Annual Weather Review 2011

Review of Weather Conditions in 2011

Northeast Monsoon Season (Jan – March)

The Northeast Monsoon started around end of November 2010 and extended into March 2011. The monsoon strengthened in January 2011 and persisted throughout the month. In January, most parts of Singapore received rainfall amounts of more than 60% above the long term average. 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. Two episodes of monsoon surges occurred during the month of 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.

The next monsoon surge occurred from 25th January and brought periods of moderate to heavy showers over Singapore between 26 and 30th January. Rainfall peaked on 30th January and the total rainfall accumulated at the Changi climate station was a record breaking 216 mm. This was the highest ever rainfall in a day for the month of January since records began in 1869. The rainfall caused several parts of the island to experience flash floods, with the eastern areas most adversely affected.

In February 2011, cross equatorial winds began to be established over the region, signifying the start of the dry phase of the Northeast Monsoon. At the same time, La Niña conditions also began to weaken in February. 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. Rainfall for the month was generally below average, particularly over eastern Singapore. In total, there were only 8 rain days with a total rainfall of 23 mm recorded at the climate station in Changi.

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.

First Inter-Monsoon Season (March - May)

Due 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. The northward migration of the equatorial trough also resulted in the return of shower activities to the region. The Sea Surface Temperatures (SSTs) over the equatorial Pacific gradually warmed in March and April, and La Niña conditions weakened considerably.

The La Niña event started to dissipate in early May and neutral conditions were established by the middle of the month. In addition, two major tropical cyclones, Tropical Storm Aere and Typhoon Songda, developed near the Philippines in May 2011. These tropical cyclones influenced the flow of winds around the region, and contributed to squally weather conditions over Singapore.

The highest rainfall of 360 mm to 400 mm (100% to 120% above average) fell over the eastern part of Singapore around Pulau Ubin. Other parts of the island largely experienced below average rainfall. An incursion of south-westerly winds led to the first “Sumatra” squall for the year affecting Singapore on 8th March 2011. Weak to moderate “Sumatra” squalls also occurred on four days in the second fortnight of March. Additionally, there were twelve days in March which saw thunderstorms in the afternoon and early evening, caused mainly by diurnal heating and sea breeze effects.

Inter-Monsoon conditions prevailed throughout the month of April and most of May 2011. As typical during the Inter-Monsoon period, weather conditions were generally warm with afternoon thundery showers, which were heavy at times. Additionally, “Sumatra” squalls affected Singapore on 6 days each in April and May. The squall lines however, were relatively weak and poorly organised.

In April, most parts of Singapore received above average rainfall, notably areas around Jurong where 380 to 460 mm of rainfall fell (130 to 170% above average). This was likely influenced by the passage of a Madden-Julian Oscillation (MJO)2 wet phase over the Maritime continent in the first half of April, which led to enhanced convection over the region. Conversely, there was suppressed convection in the first half of May 2011 as a dry phase of the MJO traversed through, and resulted in below average monthly rainfall for more than half of the island. Overall, there were 9 days in April and 3 days in May where heavy rainfall affected many parts of the island. In particular, the heavy rain that fell in the afternoon of 4th May 2011 led to flash floods in parts of northern Singapore. The highest daily rainfall for that day was 100 mm.

With the intensification of Tropical Storm Aere during the second week of May 2011, the rain band was drawn northward away from the equatorial region. Coupled with suppressed convective activity due to the passage of the MJO dry phase, dry weather conditions became prevalent over the southern ASEAN region. As a result, fair and warm weather was observed in Singapore between 6th to 8th May with temperatures exceeding 34°C. The highest maximum temperature recorded at the climate station in Changi during this period was 35.3°C on 8th May 2011.

The dry conditions also led to the emergence of scattered hotspots with localised smoke plumes over central Sumatra. Transported by the prevailing south-westerly winds over the region, slight smoke haze affected parts of Peninsular Malaysia and Singapore. The air quality in Singapore dipped into the moderate range on a few days in May and the highest PSI level of 60 was recorded on 13th May 2011. By the middle of the month, the return of wet weather conditions helped to subdue hotspot activities and led to an improvement in air quality.

