Kuala Lumpur Monsoon Activity Centre November 2011

Introduction
Since early August 2011, negative sea surface temperature anomalies have gradually
strengthened across much of the equatorial Pacific. Enhanced convection and precipitation were
weakly evident over the Maritime Continent and Australia while suppressed convections were
located over the western tropical Pacific and the dateline during October 2011. Currently, La Niña is
not as strong as it was in September 2010 and majority of ENSO models predict the continuation of
La Niña through the Northern Hemisphere winter 2011/2012. August to October 2011, witnessed 12
tropical cyclones in the Western North Pacific and South China Sea, of which three passed through
South China Sea and made landfall over Vietnam which significantly affected the regional synoptic
and weather pattern during this period.
1. Weather Conditions from July to October 2011
The Asian summer monsoon is normally associated with the rainy season over the
Southeast Asia region, particularly north of 10°N that lasts from May to October. Since mid-May,
flash flood have occasionally affected many areas in the northern part of Indochina due to active
southwest monsoon, which also caused floods over a wide area in the basins of Chao Phraya and
Mekong River. Thailand experienced the worst of the flooding, in which three months of heavy
monsoon rain have flooded a third of the country since late July.
The 925-hPa mean wind analysis (Figure 1) from July to October 2011 shows strong
southwesterly to westerly flow over Andaman Sea and South China Sea during July to September.
In July, active convection over northern part of Vietnam, Laos and Thailand was also greatly
enhanced by the influence of Tropical Depression Nock-Ten that made landfall over Vietnam on 30
July and moved into northern Thailand on 31 July before weakening as a low pressure cell. During
August to September, wet conditions continued over the Indochina Peninsula and northern Thailand
caused by the rather active monsoon trough that cut across northern Indochina between 15°N and
20°N. During the last decade of September, three tropical cyclones namely Nesat, Haitang and
Nalge made landfall over Vietnam which contributed to the unusual wet weather conditions over
these areas. In October, drastic changes in wind pattern as northeasterly to easterly winds over
Southeast Asia replaced the westerly winds signaling the withdrawal of the southwest monsoon in
the region.

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Figure 1: 925-hPa Mean Wind Analysis (ms -1 ) from July to October 2011
The time series of the 850-hPa mean zonal wind averaged between 10°N – 30°N from 1
September to 31 October 2011 (Figure 2) shows a gradual change in the wind pattern throughout
the region. Generally westerly winds were dominant over Indian sub-continent throughout
September. From early October, it is very noticeable that the easterly trade winds in the western
Pacific Ocean continued to strengthen and progress westwards. During the last decade of October,
the easterly winds were dominant from western Pacific Ocean to Bay of Bengal, signifying the
withdrawal of southwest monsoon and the transition to the northeast monsoon is underway.

Figure 2: Time Series of Mean Zonal Wind (ms -1 ) Averaged between
10°N – 30°N at 850-hPa for Sep – Oct 2011
2. Outgoing Long Wave Radiation (OLR)
The time series of OLR anomalies averaged between Equator and 10°N from 1 September
to 31 October 2011 as shown in Figure 3a indicates that negative OLR anomalies, signifying
enhanced convection are observed between 90°E and 120°E from the second pentad of September
to the end of October. During this period, the convective activity had an aperiodic oscillation of 5 to
10 days cycle, meanwhile suppressed convections are dominant from mid-October over the western
and central Pacific Ocean.
(a)
(b)
Figure 3: Time Series of OLR Anomalies (Wm -2 ) Averaged between 0° – 10°N (left panel) and 10°N – 20°N (right
panel) for September – October 2011

Wetter than normal condition over northern Thailand and Indochina region in September and
early October (Figure 3b) was mainly contributed by the westward propagation of convective
systems associated with tropical cyclones that tracked westward from the western Pacific Ocean
and made landfall over Vietnam.
The monthly distribution of OLR anomalies as shown in Figure 4, clearly shows that the
convection over Southeast Asian region was greatly enhanced during July to September, resulting
in many areas over northern, central and northeastern parts of Thailand experiencing massive
floods. Tropical cyclone activities also contributed to the wet weather conditions over this region. As
the wind pattern changed in October coupled with the southward movement of the trough, active
convection shifted closer to the Equator mainly over the Indian Ocean, Malaysian region and
Borneo Island including South China Sea. Meanwhile, suppressed convection was most prominent
over the Indian sub-continent, parts of Indochina, the Philippines and western Pacific Ocean.
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Figure 4: Distribution of OLR Anomalies (Wm -2 ) from July to October 2011

3. Weather Outlook for December 2011 to February 2012
The seasonal rainfall forecast (Figure 5) for the period of December 2011 to February 2012
generated from ECMWF and CFS seasonal forecast products depicts that Myanmar, northern
Thailand, northern Vietnam, and Laos are expected to experience normal weather conditions.
Southern Thailand, Cambodia and southern Vietnam are expected to experience above normal
rainfall. In the Philippines, central parts and Mindanao Island are expected to receive above normal
amounts of rainfall while normal weather condition is expected over Luzon Island. In Indonesia,
generally most of the provinces are expected to experience normal weather condition except for
northern Sumatra where above average rainfall amount is expected.
Over Malaysia, entire Peninsular Malaysia and Sabah are expected to be wetter than normal
while Sarawak is expected to receive normal amounts of rainfall.
Figure 5: December 2011 – February 2012 Rainfall Anomaly (%) Forecast