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.