Southwest Monsoon Season (late May – Mid Oct)

Inter-Monsoon conditions prevailed in May 2011. Towards the end of the month, Southwest Monsoon conditions began to set in over the region as the winds blew increasingly from the southeast or southwest. For the month of June 2011, most parts of the island experienced above average rainfall, especially over the central and northwest areas where 340 to 460 mm (90 to 150% above average) of rain fell. “Sumatra” squalls occurred during the pre-dawn hours and morning on six days during the month. The heaviest rainfall for the month occurred on 1st and 5th June 2011, with maximum rainfall intensity of around 90 mm/hr occurring on both days.

On the morning of 5th June, two spells of intense rainfall caused flooding over many parts of Singapore, most notably in the Bukit Timah and Tanglin areas. The downpour was caused by strong convergence of winds which led to unstable weather conditions over Singapore and the surrounding region. The highest total rainfall of 143 mm was recorded at Bukit Timah. In addition, three tropical storms developed near the Philippines in the month of June and influenced the winds over the region during that period.

Drier than normal weather conditions were experienced in July and August 2011.  The total rainfall recorded at the climate station in July and August was only 76.6mm and 81.4mm respectively. This was 50-60% below the long term average rainfall for the two months.   Interestingly, only 7 raindays were recorded in July 2011, which is the lowest number of raindays recorded for the month of July since 1942. Despite the low recorded rainfall over the climate station, five “Sumatra” squalls affected the island around end of August 2011.

Total rainfall recorded in September was also 20% below average at 136.4mm.   In the first half of October 2011, a series of “Sumatra” squalls affected the island with the highest daily rainfall of 134 mm and maximum wind gusts of 80.6 km/h recorded on 4th October 2011. Apart from “Sumatra” squalls, heavy thundery showers induced by the strong heating of land areas also occurred in the late afternoon and early evening on a few days.

The prevailing dry weather during this period resulted in a surge in hotspot activities and slight to moderate smoke haze over Sumatra and Borneo. The smoke haze affected much of the southern ASEAN region on a few days.  Under the influence of prevailing southwesterly/southeasterly winds, Singapore was not spared from the smoke haze and the 24hr air quality dipped into the moderate range on nine days between 4th July and 1st Oct.  The hazy periods were relatively short-lived as changing wind directions and shower activities helped to mitigate the situation.  The persistent dry weather conditions over Sumatra in mid September led to an escalation of hotspot activities and the ensuing smoke haze drifted across the Malacca Straits, resulting in moderate PSI readings on 3 consecutive days from 11 to 13 September 2011.  The highest 24hr PSI was recorded on 12 September at 62, which turned out to be the highest in 2011.

Second Inter-Monsoon Season (mid October – mid November)

The second Inter-Monsoon season started around mid October 2011. Singapore experienced generally wet weather conditions during the month and all parts of the island received above average rainfall. This can be attributed to the re-developing La Niña conditions, which became established by mid October, resulting in enhanced convective activities over the region. In the second half of October, the convergence of winds over the region resulted in heavy thundery showers on 21st and 27th October 2011. Daily rainfall totals of 79.4 mm and 82.8 mm were recorded on both days respectively. Flash floods were also reported in a few parts of Singapore during the heavy rain events on 21 and 27 October.

Singapore experienced showers mainly in the afternoon and early evening on many days in November 2011. This is typical of the weather during the Inter-Monsoon period. Heavy thundery showers induced by strong heating of land areas and convergence of winds resulted in heavy rainfall over a few days in the month.

Northeast Monsoon Season (mid November to December)

By the middle of November 2011, the winds started to blow consistently from a Northeasterly/Northwesterly direction as the Northeast Monsoon sets in.  The second half of November turned out to be wetter than normal with the passage of an intense wet phase of MJO.  The 850hPa zonal wind also registered easterly anomalies from the second half of October and throughout November, indicating that the winds at steering level had an anomalously strong easterly component that facilitated the development of monsoon surges.  As a result, 2011 experienced the wettest November since 1934, with 24 raindays recorded for the whole month. The climate station also recorded rainfall that was 47.2% above the long term average for the month of November. 

