Wettest November on record: What’s behind the rainy weather in Singapore?
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November 2024 was rainier than any of the years prior, with Singapore being inundated by torrential rain.
ST PHOTO: GIN TAY
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SINGAPORE – As the saying goes, “when it rains, it pours” – and that has literally been the case for Singapore in the past few weeks.
Frequent downpours, mainly due to the north-east monsoon season, have made this November Singapore’s wettest since rainfall data collection began in 1980, said the Meteorological Service Singapore (MSS).
With a total rainfall of 419mm, November surpassed the previous record of 390.1mm set in the same month in 1992, the agency said. This represents an increase of 47.4 per cent from the average November rainfall of 284.3mm in previous years.
Thundery showers fell on most afternoons in the second half of November, with record rainfall on Nov 22 triggering flash floods in Yishun and Potong Pasir.
National water agency PUB said that 127.7mm of rain was recorded between 2.15pm and 4.50pm in northern Singapore that day.
The Straits Times spoke with the MSS to examine the science behind the monsoons, and other rain-making phenomena in Singapore.
What are monsoons?
Monsoons are seasonal wind patterns that span a large geographic area, playing a significant role in influencing rainfall across regions throughout the year. They involve shifts in the direction of prevailing winds, which may blow moist air towards or away from land, thus affecting rainfall over it.
In Singapore, two main monsoon seasons shape the weather: The north-east monsoon, typically from December to March, and the south-west monsoon, from June to September. Both are part of the larger Asia-Australia monsoon, which impacts weather patterns across the globe.
During the north-east monsoon, Singapore experiences heavy rainfall and cooler temperatures, while the south-west monsoon brings comparatively drier and hotter weather.
Why does the north-east monsoon bring rain?
The north-east monsoon occurs in two phases: the wet phase, typically from December to January, and the dry phase, from February to early March.
During the wet phase, winds approach from the north-east, and blow towards the south-west.
As winter sets in over continental northern Asia, cold air occasionally surges southwards to the warmer South China Sea and moves towards the tropics.
This cold air interacts with the sea, warming up and absorbing moisture, which leads to increased rainfall over Singapore.
There will be moderate to heavy rain spells that usually last for a few days each, making December and January the wettest months of the year.
During the subsequent dry phase, the monsoon rainband moves further south to northern Australia, bringing drier conditions to Singapore, though the climate remains windy.
What other phenomena bring rain?
Besides the north-east monsoon, other large-scale and localised weather circulation patterns also influence rainfall here.
Large-scale weather patterns influence rainfall over a wide geographical range. For example, the Indian Ocean Dipole impacts rainfall over the Indian Ocean.
In contrast, localised weather circulation patterns influence rainfall over a smaller area, such as the Sumatra squalls that bring rainfall over Peninsular Malaysia and Singapore. These squalls are formed over Sumatra or the Strait of Malacca, and move eastwards to affect Singapore.
What weather conditions affect Singapore’s rainfall?
i. La Nina
La Nina is one of the three phases of the El Nino-Southern Oscillation, a global climate cycle involving changes in winds and sea surface temperatures in the tropical Pacific Ocean. These changes affect weather patterns across the globe.
During the La Nina phase, the trade winds – which are prevailing east-to-west winds near the Equator – strengthen and blow towards the western Pacific Ocean, which includes South-east Asia.
This pushes more warm water towards the region, leading to more evaporation and the formation of rain clouds. This typically results in more rain in Singapore.
According to the MSS’ website
ii. Indian Ocean Dipole (IOD)
The IOD is a natural climate cycle that influences temperature and rainfall patterns across the Indian Ocean.
In its neutral phase, winds blow from the western end of the India Ocean basin towards the east, keeping warm water concentrated around South-east Asia.
During the negative phase, however, the winds intensify towards the eastern end of the Indian Ocean.
As a result, warm water continues to accumulate around South-east Asia, resulting in cooler-than-normal water in the western Indian Ocean, and warmer-than-normal water in the east.
The warmer waters fuel the formation of rain clouds, leading to heavy rainfall over Singapore.
In the positive phase of the IOD, the winds weaken, causing the warm water around South-east Asia to move towards Africa instead. As a result, Singapore experiences drier-than-usual weather.
iii. Madden-Julian Oscillation (MJO)
The MJO is an eastward-moving phenomenon of enhanced clouds and rainfall near the Equator. It takes an average of 30 to 60 days to move around the tropics, bringing more rain than usual to Singapore when it passes over the country.
An active MJO event consists of two phases: the convective phase and suppressed phase. These appear as an eastward-moving “dipole” – akin to a magnet – with two contrasting areas: one with cloudiness and rain, and the other with clear skies.
In its convective phase, winds converge at the surface, pushing moist air upwards through the atmosphere.
At higher altitudes, the winds then spread out. This motion increases condensation, leading to rainfall over Singapore.
In the suppressed phase, the phenomenon reverses direction, thus resulting in drier weather in Singapore.
iv. Sumatra squall
“Shelf clouds”, a characteristic feature of a well-developed Sumatra squall, may sometimes appear in Singapore’s skies.
PHOTO: METEOROLOGICAL SERVICE SINGAPORE
A Sumatra squall is a line of thunderstorms that develops over Sumatra or the Strait of Malacca, often overnight.
It moves towards the east, at times producing heavy rain and gusty winds, as it crosses Peninsular Malaysia and Singapore.
While the science behind the formation of Sumatra squalls is not fully understood, factors which may lead to their formation include the heating up of Sumatra’s land in the day, and the convergence of land breezes in the Strait of Malacca at night.
Sumatra squalls have a persistent mass of cold sinking air, which rushes downward and spreads out near the ground, generating strong winds at the surface.
When this cold air interacts with the warmer air at the front of the line of storms, differences in temperature and density further strengthen these winds.
Squalls can also last longer due to rapid changes in wind speed or direction higher in the atmosphere, causing warm air to rise and new storm clouds to form.
Sumatra squalls usually occur in Singapore during the south-west monsoon and inter-monsoon periods between April and November. They can bring gusty surface winds of between 40kmh and 80kmh, although wind speeds of up to 104kmh have also been recorded.
And if your view is not obstructed, you may even see a “shelf cloud” – an elongated, wedge-shaped cloud – a characteristic feature of well-developed Sumatra squalls.
What are the challenges of weather forecasting in the tropics?
Predicting the weather in the tropics, including Singapore, is particularly challenging, according to the MSS. This is because the moist air and strong convection – the upward movement of warm air – in the region lead to frequent rain showers and thunderstorms. These weather systems typically develop and dissipate quickly, often within one or two hours.
The light prevailing winds in the tropics also make it difficult to track and predict the movement of localised storms, which are limited to a certain geographical area.
In addition, the short lifespan and small size of tropical weather systems present a challenge for meteorologists. Because of these characteristics, numerical models – which employ complex mathematical equations to simulate atmospheric conditions – tend to have lower accuracy in the tropics. They are also less able to give early warnings about weather conditions.
