1. Review of Weather Conditions
1.1 Northeast Monsoon Season (Jan – late March)
2012 started off with moderate Northeast Monsoon conditions prevailing in January, with the surface winds blowing predominantly from the north-northwest and north-northeast. A weak monsoon surge affected the region on 7-9 January 2012. This is the third surge for the season with previous 2 episodes taking place in December 2011. The passage of “Sumatra” squalls as well as the convergence of winds and intense solar heating of land areas contributed to a few episodes of heavy thundery showers on 15, 20 and 21 January. The heavy rain on 20 January caused flash floods in Little India and Thomson Road. Towards the end of January, the monsoon trough migrated southward and hovered between the equator and 150S.
During the second week of February, generally fair weather conditions were experienced under the influence of cross-equatorial and diffluent flow. Mainly dry and occasional windy conditions were observed. While it is generally dry, localised intense weather is still possible. An episode of localised intense thunderstorm in the morning over the south western part of Singapore took place on 27 February. During the event, rainfall of 102.6 mm (highest daily total for the month) fell over Ulu Pandan where flash flood was reported.;
The influence of Northeast Monsoon persisted during the first fortnight of March. The first week saw mostly isolated afternoon thundery showers, but on 8 and 9 March squall lines affected Singapore in the early hours due to an intrusion of strong westerly winds (20/25 knots) at the 850 mb level feeding into the deepening southern monsoon trough. Pressure gradient over the southern China strengthened on 10-14 March. This resulted in periods of showers on 14 March from the early hours till morning.
Despite the heavy showers, Singapore experienced below average rainfall for most parts of the island in January and February 2012. In February 2012, there were only 10 rain days with a total rainfall of 83.6mm recorded at the climate station in Changi.
1.2 First Inter-Monsoon Season (late March – mid-May)
In late March, low level winds over Singapore weakened, signifying the start of the Inter-Monsoon period. Afternoon thundery showers were seen on most days and were particularly intense on 25, 27 and 30;March. It can be attributed to the convergence between the north-easterly winds that strengthened over the; South China Sea; and the local sea breezes. Solar heating was also an indispensable ingredient for heavy thunderstorms. Daytime temperature soared to slightly above 340C on several days in April and May. The highest maximum temperature for the season was 34.90C observed at Paya Lebar on 8 May.
“Sumatra” squall developed mainly during the predawn and morning period on four days in April and ten days in May.; An un-seasonal Typhoon Pakhar triggered off an intense “Sumatra” Squall the morning on 1 April. There were six days in April which saw intense afternoon thunderstorms, with the highest rainfall of 104.2mm recorded on 10 April near Admiralty. The intense thunderstorm on that day caused flash floods around Hougang, Paya Lebar, Macpherson, Tampines and Seletar areas. During the first half of May 2012, temporary incursion of south-westerly winds resulted in a series of “Sumatra” squalls on 5-7 May. The most intense squall occurred on 5 May, leading to a high rainfall amount of 131.8mm recorded in northern part of Singapore.
The overall rainfall in April and May was above average. The total rainfall amount recorded at the Changi weather station in April and May was 45% - 55% above average. The above average rainfall was likely influenced by the passage of a Madden-Julian Oscillation (MJO) wet phase over the Maritime Continent.
1.3 Southwest Monsoon Season (mid-May – mid October)
Around second half of May 2012, the cooling of the Australian landmass in the southern hemisphere resulted in the intensification of the high pressure systems there. This in turn caused the surface winds over Singapore and our vicinity to blow predominantly from the southeast or southwest, signifying the start of the Southwest Monsoon season.
Early June saw thundery showers mainly in the morning and early afternoon. A “Sumatra” squall on the morning of 9 June brought highest rainfall of 104.2mm to Choa Chu Kang and gusty winds up to 37 knots to Jurong areas. Subsequently, presence of dry air mass and strong winds helped to subdue weather activities, resulting in fair and warm days between 12 and 19 June. For the month of June, all parts of Singapore received below average rainfall.
In the first half of July, the monsoon rain belt was hovering between 100N and 50S. A “Sumatra” squall on 3 July brought 92.8 mm of rainfall over Jurong and gusts of up to 40 knots over the Pasir Panjang area. In the second half, drier air mass and diffluent wind flow suppressed weather activities in Singapore resulting in fair weather between 21 and 27 July. Rainfall for July was generally above normal.
Rainfall in August was below average as drier conditions became established. Suppressed convection continued in September as Singapore received below average rainfall. An intense evening "Sumatra" squall on 27 September brought a highest rainfall of 100.4mm over southern part of Singapore.
