SAJC Cluster 2 Lecture 4 Notes (Tropical Cyclones & ENSO)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 4_2024/pg1 Lecture 4 Tropical Climates (III): Tropical Cyclones & ENSO and their Influence on Rainfall in the Tropics KEY QUESTION: How does synoptic scale circulations affect rainfall in the tropics?** With the completion of this lecture, attached readings and tutorial, you should be able to understand the: • Atmospheric and surface conditions necessary for the development of tropical cyclones and resultant effects on rainfall patterns • The three phases of the El Niño Southern Oscillation (ENSO) and the associated changes in rainfall over tropical Pacific **Note that this key question was also partly addressed in Lect 3 when we studied monsoons. Please connect the content for monsoons and this lecture when revising synoptic scale circulations and their influence on rainfall in the tropics. Lecture Outline 4.1 Tropical Cyclones 4.1.1 The Anatomy of a Topical Cyclone 4.1.2 Conditions for Formation and Decay of Tropical Cyclones 4.1.3 The Resultant Effects of Tropical Cyclones on Rainfall Patterns 4.1.4 The Saffir-Simpson Scale of Tropical Cyclone Intensity 4.2 El Niño-Southern Oscillation (ENSO) 4.2.1 The Three Phases of ENSO (A) Neutral phase – The Walker Circulation (B) La Niña phase (C) El Niño phase 4.2.2 Changes in rainfall over tropical Pacific due to the El Niño and La Niña The Atacama Desert is one of the most arid places on Earth (see left), but in selected years, flowers may bloom (see right), hardly the norm. This has to do with the location of the desert in South America on the east of the Pacific Ocean. How often do such instances occur, and why?

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 4_2024/pg2 4.1 Tropical Cyclones • By far the most significant atmospheric disturbances in the tropics are tropical cyclones. • These storms are known by different names in different places: hurricanes in North and Central America, typhoons in the western North Pacific, and simply cyclones in the Indian Ocean. (See Fig. 1) • Whatever the name, these are intense, revolving, rain-drenched, migratory, destructive storms that occur erratically in certain regions of the tropics and subtropics. 4.1.1 The Anatomy of a Tropical Cyclone Fig. 2 A Cross Section Through a Tropical Cyclone (Hurricane) Fig. 1 winds increasing to > 119 km/h

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 4_2024/pg3 • An intense tropical cyclone is an almost circular storm centre of extremely low pressure (LP). From the outer edge of the tropical cyclone to the centre, the barometric pressure typically drops to 950 mb or lower (see Figs 2 and 3). • Mature tropical cyclones average about 600 km across, although they can range in diameter from 150 km up to 1000 km or even more (see Fig. 2). • Because of the very strong pressure gradient (see Fig. 3), winds spiral inward at high speed. Once sustained wind speeds reach 37 km/h, tropical disturbances become tropical depressions. As wind speed increases to 63 km/h, tropical depressions are upgraded to tropical storms. Eventually, when wind speed reaches 119 km/h, tropical storms are officially referred to as tropical cyclones. It can continue to increase to higher speeds (Section 4.1.4). • As the inward rush of warm, moist surface air approaches the LP core of the storm, it turns upward and ascends in a ring of towering cumulonimbus clouds. o This wall of intense of convective activity is called the eye wall, which can exceed 16 km in height. It is here that the greatest wind seeds and heaviest rainfall occur. o Surrounding the eye wall are curved bands of clouds that trail away on spiral fashion. Near the top of the tropical cyclone the airflow is outward, carrying the rising air away from the storm centre, thereby providing room for more inward flow at the surface. • At the very centre of the storm is the eye of the tropical cyclone, in which air gradually descends and heats by compression, making it the warmest part of the storm. Here clear skies and calm winds prevail. The diameter of the eye can range from 30-50 km. • The weather pattern within a tropical cyclone is relatively symmetrical around the eye. o As the eye passes over a site, calm prevails, and the sky clears. Passage of the eye may take about half an hour, after which the storm strikes with renewed ferocity, but with winds in the opposite direction. Fig. 3 1 Ocean water above 28C is needed for the proper amount of water to evaporate. Warm water must be about 60 m deep because storms stir up the oceans, bringing up cold water from below. Fig. 4 Conditions supporting the development of tropical cyclones

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 4_2024/pg4 4.1.2 Conditions for Formation and Decay of Tropical Cyclones • Tropical cyclones occur only when a set of conditions are available (see Fig. 4; previous page). As a result, there are specific areas and times in a year in which they form and impacts experienced (see Section 4.1.3). • Warm sea-surface temperature. o Tropical cyclones develop most often in the late summer when ocean waters have reached temperatures of 26-27C or higher and are thus able to provide the necessary heat and moisture to the air. This helps air above the water to be warmed and become unstable, producing the low pressure condition to draw surrounding surface winds in. o Sufficient depth (about 60 m) of warm water is therefore also necessary because storms are capable of stirring up water from below. If the depth of warm water is insufficient, the supply of warm water will be cut when stirred by the storm, and the formation of the tropical cyclone will be weakened. • High humidity and therefore plenty of water vapour. o Tropical cyclones require an enormous supply of energy, which is provided by the latent heat released when water vapour condenses. (See Lect 3 Box 1) o The release of latent heat warms the air and provides buoyancy for its upward flight. The result is to reduce the pressure near the surface, which, in turn, encourages a more rapid inflow of air. o To get this ‘engine’ started, a large quantity of rising warm, moist air is required, and a continuous supply is needed to keep it going. • Strong Coriolis effect. o Tropical cyclones usually form only over warm oceans in 8 to 20N and S latitudes. o Although water temperatures are sufficiently high, hurricanes do not form within 5 of the equator because the Coriolis effect is too weak to initiate the necessary rotary motion for air to spiral upwards around the LP centre. • Light pre-existing winds. o Winds should be light and coming from nearly the same direction (i.e. little or no wind shear) so as to allow significant vertical development of the tropical cyclone. • As a result of the above conditions, tropical cyclones usually last 1-2 weeks (but can be short as a day, and up to 4 weeks!), and diminish in intensity whenever they: o move over cooler water that cannot supply warm, moist air; o move over land, where it abruptly loses its power source of the warm, surface waters of the ocean; or o reach a location where the large-scale flow aloft is unfavourable (i.e. either subsiding or where wind shear, i.e. a change in wind speed and/or direction, is strong).

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