For December, the recorded rainfall was close to long term averages as 850hPa zonal anomalies turned neutral (Figure 1) and diminished the easterly component of the steering level winds needed for the development of monsoon surges.  In all, Singapore experienced 3 significant Northeast Monsoon surge episodes on 11th, 17th and 23rd December with the last being the most intense and widespread.

Significant Weather Events in 2011

Monsoon Surge in January 2011

On 25th January 2011, a high pressure system intensified over China, resulting in the beginning of the second monsoon surge for the year. As the high pressure extended southwards, the 1020 hPa isobar stretched over the island of Luzon, Philippines. The steep pressure gradient caused the strengthening of north-easterly winds over the South China Sea and brought widespread moderate to heavy showers over the region from 26th to 30th January 2011 before it retreated on the 31st.

The rainfall over Singapore peaked on 30th January (Figure 3). Throughout the morning and early afternoon, there was continuous build up of heavy thundery showers to the northeast of Singapore, around the vicinity of Pulau Ubin, Pulau Tekong and Changi (Figure 6). The showers diminished in intensity as they propagated southwest towards other parts of the island.  Due to the prolonged heavy rainfall, several cases of flooding occurred, especially in the eastern region where rainfall was most intense. The highest rainfall recorded for the day was 216.2 mm at the climate station in Changi (Figure 7). This amount was also the highest ever on record for the month of January. As the monsoon surge occurred amidst peak La Niña conditions in January 2011, the enhanced convection brought about by La Niña likely contributed to the heavy rainfall induced by the surge. Satellite, radar images and observation data for the event are in Figures 3 to 7.

Heavy Rain and Flash Floods on 5 June 2011

Strong convergence of winds caused the build up of thunderstorms over Singapore in the early morning of 5th June 2011 (Figure 8). The intense downpour started off around central Singapore before spreading to the northern, eastern and southern parts of the island. After the first spell was over, a heavier deluge then advected in from the west in the late morning, and drenched the western half of the island before easing closer to midday (Figure 11).

The two spells of intense rainfall led to flooding over many parts of Singapore, most notably in the Bukit Timah and Tanglin areas. Maximum rainfall intensities of 89.6 mm/h and 87.4 mm/h were recorded at Bukit Timah and Katong respectively (Figure 12). The total daily rainfall of 143 mm recorded at Bukit Timah was also the highest in a day when compared against long term statistics for the month of June. Satellite, radar images and observation data for the event are in Figures 8 to 12.

Haze in September 2011

Although 2011 was generally a ENSO neutral to negative (La Niña) year (Figure 12), drier weather conditions in September over parts of the southern ASEAN region led to the emergence of hotspot clusters over central and southern Sumatra. Smoke plumes emanating from the fires were detected between 5th and 12th of the month. On 9th September, a total of 691 hotspots were detected in Sumatra, the highest number for the year. Smoke haze originating from these fires was carried by the prevailing low level winds to neighbouring countries. Moderate to dense smoke haze was observed to spread across the Straits of Malacca and affected parts of Peninsular Malaysia and Singapore. During this episode, air quality in Singapore deteriorated into the moderate range for several days. The highest PSI level reading of 62 for this period was recorded on 12th September 2011. This was the poorest air quality experienced by Singapore in year 2011. Satellite image of the smoke plumes and haze situation is shown in Figure 13.

In 2011, a total of 10,569 hotspots were detected in central and southern parts of Sumatra, similar to the figure of 10,297 hotspots for the same areas in 2009.  Although the 2011 figure is higher than the 4156 hotspots detected in 2010, it was still lower than the 12,014 hotspots detected in 2006, which was a year that experienced strong El Niño conditions. The haze period in 2011 was relatively mild and short-lived compared to similar periods in 1997, 1998 and 2006.