During the Southwest Monsoon season, the monsoon trough migrated northward and away from the equator. This led to drier conditions in the Indonesian region, resulting in an increase in hotspots activities in Sumatra. Between 16 and 22 June the prevailing south-westerly winds over Riau brought smoke haze across the Straits of Malacca towards the northwest states of Peninsula Malaysia. Singapore was only affected by haze towards late August when PSI in Singapore reached 53 and 59 on 27 and 28 August respectively. PSI again rose above the moderate range on 6, 7, 20 and 21 September (61, 68, 60 and 60 respectively).
Weak Southwest Monsoon conditions persisted till early October, before it gradually progressed to Inter-Monsoon conditions. The passage of the MJO dry phase inhibited convective activities for the maritime continent in early-to-mid October. Few fair and dry days were experienced in early October.
1.4 Second Inter-monsoon Season (mid October – late November)
Severe thunderstorms that developed due to strong convergence of; winds on 17 October resulted in 106.4 mm of rainfall over Thomson area and led to flash flooding at several places. Typhoon Son-Tinh formed on 23 October to the east of Philippines, tracked north-westward across South China Sea and eventually affected northern part of Vietnam on 30 October. With the dissipation of the typhoon, monsoon trough became established over our region. Intense afternoon thunderstorm occurred on 31 October where 109mm rainfall fell at Sime Road. Slightly below normal rainfall was received in October 2012.
During November 2012, the proximity of monsoon trough brought generally wet weather conditions over our region with Singapore experiencing thundery showers mainly in the afternoon and early evening. On 17 November, strong heating of land areas coupled with convergence gave rise to heavy thundery showers over many parts of the island. The highest rainfall for that day was 110.6 mm recorded over the Upper Peirce area
1.5 Northeast Monsoon Season (late November – December)
By late November, Inter-Monsoon conditions gradually gave way to the Northeast Monsoon, with surface winds strengthening and blowing from the northeast. In November, most parts of Singapore received above average rainfall.
Weak to moderate Northeast Monsoon was well-established throughout December, as the intensification of the high pressure system over the northern hemisphere resulted in cold air rushing out of Central Asia to blow towards the Maritime Continent. The monsoon surge around 23-25 December brought torrential rainfall to the east coast of Peninsular Malaysia, but produced only occasional light to moderate rain to Singapore. Another spell of monsoon surge on 30-31 December brought widespread rain to the neighbouring region.
Singapore experienced several spells of heavy thunderstorms on 31 December as a result of atmospheric instability induced by monsoon surge and afternoon heating. Serangoon and Pasir Ris registered the highest total rainfall for the day at 141.1mm and 116.4mm respectively. For December 2012, most parts of Singapore received above average rainfall.
2. Significant Weather Events
2.1 Mild Monsoon Surge - 8 January 2012
The high pressure system in China began to intensify from 6 January 2012 and reached a peak by 8 January with the 1020hPa isobar extended southwards to the south of Hainan Island, China (Figure 1). The steep pressure gradient caused strong winds of 35 knots or more over the South China Sea. The instability induced resulted in increased showers activities over our region (Figure 3).
A mild monsoon surge with occasional showers and rain in Singapore took place on 8;January.; The wind profile at Singapore showed deep northerly to north-northeasterly winds. Heavy thundery showers developed continuously to the north or northeast of Singapore since the morning till early afternoon; (Figure 5).The intensity diminished as the rain cloud propagated south towards other parts of the island. The highest rainfall fell over the eastern part of Singapore. Rainfall amount of 26.2mm was recorded at Changi station.
2.2 Heavy Rain and Flash Floods - 21 January 2012
Light winds coupled with strong afternoon heating on the land mass caused the build up of thunderstorms over Johor, Malaysia in the afternoon (Figure 8). These thunderstorms propagated south and affected Singapore in the late afternoon. The intense downpour started over northern Singapore before spreading to other parts of the island. The thunderstorms eased towards early evening (Figure 10).
Flash floods hit several parts of Singapore such as Jalan Turi off Sims Avenue The highest rainfall amount was 134.1mm at Serangoon. The maximum rainfall intensity of 88.8mm/h was recorded at Bukit Timah.
