SINGAPORE - 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 on Tuesday (March 23).

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 the Meteorological Service Singapore's Centre for Climate Research Singapore.

There has been a record 28 consecutive months of warmer-than-average mean temperatures, which began in February 2018 and ended in June last year, at which point "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 that the record-breaking temperatures during these months were due to the hotter weather during the El Nino years - 2018 and 2019 - and the positive Indian Ocean Dipole 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 - say, in the middle of the century - would be hotter than a typical El Nino year now.

The opposite climate pattern, known as La Nina , developed in the third quarter of 2020 and continued to strengthen throughout the year. It was stronger compared with previous events in 2016 and in 2017/2018.

Both 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 it is typically a drier period of the year.

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 had occurred again in September, with a monthly average temperature of 27.5 deg C, joint with September 2013 as the coolest Septembers in the last 10 years.

There were also more frequent intense heavy rainfall events over the island in 2020, with six days of very heavy hourly rainfall exceeding 70mm.

These rainfall events led to flash floods at various locations, with the highest hourly rainfall of 106.0mm recorded in August in Bedok South. The highest hourly rainfall total of 130.7mm was recorded in Ulu Pandan in July 2006.

However, despite these bouts of heavy rainfall, annual total rainfall still ranked the eighth lowest over the past 30 years.

In addition, the long-term warming trend overshadows the cooling effect La Nina has on Singapore's temperatures, as the average temperature during south-west monsoon season was only slightly above average.

Prof Koh said that the difference in Singapore's mean temperature between El Nino and La Nina years is usually around 0.5 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 Niña 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 chance of heat illnesses especially among the elderly population.

To mitigate this, innovative land use design, such as the use of blue and green spaces, ventilation corridors, and optimal orientation of housing in new towns, can help keep the heat stress under control, he added.