Heavy Rain and Flash Floods (21 Oct 2011)

The presence of a zone of convergence along the Malacca Straits coupled with a circulation east of Singapore (Figure 14) induced the development of scattered thunderstorms in the seas surrounding Singapore from 0800 hrs (Figure 17).  Isolated showers began affecting parts of Singapore from 1100hrs. By midday, thunderstorms had developed over southern Singapore and intensified as convective heating of the land enhanced the vertical development of the thunderstorm clouds. By 1300hrs, the thunderstorms had spread to the north west of the island where it further intensified and brought the heaviest rainfall to that area. 

Rainfall for the day was particularly heavy over the southern and northwestern parts of the island. The highest rainfall recorded was at Bukit Panjang in which 79.4mm of rainfall fell in an hour (Figure 18). On that day, flash floods were reported to have taken place in parts of Bukit Panjang and Upper Serangoon Road.  Satellite, radar images and observation data for the event are in Figures 14 to 18.

Heavy Rain and Flash Floods on 23 December 2011

As a result of strong convergence of winds that was exacerbated by convective heating, scattered showers started to develop over the island from around midday on 23 December (Figure 19).  The western part of the island experienced the first bout of heavy thundery showers from 1245hrs to 1415hrs.  Subsequently, several spells of torrential rain affected the central and southern regions between 1400hrs to 1730hrs. The heaviest rain fell over Orchard Road, with a maximum intensity of 39.4 mm of rain in half an hour was recorded (Figure 23).

For the three hour period between 1420hrs to 1720hrs, a total of 152.8 mm of rain fell on Orchard Road and resulted in flash floods. Additionally, flash floods were also reported at various parts of the island as well.

At this time, the signal of the MJO wet phase was evident over the Maritime Continent and likely to have enhanced the convection over the region. La Niña conditions, which re-established in August 2011, would also have contributed to further enhance convective activity. The combination of the above factors consequently led to the heavy rainfall occurring on 23 December 2011. Satellite, radar images and observation data for the event are in Figures 19 to 23.

Climatic Features

Weak to moderate La Niña conditions prevailed for most of 2011 although there were around five months of neutral conditions between May and September. The moderate to strong La Niña conditions at the start of the year weakened gradually towards May and strengthened again after September. This is depicted by the Oceanic Niño index (ONI) in Figure 26.  Another indicator, the Indian Ocean Dipole Mode Index (DMI) (Figure 27), which focuses on convective anomalies over the Indian Ocean, was negative for January, March and June and mostly positive for the rest of the year.  In particular, the warmer anomalies over the eastern Indian Ocean and the western Pacific Ocean (as indicated by the negative DMI and ONI) worked in tandem and resulted in higher than average rainfall for the months of January, March and June.

Between July and September, ONI conditions were close to neutral and the effect of the intensifying positive DMI dominated.  This resulted in lower than average rainfall recorded at the climate station.  In the last three months of the year,   La Niña conditions intensified and positive DMI weakened and resulted in anomalously high rainfall for Singapore.   In general, the percentages of rainfall distribution from mean (Figure 25) is in agreement with both the ONI and DMI.

On the whole, the annual rainfall over most parts of the island was above average in 2011. A monsoon surge coinciding with peak La Niña conditions contributed to a particularly wet January. As the Northeast monsoon switched into the dry phase and La Niña conditions began to weaken, February turned out to be an exceptionally dry month. The climate station at Changi recorded an annual rainfall of 2524.2 mm, which was 8% above the long term average of 2341.1 mm.

Total rainfall recorded was the highest over central parts of Singapore (Figure 28) with Pandan Reservoir registering more than 4600 mm of rainfall this year. Northwest and central Singapore experienced rainfall that was up to 85% above long-term averages but south-western, and eastern parts of Singapore experienced rainfall that was up to 15% below long-term averages (Figure 29). A likely cause of higher rainfall over Northwest and central Singapore could be due to increased occurrences of afternoon convective rainfall mainly affecting the north, west and central parts of Singapore.  These regions are particularly susceptible to thunderstorms that move down the west coast of Johor in the afternoon under the influence of north-westerly winds.  