2.3 Intense “Sumatra” Squalls – 5, 6, 7 May 2012
In May 2012, the strengthening of cross-equatorial south-westerly winds associated with the development of the northern monsoon trough triggered “Sumatra” squall over the Straits of Malacca (Figures 13, 14, 15). The thunderstorms were steered towards Singapore and brought heavy downpour in the early hours of 5 May (Figure 19). Highest total rainfall of 131.7mm and 114.6mm fell over Woodlands and Bukit Panjang respectively. Strong wind gusts of 40.6 knots (75.1 km/hr) were also registered in Woodlands and the West Coast. Flash floods were reported in Bukit Timah and flood waters reached a depth of up to 25cm along Bukit Timah Road and Dunearn Road. On the following days i.e. 6 – 7 May, “Sumatra” squalls continued to hit the island in the early afternoon and early morning periods respectively, but lower amount of rainfall was recorded (Figures 20, 21). These intense squall lines that lashed the island three days in a row had caused numerous trees to be uprooted in Changi Beach Park.
2.4 Haze in September 2012
With Southwest Monsoon conditions prevailing in September, the monsoon rain belt was situated north of the equator and this led to drier conditions in Indonesia. The condition was further aggravated by the passage of MJO dry phase. This resulted in the escalation of hotspot activities with localised smoke plumes scattered over the southern Sumatra. In the second half of September 2012, hotspot clusters emanating moderate smoke haze persisted in South Sumatra and affected the Jambi province. On 18 September (Figure 24), a total of 631 hotspots were detected in Sumatra. PSI readings rose to moderate range on a few days and peaked at 68 on 7 September. The highest number of hotspot for the year was at 681 detected on 27 July (Figure 25).
2.5 Heavy Rain and Flash Floods - 17 October 2012
Generally weak prevailing winds from the surface to 700hPa levels and strong local convergence of winds led to development of quasi-stationary and long-lived (more than an hour) thunderstorms over the central part of Singapore in the afternoon of 17 October (Figure 29). The thunderstorms intensified and affected many areas between 1430LT and 1600LT. The nearly stationary thunderstorms reached maturity and produced heavy rain over same area around Thomson, resulting in flash flooding at several places. The maximum hourly rain intensity reached 85 mm/hr and the total at 106.4 mm.
2.6 Heavy Rain and Waterspout at Pasir Panjang Ferry Terminal - 15 December 2012
On 15 December, weak thunderstorms developed around midday over southern Singapore and moved towards the south-western coast of Singapore (Figure 34). As the thunderstorm moved over the waters off Pasir Panjang Ferry Terminal, it intensified and spawned a waterspout with visible funnel around 1430LT that lasted for up to tens of minutes. The deep easterly winds and unstable atmospheric conditions that were aggravated by afternoon solar heating resulted in such severe thunderstorms. The highest rainfall was over the West Coast, which registered a total of 84.6mm. The strongest gust measured was 34.8 knots at the nearest station at Tuas.
3. Climatic Features
La Niña conditions predominated in the tropical Pacific Ocean in early 2012 as depicted by the Oceanic Niño Index (ONI) in (Figure 36). The conditions gradually eased towards ENSO-neutral in March and April and reached a borderline neutral/ weak El Niño conditions briefly in October. For the rest of year 2012, oceanic and atmospheric anomalies reflected ENSO-neutral conditions.
The Indian Ocean Dipole Mode Index (DMI) (Figure 37) is another indicator which focuses on convective anomalies over the Indian Ocean. DMI was largely positive during 2012, except for February, March and April. Between May and December, the intensifying positive DMI likely to have contributed to drier weather conditions over the Maritime Continent, while for March and April the negative DMI likely to have contributed to wetter conditions. In general, DMI displayed good correlation with the percentages of 2012 monthly rainfall difference from mean for the climate station (Figure 38).
The passage of an intense Madden-Julian Oscillation (MJO) (Figure 39) dry phase in February and June may have a suppressing effect on the convective activities around Singapore, resulting in monthly rainfall that was lower than the long-term average. During the first half of March and December, Singapore saw enhanced convection activities and above average rainfall brought forth by the MJO wet phase.
The 2012 anomalous 850-hPa zonal wind averaged between 50N-50S(Figure 40) indicated positive anomalies (westerly anomalies) between January and April. It then turned negative (easterly anomalies) from May till November, before reverting to positive anomalies in late November and December. For the most part of 2012, westerly anomalies were experienced during the Northeast Monsoon and easterly anomalies were experienced during the Southwest Monsoon. These scenarios gave rise to a mild Northeast Monsoon and Southwest Monsoon with lower than average rainfall.
