Every year, Singaporeans trade the scorching sun for torrential rains when the monsoon season arrives. Streets glisten under constant showers, temperatures dip slightly, and talk of “monsoon surges” and “Sumatra squalls” pops up on the news. What’s really going on with Singapore’s monsoons? In simple terms, a monsoon is not a single storm but a seasonal wind pattern – and in our region, those winds bring dramatic shifts in rainfall. Let’s break down the science of Singapore’s monsoon, why we have two monsoon periods, and how this year’s monsoon compares to those in the past.
What Exactly Is a Monsoon?
The word monsoon comes from the Arabic mausim, meaning “season.” It refers to a seasonal reversal in wind direction. In many parts of Asia, monsoons are the defining feature of climate – they create a wet season and a dry season each year. The classic example is the Indian subcontinent, where the southwest monsoon brings summer rains and the northeast monsoon brings a drier winter. Singapore, being near the equator, experiences a less extreme but still distinct monsoonal pattern.
Here, we have two monsoon seasons:
- Northeast Monsoon (Wet Season): Generally from December to early March, winds blow from the northeast. This is our wetter monsoon. It often brings steady rains and the occasional “cool spell” when winds from the South China Sea set in for days (the monsoon surges). December and January typically see frequent showers, sometimes continuous over multiple days when surges occur.
- Southwest Monsoon (Drier Season): Roughly from June to September, winds come from the southwest. Paradoxically, this monsoon is usually less wet for Singapore – it’s often characterized by early morning thunderstorms (like the famous Sumatra squalls that roll in from Sumatra across the Straits) and slightly drier, hazier afternoons. July is often one of the drier months of the year for us.
Between these monsoons, we have inter-monsoon periods (around April-May and October-November) where winds are lighter and more variable, and thunderstorms tend to pop up in the afternoons due to daytime heating.
So why do these wind patterns flip? It’s all about differential heating of land and sea. In simpler terms, continents and oceans heat up and cool down at different rates with the seasons. During our Northern Hemisphere winter (Dec-Jan), the Asian landmass (China, etc.) is colder than the surrounding oceans. Winds blow outward from the high-pressure cold areas over Asia towards the warmer ocean – hence northeasterly winds come down towards Southeast Asia. Those winds pick up moisture over the South China Sea and drop it on us as rain. Six months later, in June, Asia is warmer than the oceans, so the winds reverse, coming from the ocean (southwest) towards the land – bringing a different pattern of weather.
In Singapore’s context, even the “dry” monsoon brings a fair bit of rain (we’re in the tropics, so thunderstorms can happen anytime). But you might notice differences: Northeast Monsoon rain can be steady and longer-lasting (stratiform rain), while Southwest Monsoon rain often comes in short, intense bursts (thunderstorms).
Why Does It Rain So Much During the Northeast Monsoon?
During the Northeast Monsoon, two key phenomena cause rain: monsoon surges and the shear line. A monsoon surge is when a surge of cooler air from the north intensifies the monsoon winds, as mentioned earlier. This typically results in days of continuous rain and windy conditions. Singapore averages 2–4 monsoon surges each NE monsoon season (Meteorological Service Singapore). When you hear the weatherman say a surge is coming, you know to expect a very wet spell.
The “shear line” is a bit technical: it’s a boundary where winds from northeast and east (for example) converge. These often set up over the equatorial region during monsoon season. A shear line can trigger bands of rain and thunderstorms as the converging winds force air upward. Think of it like a mini cold front in the tropics. If you’ve ever woken up to a pre-dawn thunderstorm that isn’t a Sumatra squall, it could be due to one of these monsoon shear lines hovering over us.
There’s also the role of the Madden-Julian Oscillation (MJO) – an intraseasonal climate pattern. The MJO is like a pulse of storminess that moves eastward around the equator every 30-60 days. When the MJO is in our region, it can enhance rainfall (NOAA Climate.gov). Meteorologists sometimes mention the MJO when explaining why a particular fortnight is rainier than another, even within the same monsoon.
All these factors mean the Northeast Monsoon isn’t a steady state; it has active phases (lots of rain) and occasionally “dry” phases (where for a week or two, rainfall might ease, especially toward late January or February). Historically, January tends to be the peak of rainfall. For instance, January 2021 broke records with 648.4 mm of rain in the first half of the month alone (Meteorological Service Singapore).
Adapting and Understanding
Understanding the science behind our monsoons helps in appreciating why each year can feel different. The mechanics (wind patterns) remain consistent, but their strength and timing vary due to global patterns like La Niña/El Niño, as well as random variability. Singapore’s location near the equator also means we get thunderstorms year-round; we don’t have a true “dry season” the way Bangkok or New Delhi does. Even in our drier monsoon, an average month can have 12-15 rain days.
As climate change progresses, scientists are closely watching monsoonal behavior. The concern is that the Asian monsoons could become more erratic – possibly delivering the same or slightly more rain overall, but in more intense bursts with longer dry spells in between. Historical data from the 1980s compared to now shows a slight uptick in total annual rain and in the number of days with very heavy rain (Meteorological Service Singapore).
In short, Singapore’s monsoons are a tale of two seasons: the Northeast Monsoon which drenches us, and the Southwest Monsoon which gives a bit of a breather (albeit with some dramatic storms). This year’s monsoon might feel extra intense because of factors like La Niña amplifying the rains, but it’s all part of the broader monsoonal rhythm that has existed for millennia in this region.
So the next time the monsoon clouds gather, take a moment to appreciate the science behind the storm. It’s not just random bad luck – it’s the complex dance of seasonal winds, ocean temperatures, and geography at play.