The 2011 anomalous 850-hPa zonal wind average between 5°N-5°S indicates that at Singapore’s longitude (around 100E), it was positive (easterly anomalies) between January and May, turned negative from June to November and became positive again in December.  Generally for the year of 2011, westerly anomalies for the 850hPa winds were experienced during the Northeast Monsoon and easterly anomalies was experienced during the Southwest Monsoon. As a result, both the Northeast and Southwest monsoon were relatively mild this year.

Links to Learning Resources:

• Understanding the Weather
• Weather Phenomena
• Weather Monitoring Systems
• Weather and Climate of Singapore

Additional Notes:

1. During a La Niña episode, the sea surface temperatures (SST) across the equatorial central and eastern Pacific Ocean are observed to be cooler than normal while the converse is happening at the western Pacific. These anomalous SST induce enhanced convective activity over the southern ASEAN region and hence weather conditions are generally wetter than normal during a La Niña event.

2. The Madden-Julian Oscillation (MJO) is an atmospheric disturbance that travels eastward around the tropics with a cycle on the order of 30-60 days. The MJO affects tropical precipitation, with the wet phase enhancing precipitation and the dry phase suppressing it.

Figure 1: Time-longitude section of anomalous 850-hPa zonal wind averaged between 5N-5S (source: CDAS/Reanalysis). Contour interval is 2 ms-1. Anomalies are departures from the 1981-2010 base period pentad means. The data are smoothed temporally by using a 3-point running average. Blue shading and dashed contours indicate easterly anomalies. Orange shading and solid contours indicate westerly anomalies. Singapore is located at approximately 100E.

Figure 2: MJO fields (bold and dotted lines) superimposed on Outward Longwave Radiation (OLR) anomalies.  Bold blue lines indicate a wet phase of MJO while dotted lines indicate a dry phase of MJO. Blue contours indicate negative OLR anomalies while orange contours indicate positive OLR anomalies.

Highest Daily Rainfall for Jan 2011 (30th Jan 2011)

Figure 3: 3-channel composite MTSAT satellite imagery at 1130 LT on 30th Jan 2011 showing a monsoon surge bringing intense thunderstorms over Singapore and Peninsular Malaysia.

Figure 4: Gradient wind analysis (from Bureau of Meteorology) of synoptic situation in South East Asia region on 30th Jan 2011. A monsoon surge affecting the region at this time intensified the prevailing northeasterly wind flow over the South China Sea. The winds reduced speed and converged near the equator, leading to intense rainfall over the region.

Figure 5: Vertical wind profile from wind profiler located at Changi Airport on 30th Jan 2011 between 08:00LT and 12:00LT.  Northeasterly winds associated with the monsoon surge predominate throughout. Winds became more variable after 11:00LT due to intense rainfall over Changi.

Figure 6: Radar images at half-hourly intervals between 0903-1133LT on 30th Jan 2011 showing the buildup of intense thunderstorms to the northeast of Singapore during the monsoon surge.

Figure 7:  Rainfall data from the automatic weather stations located island wide on 30th Jan 2011. Total rainfall (top) and hourly rainfall intensity (bottom) for that day.

Heavy Rain Event (5th Jun 2011)

Figure 8: 3-channel composite MTSAT satellite imagery at 1130 LT on 5th Jun 2011 showing intense thunderstorm development over Singapore and Johor induced by strong convergence of winds.

Figure 9: Gradient wind analysis (from Bureau of Meteorology) of synoptic situation in South East Asia region on 5th Jun 2011. The convergence of winds over the region around Singapore led to the unstable weather conditions that day.

Figure 10: Vertical wind profile from wind profiler located at Changi Airport on 5th Jun 2011 between 08:00LT and 12:00LT.  The winds were southwesterly to westerly from the surface to 700hPa, turning southeasterly aloft.

Figure 11: Radar images at half-hourly intervals between 0800-1030LT on 21st Oct 2011 showing the build-up of intense thunderstorms over Singapore. Intense rainfall first affected the eastern half of the island in the early morning, before another heavy spell advected in from the west.

