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). Fro