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Unseasonally wet weather between June and September in 2020: MSS annual review

Unseasonally wet weather between June and September in 2020: MSS annual review

A woman pushes a pram on the road during heavy rain in Singapore. (File photo: Jeremy Long)

SINGAPORE: A hot and dry start to 2020 gave way to unseasonally wet weather during the southwest monsoon season, with record rainfall in June and September, said the Meteorological Service Singapore (MSS) in its annual weather review released on Tuesday (Jan 19).

The southwest monsoon season in the third quarter is typically drier than the rest of the year, but the total rainfall for June to September 2020 was 30 per cent above the long-term average, said MSS.

The two highest islandwide average monthly rainfall totals were in June and September. The June rainfall of 310.1mm was 99 per cent higher than the month's long-term monthly average, while September's 302.4mm was 63 per cent above the norm.

Commentary: Why that unusually high rainfall in Singapore during the last summer monsoon may be our new normal

Meanwhile, from January to March, the monthly rainfall was more than 30 per cent below average.

There were also more frequent intense heavy rainfall events in 2020, with six days of heavy downpours. This was the most number of days since 2013.

A WET JUNE, SQUALLS IN OCTOBER

June 2020 was the wettest June in 10 years, with total rainfall of 233.8mm just at the Changi climate station. ​​​​​​​It rained 21 days that month, the highest in the last 30 years.

The chart below shows the rainfall recorded at the Changi climate station, not islandwide.

The heavy thundery showers on several days, particularly on Jun 23, brought intense rainfall and contributed to flash floods across Singapore.

READ: Singapore experienced its second coolest June in 20 years, wettest in a decade

This was mostly due to strong solar heating of land areas, combined at times with the passage of Sumatra squalls.

In October, 14 Sumatra squalls passed over Singapore, the highest number for October since 2010, partly due to several tropical cyclones over the South China Sea and western Pacific Ocean.

On average, Singapore experiences 45 Sumatra squalls a year, with most passing between April and November. In 2020, about 50 Sumatra squalls crossed over Singapore.

8TH HOTTEST YEAR

While cooler than prior years, 2020 was still the eighth hottest year on record. The annual mean temperature was 28 degrees Celsius, 0.5 degrees above the long-term average of 27.5 degrees Celsius.

This comes after a sweltering 2019, when the mercury hit 28.4 degrees Celsius on average, equaling record temperatures for 2016.

The mean temperature for the last 10 years from 2011 to 2020 was 27.94 degrees Celsius, just marginally lower than the previous warmest decade from 2010 to 2019 (27.95 degrees Celsius).

No long-term monthly temperature records were broken in 2020, but in most months, the temperature stayed above the 30-year mean monthly average.

Besides being unseasonally rainy, June and September were also cooler than normal. June 2020 was the second coolest for the month in 20 years (28.1 degrees Celsius), while September was the coolest in 10 years (27.5 degrees Celsius), tying with September 2013.

LA NINA

The wetter second half of the year can be attributed in part to La Nina conditions that developed in the third quarter of 2020 and continued to strengthen during the rest of the year.

La Nina occurs when the sea surface temperature in the eastern tropical Pacific is unusually cold and strong winds blow warm seawater away from South America and towards Indonesia.

It's the opposite phase of the El Nino effect; and while El Nino brings hot and dry weather to Southeast Asia, its sister carries rain.

Both are caused by changes in ocean temperatures and atmospheric pressure.

La Nina events tend to have a stronger effect on Singapore’s rainfall during the southwest monsoon season, where they bring more rain, said MSS.

Source: CNA/hm(rw)

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