Figure 12:  Rainfall data from the automatic weather stations located island wide on 5th Jun 2011. Total rainfall (top) and hourly rainfall intensity (bottom) for that day.

Figure 13: NOAA-18 image of the smoke plumes emanating from hotspots in southern Sumatra and transported over to our region by the prevailing winds

Heavy Rain Event (21st Oct 2011)

Figure 14: Colourised MTSAT imagery (left) and 3-channel composite MTSAT satellite imagery at 1230 LT showing intense thunderstorms induced by a circulation in the South China Sea, east of Singapore.

Figure 15: Gradient wind analysis (from Bureau of Meteorology) of synoptic situation in South East Asia region on 21 Oct 2011. The monsoon trough was located in close vicinity to Singapore and the presence of a circulation east of Singapore induced the convergence of winds and development of a line of thunderstorms along the west coast of Malaysia and Singapore.

Figure 16: Vertical wind profile from wind profiler located at Changi Airport on 21st October 2011 between 11:00LT and 15:00LT. Light and variable winds predominate from the surface to 700hPa, facilitating the development of deep convective clouds and intense thunderstorms.

Figure 17: Radar images at half-hourly intervals between 1130-1400LT on 21st Oct 2011 showing the build-up of intense thunderstorms over Singapore with rainfall heaviest at the northwest and south of the island. The zone of convergence stretched from the west coast of Peninsular Malaysia to Bintan Island.

Figure 18: Rainfall data from the automatic weather stations located island wide on 21st Oct 2011. Total rainfall (top) and hourly rainfall intensity (bottom) for that day.

Heavy Rain Event (23rd Oct 2011)

Figure 19: Colourised MTSAT imagery (left) and 3-channel composite MTSAT satellite imagery at 1630 LT showing intense thunderstorms developing over Singapore and the Malacca Straits as a Northeasterly surge of winds converged on Singapore and the West coast of Peninsular Malaysia.

Figure 20: Gradient wind analysis (from Bureau of Meteorology) of synoptic situation in South East Asia region on 23 Dec 2011. A strong surge of Northeasterly winds affected Vietnam, Peninsular Malaysia and Singapore, driven by an intense high pressure system over Siberia and China. As a result, intense thunderstorms developed over these regions and the South China Sea as the winds slowed and converged closer to the equator.

Figure 21: Vertical wind profile from wind profiler located at Changi Airport on 23rd December 2011 between 11:00LT and 17:00LT. A significant increase in wind speed was observed starting from 1300LT when the surge of strong Northeasterly winds swept across Singapore and induced the development of intense thunderstorms as they converged over the island.

Figure 22: Radar images at hourly intervals between 1230-1730LT on 23rd Dec 2011 showing the northeast monsoon surge affecting Southern Johor, Singapore and Batam Island. The last surge of thunderstorms was the most intense and affected much of southern and central Singapore around 1630LT.

Figure 23: Rainfall data from the automatic weather stations located island wide on 23st Dec 2011. Total rainfall (top) and hourly rainfall intensity (bottom) for that day.

Figure 24: Monthly statistics of rainfall, maximum and minimum temperature at the climate station located in Changi.

Figure 25: Percentage deviation of rainfall from the mean (1869-2010). The correlation with the Oceanic Niño Index was relatively strong in 2011 with the exception of the months of February, May and December.

Figure 26: The Oceanic Niño Index for 2011 (Source; NOAA NWS CPC). Red: El Niño conditions; Black: Neutral conditions; Blue: La Niña conditions.

Figure 27: The Indian Ocean Dipole Mode Index (source: JAMSTEC). Positive (red) indicates warmer anomalies in the western Indian Ocean and cooler anomalies in the eastern Indian Ocean. Negative (blue) indicates cooler anomalies in the western Indian Ocean and warmer anomalies in the Eastern Indian Ocean. South East Asia is located at the Eastern Indian Ocean.

Figure 28: Isohyets of annual rainfall distribution in 2010 for Singapore

Figure 29: Percent deviation of annual rainfall in 2010 from the long term means for Singapore.