values. However, parts of Asia such as Malaysia, northern
Luzon and southern Mindanao in Philippines, northern Thailand, central Indochina and
Sumatra (except the central areas) as well as eastern Indonesia still received above normal
amounts of rainfall.
Figure 1: 925 hPa Mean Wind Pattern for November 2008
The 925 hPa mean wind pattern for November 2008 (Figure 1) depicting the
northeasterly winds with speeds around 20 to 25 knots prevailed mainly over the northern
South China Sea (as depicted by the blue colour shaded area). These northeasterly winds
have slackened significantly over central South China Sea and turn cyclonically over
southern South China Sea.
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Figure 2: Time Series of 925 hPa 10 – 30ºN Zonal Winds (m/s) for November 2008
Time Series of 925 hPa zonal winds for November (Figure 2) depicts a broad strong
easterly winds with speeds greater than 12 m/s (25 knots) as depicted by the dark blue
colour dominating the western Pacific Ocean and South China Sea during the period 18 to
25 November 2008. Such wind conditions coupled with the cold air surges during 18 to 25
November 2008 brought heavy precipitation that lead to widespread floods over the
northeastern part of Peninsular Malaysia as well as southern Thailand.
3.
Madden-Julian Oscillation (MJO)
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Figure 3: Sequence of Satellite Imageries (10ºN – 10ºS) for November 2008 shows
the Passage of MJO
Time-longitude sequence of satellite imageries (Figure 3) shows the cloud cluster
passage of the wet phase of MJO during the period 15 to 26 November 2008 across the
eastern tropical Indian Ocean as well as the Malaysian-Indonesian region. This wet phase of
MJO had caused heavy rain over eastern Indonesia and also enhanced further the
precipitation induced by the cold air surges and the easterly winds which in turn had caused
floods especially over the eastern part of Peninsular Malaysia and southern Thailand.
4.
Dec-Jan-Feb 2008/2009 Rainfall Anomaly Percentage Outlook
Figure 4: December-January-February 2008/2009 Rainfall Anomaly Percentage Forecast
Over the next three months, near-normal sea surface temperatures (SSTs) are
expected to persist over central Pacific Ocean while the low-level easterly winds and upperlevel westerly winds would remain normal across the central equatorial Pacific. These SSTs
and wind conditions indicate that ENSO-neutral conditions would continue into early 2009.
Based on the rainfall anomaly percentage forecast (Figure 4) for the period January
to March 2008 generated by data from ECMWF and JMA seasonal forecast products,
southwestern Myanmar, southern Cambodia, central and southern Vietnam, southern
Thailand as well as western Philippines would receive above normal amounts of rainfall.
Other areas in these regions are expected to receive normal amounts of rainfall.
In
Indonesia, entire region except western part of Sulawesi is predicted to receive normal
amounts of rainfall. The western Sulawesi is predicted to receive below normal amounts of
rainfall. Entire Brunei is expected to experience normal weather conditions. In Malaysia,
above normal rainfalls are expected almost over entire Peninsular Malaysia while Sarawak
and Sabah are expected to receive normal amounts of rainfall.