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SINGAPORE: June to September this year was more humid than normal. This is partly due to the intense moisture-laden winds that blow from the tropical Indian Ocean and the Java Sea towards Southeast Asia.
According to the Singapore Meteorological Service (MSS) report, June was the wettest and September the second wettest in the last 10 years.
Throughout this season, increased rainfall was observed in the western part of Singapore.
Monthly precipitation anomaly (in%) in June 2020 (left) relative to the 1981-2010 average. A positive anomaly (blue) implies that the precipitation is greater than the long-term average. (Chart: MSS, Singapore)
Monthly precipitation anomaly (in%) in September 2020 relative to the 1981-2010 average. A positive anomaly (blue) implies that the precipitation is greater than the long-term average. (Chart: MSS, Singapore)
Aside from an increase in seasonal average rainfall, Singapore also experienced more rainy days during this period.
The Changi weather station experienced a total of 73 rainy days in the season compared to the long-term average of 53 days over the past three decades, making it the wettest season in terms of the most rainy days. at the Changi weather station since 1982.
During this season there were several events with extremely heavy rains, such as June 23, August 23 and September 12.
Despite its extensive drainage system, several flash flood events still occurred, such as the filling of the Bukit Timah canal on September 12.
Time series of seasonal anomalies of rainy days in the Changi climate station during June-September 1982-2020. The seasonal anomaly is the difference between the total number of rainy days during June-September 2020 and the long-term average of the seasonal rainy days. (Graphic: Authors)
PATTERNS OF THE SOUTHWEST MONSOON TRADITIONALLY CONTRIBUTE TO RAIN FROM JUNE TO SEPTEMBER
While the June to September anomaly is curious, it is worth reminding us that Singapore receives a lot of heavy rainfall. Being a tropical country, Singapore experiences rainfall throughout the year.
The rains affect all Singapore residents, which has a great impact on our daily activities and our travels. Singapore’s rains are largely influenced by the northeast (winter) and southwest (summer) monsoons.
While the winter monsoon (December to March) causes the highest rainfall of the year, the Southwest monsoon (June to September) also plays an important role in Singapore’s climate.
The weather and rainfall in Southeast Asia during June to September are largely controlled by wind flow patterns associated with the Southwest monsoon system.
In Singapore, the low-level wind generally blows from the south or southeast during June to September. The combination of the moisture-laden wind from the warmer ocean and the monsoon rain band near the equator causes moderate to heavy rains in the morning and afternoon.
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THE ROLE OF SUMATRA SQUALL
But the question remains why is this increase in rainfall happening and how concerned we should be.
In addition to the southwest monsoon, the rainfall in Singapore during June to September is dependent on the number of Sumatran storm events.
Sumatra Squall is an organized storm line that generally develops over the island of Sumatra or the Straits of Malacca at night, then moves eastward to affect Singapore and Peninsular Malaysia.
Typically, the Sumatra storm can bring about one to two hours of rain before dawn or early morning thunder.
Interestingly, this year, Singapore has experienced a higher number of storm events than Sumatra which plays a significant role in the higher amount of rainfall in Singapore and several flash flood events.
CONDITIONS OF LA NINA IN THE TROPICAL PACIFIC OCEAN
Singapore’s seasonal rainfall varies from year to year and is also largely controlled by the El Niño Southern Oscillation (ENSO).
Moderate El Niño conditions mean drier and warmer days for Singapore.
ENSO has three phases, El Niño, La Niña, and a neutral phase. During a La Niña episode, the cooler-than-normal sea surface temperature over the eastern tropical Pacific Ocean is accompanied by an increase in the sea level pressure gradient from east to west that drives the trade winds.
The opposite occurs with the episodes of Niño.
A La Niña condition, like the one Singapore is experiencing in recent days, helps frequent and intense convections and deep rains develop over the maritime continent.
Developing La Niña conditions likely played a role in increased rainfall during the latter part of the season (June to September) in 2020.
However, the higher seasonal rains during the early 2020 season (in June) were unlikely to be due to La Niña, as La Niña had not developed properly at that time.
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CLIMATE CHANGE
Like all parts of the world, Singapore is also experiencing the impact of climate change. The wettest conditions in Singapore in recent decades is one of the key indicators of climate change in this region.
Total annual rainfall for Singapore since 1980 has increased to an average of 67mm per decade (through 2019), while the annual number of days with very heavy rainfall (hourly rainfall exceeding 70mm) has increased by 0, 2 days per decade.
Despite occasional flash floods during heavy rains, Singapore’s infrastructure is more than capable of draining most of these rains.
However, rising sea levels in this region may pose new challenges in the coming decades, if the trend of increasing rainfall continues. Are we ready for a wetter Singapore in the future?
Dr. Dhrubajyoti Samanta is a Principal Investigator at the Asian School of the Environment at Nanyang Technological University (NTU). Professor Benjamin P. Horton is the director of NTU’s Singapore Earth Observatory.
