Singapore's temperatures rising despite La Nina's cooling effect

An extra-cool spell triggered by the weather phenomenon known as La Nina was not enough to offset soaring temperatures in Singapore, said the weatherman in the country's annual climate assessment report yesterday.

Last year's annual mean temperature of 28 deg C was half a degree higher than the long-term average, making 2020 the eighth warmest year on record, according to Meteorological Service Singapore's Centre for Climate Research Singapore.

There have been a record 28 consecutive months of warmer-than-average mean temperatures that began in February 2018 and ended last June, when "unseasonably wet conditions" contributed to cooler temperatures all over the island.

Associate Professor Koh Tieh Yong, a weather and climate scientist at the Singapore University of Social Sciences, said the record-breaking temperatures in those months were due to the hotter weather during the El Nino years - 2018 and 2019 - and the positive Indian Ocean Dipole climate phenomenon in the second half of 2019.

"This is part of the natural variability in the tropical atmosphere-ocean system, which is superimposed on a long-term global warming trend," he said.

He expects that future El Nino years - in the middle of the century, for example - would be hotter than a typical El Nino year now.

The opposite climate pattern, La Nina, developed in the third quarter of last year and continued to strengthen. It was stronger compared with previous La Nina events in 2016 and 2017/2018.

La Nina and El Nino are brought about by changes in atmospheric pressure and sea surface temperatures in the tropical Pacific Ocean.

Based on historical data, La Nina events tend to have a stronger effect on Singapore's rainfall during the south-west monsoon season, when they bring wetter-than-average conditions.

The south-west monsoon season is usually from June to September and is typically a drier period.

However, the two highest islandwide average monthly rainfall totals of 310.1mm and 302.4mm occurred in June and September last year.

These values were 99 per cent and 63 per cent above their respective long-term monthly averages.

For the whole south-west monsoon season last year, islandwide total rainfall was 30 per cent above the long-term average for June to September, making it the third-wettest period since 1981.

Last June, the monthly average temperature was 28.1 deg C, 0.2 deg C below the long-term average and the second-coolest June in the past 20 years.

Similar temperature conditions occurred last September, with a monthly average temperature of 27.5 deg C, making that month and September 2013 the coolest Septembers in the last 10 years.

There were also more frequent intense heavy rainfall events over the island last year, with six days of very heavy hourly rainfall exceeding 70mm.

These events led to flash floods at various locations, with last year's highest hourly rainfall of 106mm recorded in August at Bedok South.

Singapore's highest hourly rainfall of 130.7mm was recorded at Ulu Pandan in July 2006.

Despite the bouts of heavy rain, annual total rainfall last year ranked the eighth lowest in 30 years.

The long-term warming trend overshadows La Nina's cooling effects on Singapore's temperatures, with the average temperature during last year's south-west monsoon season slightly above average.

Prof Koh said the mean temperature difference in Singapore between El Nino and La Nina years is usually about 0.5 deg C to 0.6 deg C.

"With the observed secular warming trend of about 0.25 deg C per decade in Singapore over the past 70 years, we may expect that by the middle of this century, a typical 'cooler' La Nina year will be hotter than a typical 'warmer' El Nino year now."

He added that as La Nina typically brings more rainfall to Singapore, the future hot and humid La Nina years could increase the likelihood of heat illnesses, especially among the elderly population.

Innovative land use design, including the use of blue spaces, which refer to bodies of water, and green spaces, ventilation corridors and optimal orientation of housing in new towns, can help keep heat stress under control, he said.

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A version of this article appeared in the print edition of The Straits Times on March 24, 2021, with the headline Singapore's temperatures rising despite La Nina's cooling effect. Subscribe