The La Niña event started to dissipate in early May and neutral conditions were established by the middle of the month. The Sea Surface Temperatures (SSTs) over the equatorial Pacific gradually warmed in March and April, and La Niña conditions weakened considerably. The northward migration of the equatorial trough also resulted in the return of shower activities to the region. Northeast monsoon conditions started to weaken during the first fortnight of March 2011, as the low level winds in the region turned light and variable.ĭue to light and variable wind conditions, Singapore experienced thundery showers mainly in the afternoon caused by convergence of winds and strong convective heating of land areas. In total, there were only 8 rain days with a total rainfall of 23 mm recorded at the climate station in Changi. Rainfall for the month was generally below average, particularly over eastern Singapore. The equatorial trough was situated far south of the equator around this time, and coupled with occasional dry surges, led to fair and windy conditions over the region. At the same time, La Niña conditions also began to weaken in February. In February 2011, cross equatorial winds began to be established over the region, signifying the start of the dry phase of the Northeast Monsoon. The rainfall caused several parts of the island to experience flash floods, with the eastern areas most adversely affected. This was the highest ever rainfall in a day for the month of January since records began in 1869. Rainfall peaked on 30th January and the total rainfall accumulated at the Changi climate station was a record breaking 216 mm. The next monsoon surge occurred from 25th January and brought periods of moderate to heavy showers over Singapore between 26 and 30th January. The high pressure region over China started to intensify from 6th January 2011 and this resulted in a surge that brought mainly passing showers over Singapore for several days before weakening by 12th January. Two episodes of monsoon surges occurred during the month of January. The enhanced convective activity was likely due to the prevailing La Niña1, which was established by 4th quarter of 2010 and was at its peak in early 2011. In January, most parts of Singapore received rainfall amounts of more than 60% above the long term average. The monsoon strengthened in January 2011 and persisted throughout the month.
The Northeast Monsoon started around end of November 2010 and extended into March 2011.
Review of Weather Conditions in 2011 Northeast Monsoon Season (Jan – March)
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