TMJC 2025 JC1H2 C2 Lecture 4 Tropical Cyclones

Higher 2 (9173) Tropical Environments TMJC Lecture 4 Tropical Cyclones 2025 Page 1 of 12 LECTURE OBJECTIVES • Explain the spatial and temporal patterns of tropical cyclones occurrence • Explain the surface and atmospheric conditions that lead to the formation of tropical cyclones (A) WHAT ARE TROPICAL CYCLONES Tropical cyclones are intense storms that originate over warm tropical seas, characterised by a low-pressure center surrounded by swirling spiral bands of thunderstorms that produce torrential rain and sustained winds of at least 119 km/hour. They are another mechanism by which surplus energy is transferred away from the tropics Fig. 1 - Aerial view of Hurricane Sandy at the east coast of the USA on October 27, 2012 Tropical cyclones are known as hurricanes and typhoons in different parts of the world. They are known as hurricanes over the North Atlantic Ocean, cyclones over the Indian Ocean and typhoons over the Pacific Ocean.

2 (B) The Structure of a Tropical Cyclone Structure Tropical cyclones have distinctive sections: the eye, eyewall and rainbands (Figs. 3-6) The centre of a tropical cyclone is called the eye. The eye is 30 -50km in diameter, has very low pressure, light winds, no clouds and no rain because of subsiding air in the eye. Due to the calm, sunny and clear conditions at the eye of the storm, it gives a deceiving impression that the storm has passed. The eye wall is a band around the eye , where there is spiralling rising air, very strong winds around 160km/hr, storm clouds and torrential rain. The greatest wind speeds and rainfall intensity within the tropical cyclone are found in the eye wall. Rainbands are found away from the eye wall. Spiralling rising air is also found here. However, wind speed and rainfall intensity is lower than in the eye wall, and temperatures are higher. Note: 1. Air is only subsiding at the eye, while rising at different parts of the tropical cyclone. 2. Temperature is highest and pressure is lowest at the eye. 3. Heaviest precipitation and greatest wind speed is at the eye walls. 4. Due to the tropical cyclone spinning anti-clockwise in the Northern Hemisphere, upon contact with the tropical cyclone, wind direction is from the north-west, followed by calm conditions at the eye, followed by wind direction from the south -east on the other side of the tropical cyclone (i.e. wind directions are reversed on opposite sides of the tropical cyclone). Size The diameter of the whole tropical cyclone may be as much as 800km across in diameter, and in terms of height it normally reaches all the way to the tropopause. Associated As the storm is passing over land, the weather phenomenon experienced is strong wind blowing from a prevailing direction bringing with it rain and storm surges. The high

3 Weather conditions intensity rainfall may exceed 500mm in 24 hours (Note: For reference, Singapore's average total annual rainfall is 2340mm). This is followed by a period of calmness when the eye passes before the return of destructive prevailing winds and intense rain again. However, this time the prevailing wind will be from the opposite direction.

4 (C) Spatial and Temporal occurrence of Tropical Cyclones Location Tropical cyclones normally form 5-200 North & South of the equator. They do not form at the equator because sufficient Coriolis force is required to cause the winds to spin. Also do not form at higher latitudes because there is insufficient heating of the ocean to cause intense instability and high levels of evaporation. T hey generally form over the Western parts of oceans where surface temperatures are warmest (Recall: Walker circulation). Most of these systems actually form between 10-30o away of the equator, and 87% form no farther away than 20 o latitude N/S. Because the Coriolis effect initiates and maintains tropical cyclone rotation, tropical cyclones rarely form or move within about 5 o of the equator, where the Coriolis effect is weakest. It seldom form beyond 20 o because of the STHPB. Course Tropical cyclones generally move westwards and polewards , often on erratic and unpredictable courses. This is due to the easterly trade winds at the low latitudes which blow from east to west. They also tend to move away from the equator due to the Coriolis force. Duration The average lifespan of a tropical cyclone is 7 to 14 days . The tropical cyclone rapidly dies off once the source of heat is removed such as when it moves over colder water, or a land surface leading to insufficient supply of moisture. Timing Tropical cyclones occur during the summer and early fall, as these months are the hottest months of the year allowing the sea surface temperature to be warm enough Thus, tropical cyclones develop from June to October in the northern hemisphere, and from December to May in the southern hemisphere. Direction of rotation Tropical cyclones in the northern hemisphere will spin anti -clockwise in the northern hemisphere and clockwise in the southern hemisphere, due to the Coriolis force.

5 Examples of Spatial and Temporal distribution of tropical cyclones Hurricane Katrina (August 2005) Northern Hemisphere → Approaching New Orleans Cyclone Idai (March 2019) Southern Hemisphere → Approaching Mozambique

6 (D) How do tropical cyclones form? Tropical cyclones start as small-scale tropical depressions, an area of low pressure due to warm air rising. Thunderstorms result, which persist for at least 24 hours, and may develop into tropical storms with greater wind speeds of up to 118km/hour. Only about 10% of tropical depressions become tropical cyclones, with wind speeds of at least 119km/hour. In order for tropical cyclones to form, there are several essential conditions that must be present first: (Note: When explaining formation of tropical cyclones, you must state the essential conditions required first.) Surface and Atmospheric Conditions required for the Formation of Tropical Cyclones 1) Ocean water that is heated to at least 28 0C to a depth of 60m (note: some books state 26 0C/270C and 50km/70km depth, both are acceptable but be consistent in your answers). The high temperature is essential because waters of such temperature will cause the overlying air to become unstable and rise to form clouds. Furthermore, the high water temperatures will cause a high rate of evaporation of the ocean water and make the air more humid. The large depth of high temperature of ocean water is required as the storm winds will stir up the ocean water and bring deeper cooler waters to the surface. Thus, the deeper water needs to be sufficiently heated in order to continue the formation of the tropical cyclone. As a result, tropical cyclones usually form over tropical waters where there is sufficient heating of the ocean waters to a great depth. In fact, two - thirds of all tropical cyclones form between 100 and 200 North / South of the equator. 2) There must be sufficient Coriolis force acting on the winds . This condition is essential because sufficient Coriolis force enables the winds to be deflected and spiral around a low-pressure centre. Thus, tropical cyclones rarely form at less than 5 degrees North / South, due to the insufficient Coriolis force at such low latitudes below 5 degrees (Recall: The greater the latitude, the greater the Coriolis force). 3) Air up to about 5500m needs to have a humidity of at least 75% when it is pulled into the storm. This condition is essential because there must be a large supply of moisture to provide the latent heat of condensation to drive the storm formation and provide heavy rainfall. 4) Pre-existing winds (those not created by the storm) should be coming from nearly the same direction and speed at all altitudes. This means that there must not be strong winds that are disrupting the organised p