The tropical equatorial climate experienced in Singapore is known for its high amounts of rainfall and high temperatures year-round. However, the weather over the past few nights has been characterised by relatively colder temperatures of about 21oC – 23oC over many parts of Singapore.

How may we understand this phenomenon within the context of weather and climate?

Understanding how pressure and wind affect temperature

On 10th January 2018, the National Environment Agency (NEA) issued an update on its website stating that a monsoon surge has been present in the South China Sea. This being January, the Northeast Monsoon dominates the weather in south, east and southeast Asia. Given the little protection afforded by the southern tip of Peninsular Malaysia which surrounds Singapore’s northern coastline, Singapore typically receives relatively more moisture during the initial parts of the Northeast Monsoon period compared to other parts of the year, blown in by northeasterly winds which pick up moisture from the South China Sea.

In turn, this higher moisture content, coupled with local conditions of atmospheric instability arising from the interplay between a few types of vertical temperature differentials known as lapse rates, translates into an increase in the frequency and magnitude of rainfall during the November – January period in Singapore.

As seen in the isobar map of the South China Sea (Figure A), there is a noticeable high pressure zone at the surface of the South China Sea with a large pressure gradient (red isobars). The effect of this gradient through the Pressure Gradient Force, coupled with the Coriolis effect which causes a deflection of winds to the right relative to the downwind direction in the Northern Hemisphere, is probably causing winds with higher wind speeds to blow out from the part of the South China Sea adjacent to northern Philippines down towards Peninsular Malaysia and Singapore.

These winds can be seen in the wind speed map below (Figure B), which indicates the northeasterly direction of the winds blowing across the South China Sea as well as the higher wind speeds within the region which range from 45 to 65 km/hr, compared to nearby vast areas of water such as the Indian Ocean or the western Pacific Ocean over which there are maximum winds speeds with half or one-third the magnitude of those over the South China Sea. Another interesting pattern which the wind map elucidates is the relatively lower wind speeds in the Gulf of Thailand, the Gulf of Tonkin, and the Sulu Sea. These areas, unlike Singapore, are shielded by landmasses which prevent them from receiving winds with higher speeds and moisture.

With wind being a key agent by which moisture is transported over the oceans, these apparent conditions of lower wind speeds is the rainshadow effect of landmasses at play during the period of the Northeast Monsoon.

What might this mean for temperature lapse rates and, hence, rainfall in Singapore?

Atmospheric instability which drives cloud formation and, hence, rainfall, comes about when air parcels carrying moisture are cooling at a slower rate than the environmental lapse rate which is the vertical variation of temperature from the ground level. With relatively lower temperatures of 21.7oC – 22.8oC at night (Figure C), there is a possibility that the environmental lapse rate is higher than usual, meaning that the environment might be cooling faster than usual to the point where the temperature of the environment is cooler than that of dry and saturated air parcels (indicated by the Dry Adiabatic Lapse Rate and Saturated Adiabatic Lapse Rate respectively) in it, resulting in prolonged rainfall throughout the night.

Of course, this note on lapse rates in Singapore is merely speculative, so long as there is no readily available real-time data on vertical variations in temperature for reference.

A question of the effect of anthropogenic climate change?

One question to be asked about the cooler temperatures is whether this might be the result of anthropogenic climate change. The short answer is maybe. The slightly less short answer is that there is some evidence to show that monsoon variability in Asia has been affected by anthropogenic climate change, but the literature is certainly not conclusive. A longer-term study of temperature lows and highs is probably necessary in order to search for significant patterns.

In any case, the difficulty in answering whether climate change can be said to have resulted in the cooler temperatures and heavier rainfall experienced in Singapore over the past few days is that a plethora of time-specific and localised factors need to be accounted for before monsoon variability in the South China Sea can conclusively be linked to the variation of temperature and rainfall in Singapore. The temporal challenge lies in the fact that such a long-term study would have to look at data over a protracted period of time, while the spatial challenge lies in the fact that the monsoon involves the effect of pressure patterns roughly 2000 km away from Singapore on the nation’s local precipitation and temperature patterns, which would render the consideration of other factors such topography or seasonal changes in sea surface temperature, for example, necessary.

Regardless of whether the colder conditions in Singapore are a sign of changing climatic conditions in our region or whether they are the result of quickly changing weather patterns, the spatial scale on which the Northeast Monsoon operates and causes different weather phenomena in different places including those much further afield from the high pressure zone in the South China Sea, such as Singapore, is certainly noteworthy.