S’pore’s year-end rains to coincide with two rain-bearing systems, but wetter weather unlikely

SINGAPORE - Two different rain-bearing climate phenomena are expected to be felt in Singapore by December, when the country usually experiences its wettest weather.

But their arrival may not increase total rainfall over Singapore during the year-end holidays, experts told The Straits Times, as these events tend to influence rainfall during other parts of the year instead.

The Republic’s usual rainy season in December and January is caused by the

wet phase of the north-east monsoon

, when winds blow from the cool Northern Hemisphere towards the equator.

This period will also usually see one to three episodes of monsoon surges, which results in higher rainfall over Singapore,

as was the case in January

.

These surges refer to bursts of cold air from the north-east, such as Central Asia. As the cool air flows over the warm waters of the South China Sea, it picks up moisture, bringing widespread rainfall, strong winds and cooler weather to Singapore.

But ST checks show that in 2025, the rainy season will coincide with the

return of La Nina

and the negative phase of the Indian Ocean Dipole (IOD).

Both climate phenomena tend to result in higher rainfall over Singapore, owing to changes in atmospheric pressure and sea surface temperatures across the Pacific and Indian oceans. The Republic sits between both ocean basins, and can be influenced by weather patterns there.

The Meteorological Service Singapore under the National Environment Agency noted on its website that Singapore is on “La Nina watch”, which it says is expected to persist until at least the end of 2025.

In response to ST’s queries, a spokesperson for the Met Service said it is likely that La Nina conditions have already been established, based on preliminary assessments of sea surface temperatures in October.

Separately, the Met Service also noted on its website that a negative IOD is present, and that it is expected to persist until December.

Professor Matthias Roth from NUS’ department of geography said: “A developing La Nina paired with a negative IOD typically enhances rainfall, making the November to December period, and the early north-east monsoon season, wetter.”

These climate phenomena have a wide footprint, and can influence rainfall patterns differently across the entire ocean basin.

In Singapore, the Met Service told The Straits Times that La Nina events usually have no noticeable effects on Singapore’s total rainfall during the December to January period.

Instead, they increase the likelihood of wetter conditions during the dry phase of the north-east monsoon season,

between February and March

. During this phase, the monsoon rainband moves farther south towards northern Australia, causing Singapore to experience slightly drier conditions.

Similarly, negative IOD events “tend to have little effect on Singapore’s rainfall during the north-east monsoon season”, the Met Service spokesperson added.

The spokesperson added that negative IOD events usually increase the likelihood of wetter conditions

during the second inter-monsoon period

, which is from October to November.

Inter-monsoon periods, or the transitions between the monsoon seasons, occur gradually, generally over a period of two months. In Singapore, there are two inter-monsoon periods each year: late March to May, during the transition from the north-east to the south-west monsoon season, and October to November, when the reverse happens.

“Since negative IOD events end before the rainy season reaches the Southern Hemisphere, they tend to have little effect on Singapore’s rainfall during the north-east monsoon season,” the Met Service spokesperson added.

Dr Koh Tieh Yong, who specialises in atmospheric physics at the National University of Singapore, said that this La Nina is predicted to be weak, and that the impacts attributed to it are not expected to be pronounced.

He added: “Having said so, other influences can still bring severe weather to Singapore quite apart from La Nina... So a weak La Nina does not imply the lack of severe weather per se.”

For instance, the Madden-Julian Oscillation – an eastward-moving pulse of enhanced cloud and rainfall near the equator – can also bring more rain over Singapore when it “cycles” over the Republic.

Some relief in the form of cooler temperatures are also on the cards for the upcoming holiday season.

The Met Service said that La Nina events tend to have a moderating effect on Singapore’s temperature, compared with the long-term warming trend. For example, La Nina conditions in December tend to lead to a less-warm hot season during March to May, it added.

But experts also say that climate change could gradually weaken La Nina’s cooling effect.

“Due to long-term warming, recent La Nina years are warmer than El Nino years from the 1980s,” added the spokesperson.

El Nino is a “twin” climate phenomenon to La Nina, causing drier weather and warmer temperatures over Singapore.

Dr Koh added: “Global warming has indeed resulted in higher annual mean temperature during La Nina years compared with more than three decades ago.”