SINGAPORE: Last year's southwest monsoon season between June and September was the third wettest for Singapore since 1981, with total rainfall across the island coming in at 30 per cent above the long-term average for the period, according to an annual climate assessment released on Tuesday (Mar 23).
The southwest monsoon season is typically the drier period of the year, however, June and September 2020 recorded the two highest islandwide average monthly totals of 310.1 mm and 302.4 mm respectively, said the report by the Meteorological Service Singapore (MSS).
This was 99 per cent and and 63 per cent above their respective long-term monthly averages.
Previously, MSS reported that June 2020 was the second coolest June in 20 years and the wettest in 10. It ended a record 28 months of above-average monthly temperatures since February 2018.
In Tuesday’s report, the MSS said it rained 21 days in June, the highest in the last 30 years. It surpassed the previous high of 19 rain days in June 2010.
“The wet weather in June 2020 was mostly due to strong solar heating of land areas, at times coupled with large-scale convergence of winds in the surrounding region, and to the passage of Sumatra squalls on some days,” said the report.
“The heavy thundery showers on several days particularly on 23 June 2020 brought intense rainfall and contributed to flash floods at various locations in the island.”
A Sumatra squall is an organised line of thunderstorm developing over Sumatra or the Straits of Malacca and moves eastwards to affect Peninsular Malaysia and Singapore. According to the climate assessment, the squall line typically brings one to two hours of thundery showers.
WETTER-THAN-AVERAGE SOUTHWEST MONSOON SEASON
La Nina conditions developed in the third quarter of 2020, which contributed to the wetter-than-average southwest monsoon season last year.
“Based on historical observations, La Nina events tend to have a stronger effect on Singapore’s rainfall during the southwest monsoon season, where they bring wetter- than-average conditions,” said the report.
El Nino and La Nina are two opposing climate patterns in the Pacific Ocean that affect weather worldwide. They are part of the El Nino-Southern Oscillation.
El Nino and La Nina events tend to develop between May and October and they tend to influence the southwest monsoon conditions in Southeast Asia, said the report. However, not all southwest monsoon seasons are influenced by an El Nino or La Nina event.
COOLEST DAY, HIGH OCCURRENCE OF SUMATRA SQUALLS
The coolest day last year was on Sep 16, when the temperature fell to 20.9 degrees Celsius at Newton.
This was due to a Sumatra squall developing under the "indirect influence" of Tropical Storm Noul over the South China Sea.
The "widespread and intense thundery showers" over Singapore followed by light to moderate rain cooled the air and caused the temperature to fall to 20.9 degrees Celsius between 3.30am and 4am.
October also saw 14 Sumatra squalls passing over Singapore, the highest number for October since 2010.
This was partly attributed to the presence of several tropical cyclones over the South China Sea and western Pacific Ocean, which altered wind flow over equatorial Southeast Asia.
This created favourable conditions for the squalls to develop and move east.
READ: Commentary - Why that unusually high rainfall in Singapore during the last summer monsoon may be our new normal
EIGHTH WARMEST YEAR IN SINGAPORE
The climate report also showed that 2020 was the eighth warmest year in Singapore based on records since 1929.
With an annual mean temperature of 28 degrees Celsius, it is on par with 2005, being 0.5 degrees Celsius higher than the 1981-2010 long-term average.
Globally, significant parts of the world also recorded above average temperatures last year. The global average temperature for 2020 was about 0.5 degrees Celsius above the 1981-2010 long-term average and 1.2 degrees Celsius above the pre-industrial baseline of 1850-1900.
This was one of the top three warmest years on record globally together with 2016 and 2019.
“Since (the) 1980s, each decade has been warmer than the previous one, highlighting the persistent long-term climate change trend,” said the report.