- Learn | Tropical Circulation
Learn | Tropical Circulation
Tropical Circulations are the primary drivers of weather systems in Singapore
Hadley cells are circulations of air at the tropics consisting of rising air close to the equator and sinking air at the subtropics, around 30° north and south of the equator. They are driven mainly by strong heating from the sun near the equator which causes air near to rise. As the rising air reaches near the Tropopause 10-15 km above the surface, it moves polewards, cools and sinks near the subtropics. The sinking air flows back to the equator as trade winds to complete the circulation.
The trade winds from the Northern and Southern hemispheres meet close to the equator in an area known as the Intertropical Convergence Zone (ITCZ). The strong convergence of warm moist winds in this area result in intense thunderstorms development and the location of the ITCZ can be picked out on a satellite image by a band of thick white clouds in close proximity to the equator.
NE Monsoon (November–March)The NE monsoon typically onsets around November and persists till March each year. During this period, the Northern Hemisphere is experiencing winter and cold conditions develop over much of Continental Asia, Siberia and China. Consequently, high pressure systems in the Northern Hemisphere result in cold subsiding air that flow southwards towards the warmer low pressure areas of the Southern Hemisphere. As the winds, move over the South China Sea, they warm and pick up moisture, depositing them as they approach the equator as spells of rain and thunderstorms. The Monsoon winds are usually in the Northeast or Northwest directions as they cross the equator into the Southern Hemisphere. The start of the Northeast Monsoon between November and mid-January usually results in the wettest months of the year while the tail end of the Northeast Monsoon between end January and mid March are the driest months of the year as the ITCZ migrates to the Southern Hemisphere.
Typical windflow during the Northeast Monsoon. When winds approach the equator, they are usually blowing from the Northwest or Northeast
SW Monsoon (June- September)
The Southwest Monsoon develops during the Southern Hemisphere winter when cold subsiding winds move out of high pressure systems in the Southern Hemisphere towards the low pressure systems in the warmer Northern Hemisphere. The winds approach the equator from the Southwest or Southeast direction and sometimes result in Sumatra Squalls or periods of drier weather that can lead to forest fires and haze in the region.
Typical windflow during the Southwest Monsoon. When winds approach the equator, they are usually blowing from the Southwest or Southeast
Inter-monsoon periods (1st Inter-Monsoon: April–May, 2nd Inter-Monsoon: October-November)
The periods between the NE and SW monsoons are known as Inter-Monsoon periods. There are two inter-monsoon periods, Inter-monsoon periods are generally characterised by light winds and warm temperatures. There are two inter-monsoon periods: the first one is from April – May and the second from October-November.
Typical windflow during an Inter-Monsoon period. Winds around the equator are usually light and variable
A tropical cyclone is the generic term for a rapidly rotating organised convective storm system characterised by a low pressure centre, strong winds, a closed cyclonic circulation, and a warm core at the centre in the middle and upper troposphere. A tropical cyclone develops in stages. It begins as a tropical depression with maximum sustained wind speeds of less than 63km/h. The system is classified as a tropical storm when the maximum sustained wind speed is more than 63 km/h but less than 119km/h. At maximum sustained wind speeds exceeding 119km/h, the tropical cyclone is termed a hurricane, typhoon or tropical cyclone, depending on the ocean basin where it develops. Tropical Cyclones are known as Hurricanes in the Northeast Pacific and North Atlantic. They are known as Typhoons in the Northwest Pacific and are simply known as tropical cyclones elsewhere. Tropical cyclones in the western North Pacific and South China Sea can form throughout the year but are more common between May and November. In the north Indian Ocean, tropical cyclones usually form between April and December. A tropical cyclone typically lasts between 3 to 7 days, with some weak ones dissipating before they intensify or hit land. If they develop and stay in favourable warm seas, they can last up to weeks.
Typhoon Hagupit was the most intense tropical cyclone to affect the Philippines in 2014. Here it is seen with a well-formed eye at its core and bands of thunderstorm clouds extending thousands of kilometres. (Image credit: NASA)
The main requirements for tropical cyclone formation include sufficiently warm sea surface temperatures, atmospheric instability, low to moderate vertical wind shear, and enough Coriolis force (caused by the rotation of the Earth) to sustain a low pressure centre. In general tropical cyclones do not occur near to the equator as the Coriolis force is weak or negligible here. The development of Tropical Storm “Vamei” near Singapore in December 2001 was the result of a strong and persistent Northeast Monsoon wind surge interacting with a circulation system in the southern part of the South China Sea, which created a large background cyclonic vorticity near the equator. The simultaneous occurrence of the two interacting systems, leading to the development of a typhoon near the equator, is rare.
There is not enough evidence to suggest that climate change will increase the likelihood of tropical storms occurring in the region surrounding Singapore However, it is possible that climate change could lead to an increase in both storm intensity and rainfall rate for tropical cyclones further north in the SouthEast Asia region.
Tropical cyclones are assigned names to help in the quick identification of storms in warning messages as names are easier to remember than numbers and technical terms. Assigning names to tropical cyclones also makes it easier for the media to report on them, heightens interest in warnings and increases community preparedness. The latest list of tropical cyclone names could be found here (link to wmo page).
Tropical cyclones in the Northwest Pacific and South China Sea are named by the surrounding regional countries. They are selected based on the familiarity with the people in the region and are not in any alphabetical order or named after any person. Singapore does not contribute to the naming of tropical cyclones as cyclones rarely affect Singapore unlike the other South East Asian countries.
The names of significant tropical cyclones in the North Atlantic Ocean, Pacific Ocean, and Australian region are retired from the naming lists and replaced with another name.
The El Niño phenomenon is a non-regular occurrence in the tropical pacific region where warmer waters develop over the Eastern Tropical Pacific Ocean along the coast of South America. In South East Asia, this brings drier weather conditions and increases the risk of forest fires and smoke haze developing. The La Niña phenomenon is a reverse of the El Niño where cooler waters develop over the Eastern Tropical Pacific Ocean along the coast of South America. In South East Asia, higher than normal rainfall that occurs during a La Niña episode may result in an increased occurrence of floods.