Broadly speaking, in 2012, rainfall was above average for most parts except the western and eastern ends of the island (Figures 41 and 42). The highest rainfall of more than 2950mm (25% - 35% above the average) fell mainly over the northern part of Singapore. A total rainfall of 2159.9mm was recorded at Changi in 2012, which was 7.8% below the long-term average of 2342.3mm. March and December were wettest with monthly rainfall at 313.4mm and 363.4mm respectively (Figure 43).
Mild Monsoon Surge - 8 January 2012
Figure 1: Mean Sea Level Pressure analysis (from Bureau of Meteorology, Australia) on 8 January 2012, showing the isobar of 1020hPa extended to the south of Hainan Island, China.
Figure 2: Gradient wind chart (from Bureau of Meteorology, Australia) of synoptic situation in South East Asia region on 8 January 2012. A mild monsoon surge brought north-easterly winds of 30 knots or more over the South China Sea. Strong speed convergence led to intense rainfall in our region.
Figure 3: 3-channel composite MTSAT satellite imagery at 1130LTC on 8 January 2012 showing increasing showers/ thunderstorms activities over the South China Sea (in square box) due to a monsoon surge. The mild surge brought only moderate rainfall to Singapore.
Figure 4: Vertical wind profile at Changi Airport on 8 January 2012 between 1000LT and 1400LT. Strong winds blowing from north to northeast predominate throughout the whole period. This wind pattern was responsible for the continuous steering of thunderstorms cloud towards Singapore.
Figure 5: Radar images at half-hourly intervals between 1100 – 1330 LT on 8 January 2012 showing continuous build-up of thunderstorms to the north and northeast of Singapore during the monsoon surge.
Figure 6: Rainfall data from the automatic weather stations located island wide on 8 January 2012. Rainfall intensity (top) and total rainfall (bottom) for the day.
Heavy Rain Event - 21 January 2012
Figure 7: 3-channel composite MTSAT satellite imagery at 1730LT on 21 January 2012 showing intense thunderstorms development over Singapore and Johor in the late afternoon.
Figure 8: Gradient wind chart (from Bureau of Meteorology, Australia) of synoptic situation in South East Asia region on 21 January 2012. Light winds with intense afternoon heating led to development of severe thunderstorms in the late afternoon.
Figure 9: Vertical wind profile at Changi Airport on 21 January 2012 between 1300LT and 1730LT. Light north-westerly wind condition supported by ample supply of warm moist air had provided a suitable environment for the growth of afternoon air mass thunderstorms.
Figure 10: Radar images at half-hourly intervals between 1600 – 1830 LT on 21 January 2012 showing severe thunderstorms over Singapore. The storms affected the northern part first before moving south and brought heavy rainfall to the whole island.
Figure 11: Rainfall data from the automatic weather stations located island wide on 21 January 2012. Rainfall intensity (top) and total rainfall (bottom) for the day.
Intense “Sumatra” Squalls – 5, 6, 7 May 2012
Figure 12: FY2 Infrared channel satellite imagery on (top) 5 May 2012 midnight till 0200 LT (left to right); (centre) 6 May 2012 at 1000LT, 1200LT and 1400LT (left to right); (bottom) 7 May 2012 0600 LT – 0800LT showing thunderstorms that developed over the Sumatra and Straits of Malacca moving eastward to Singapore.
Figure 13: Gradient wind chart (from Bureau of Meteorology, Australia) of synoptic situation in South Asia region on 4 May 2012 12 UTC. South-westerlies with convergence of winds triggered the build-up of “Sumatra” Squall along the Straits of Malacca. The thunderstorms brought heavy rainfall in the early hours (0100 – 0400 LT) of 5 May 2012.
Figure 14: Gradient wind chart (from Bureau of Meteorology, Australia) of synoptic situation in South Asia region on 6 May 2012. “Sumatra” Squall formed and adverted in by the south-westerly winds to Singapore in the early afternoon.
Figure 15: Gradient wind chart (from Bureau of Meteorology, Australia) of synoptic situation in South Asia region on 7 May 2012. A milder squall line, accompanied with gusty winds, formed in the early morning, bringing less rainfall compared to previous days.
Figure 16: Vertical wind profile at Changi Airport on 5 May 2012 between 0100LT and 0400LT. South-westerly winds were observed from surface to 10000ft before the squall line hit Changi. Winds of 30 – 40 knots were recorded during the passage of the thunderstorms.
Figure 17: Vertical wind profile at Changi Airport on 6 May 2012 between 1100LT and 1500LT.The westerly winds at the low level strengthened significantly when “Sumatra” Squall hit Singapore.
Figure 18: Vertical wind profile at Changi Airport on 7 May 2012 between 0600LT and 1000LT. A marked change in wind speed and wind direction can be seen at 0800 LT when the “Sumatra” Squall approached Changi airport. North-westerly winds of up to 25 – 30 knots were observed at the low level.
Figure 19: Radar images at half-hourly intervals between 0100 – 0330 LT on 5 May 2012 showing the build-up of intense “Sumatra” Squall over the western part of Singapore in the early hours and moved eastwards across the island.
Figure 20: Radar images at half-hourly intervals between 1200 – 1430 LT on 6 May 2012 showing patches of thunderstorms formed in the southern Johor, Malaysia in midday. The thunderstorms were then adverted by the westerly winds and brought heaviest rainfall over the northern half of the island.
Figure 21: Radar images at half-hourly intervals between 0600 – 0830 LT on 7 May 2012 showing a line of thunderstorms cells off the western coast of Singapore in the early morning and swept across the island in less than an-hour’s time.
Figure 22: Rainfall data from the automatic weather stations located island wide on 5 May 2012. Rainfall intensity (top) and total rainfall (bottom) for the day.
Figure 23: Rainfall data from the automatic weather stations located island wide on 6 May 2012. Rainfall intensity (top) and total rainfall (bottom) for the day.
Haze in September 2012
Figure 24: NOAA-18 satellite image of the smoke plumes emanating from hotspots (total 631) in Sumatra and transported over to the neighbouring region by the prevailing winds on 18 September 2012.
Figure 25: NOAA-18 satellite image of scattered hotspots (total 681) over central and southern Sumatra. The smoke plumes were transported over to Straits of Malacca and Peninsula Malaysia by the prevailing winds on 27 July 2012.
Heavy Rain and Flash Floods - 17 October 2012
Figure 26: 3-channel composite MTSAT satellite imagery on 17 October 2012 at 1530LT showing intense afternoon thunderstorms development over Singapore and Johor induced by strong convergence of winds and heating.
Figure 27: Gradient wind chart (from Bureau of Meteorology, Australia) of synoptic situation in South Asia region on 17 October 2012. Light and variable wind conditions with strong convergence over the region around Singapore led to unstable weather conditions that day.
Figure 28: Vertical wind profile at Changi Airport on 17 October 2012 between 1400 – 1800LT. The winds were initially south-easterly (sea breeze) at near-surface level, but kept changing throughout the day. It became light north-westerly as the weather system affected Changi station at around 1730LT. The winds aloft, however, remained light and blew from the northeast during the day.
Figure 29: Radar images at half-hourly intervals between 1500 – 1730 LT on 17 October 2012 showing a locally developed thunderstorm over the western and central Singapore in the early afternoon.
Figure 30: Rainfall data from the automatic weather stations located island wide on 17 October 2012. Rainfall intensity (top) and total rainfall (bottom) for the day.
Waterspout at Pasir Panjang Ferry Terminal - 15 December 2012
Figure 31: 3-channel composite of MTSAT satellite imagery at 1430LT on 15 December 2012
Figure 32: Gradient wind chart (from Bureau of Meteorology, Australia) of synoptic situation in South Asia region on 15 December 2012. The surrounding region and Singapore were mainly under the influence of light converging easterly to east-southeast winds.
Figure 33: Vertical wind profile at Changi Airport on 15 December 2012 between 1200LT and 1600LT depicting a shallow layer of north-easterly winds below 3000ft, turning more easterly aloft with greater wind speed as well.
Figure 34: Radar images at half-hourly intervals between 1330 – 1600 LT on 15 December 2012 showing a localised thunderstorm developed over the south-western coast of Singapore.
Figure 35: Rainfall data from the automatic weather stations located island wide on 15 December 2012. Rainfall intensity (top) and total rainfall (bottom) for the day.
Figure 36: The Oceanic Niño Index for 2012 (Source: NOAA NWS CPC). Black: Neutral conditions; Blue: La Niña conditions.
Figure 37: 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.
Figure 38: Percentage difference of 2012 monthly rainfall from the mean (1869-2011) for the climate station at Changi.
Figure 39: MJO fields (bold and dotted lines) superimposed on Outward Long wave 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.
Figure 40: 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 41: Percentage difference of annual rainfall in 2012 from the long-term means for Singapore.
Figure 42: Isohyets of annual rainfall in 2012 for Singapore.
Figure 43: Monthly statistics of rainfall, maximum and minimum temperature at the climate station located at Changi.