Lecture Notes on ENSO and Hurricanes

Southern Oscillation and ENSO (El Niño-Southern Oscillation)

• Definition of ENSO: A climate phenomenon involving fluctuations in ocean temperatures and atmospheric pressure in the Pacific Ocean.
• Key Components:
  • El Niño: Periodic warming of ocean surface temperatures in the central and eastern Pacific, causing changes in weather patterns.
  • La Niña: Opposite phase characterized by cooler ocean surface temperatures.

Characteristics of El Niño

• Ocean-Wind Interaction:
  • Reversal of normal pressure and wind patterns across the Pacific Basin (Western Pacific low pressure, Eastern Pacific high pressure).
  • Changes lead to warmer Pacific Ocean temperatures which affect weather patterns globally.
• Duration:
  • Lasts approximately 3 to 6 months; can extend to over a year.
• Detection:
  • Observed through sea surface temperatures and atmospheric pressure metrics using satellite technology and pressure stations in key locations (e.g., Tahiti, Darwin, Peru).

Normal vs. El Niño Conditions

• Normal Conditions:
  • High pressure off the coast of Peru leads to colder ocean waters due to upwelling, nutrient-rich fisheries, and typical dry weather.
  • Low pressure in the Western Pacific (Indonesia) produces warm sea conditions and significant precipitation.
• El Niño Conditions:
  • Low pressure systems shift towards the Eastern Pacific, reducing upwelling and replacing cold water with warmer water.
  • Impacts: Drought in Western Pacific areas, increased precipitation and flooding in Eastern Pacific (e.g., Peru).

Predictive Aspects of ENSO

• Teleconnections:
  • ENSO influences weather patterns not only in the Pacific Basin but also in regions such as the Atlantic and Indian Oceans, albeit to a lesser extent.
• Global Climate Impact:
  • Changes in temperature and precipitation patterns can affect agricultural practices and increase natural disasters (e.g., flooding, drought).

Southern Oscillation Index (SOI)

• Measurement:
  • SOI computes the pressure difference between Tahiti and Darwin, providing insight into ENSO conditions.
• Indicators:
  • A drop in pressure at Tahiti indicates the onset of El Niño, while an increase suggests La Niña or neutral conditions.

La Niña Overview

• Characteristics:
  • Enhanced normal conditions with stronger trade winds and lower sea surface temperatures.
  • Often results in wetter conditions in the Pacific, contrasting the drier conditions resulting from El Niño.

Global Climate Effects of ENSO

• El Niño's Influence:
  • Increased rainfall in the southern U.S.; drier conditions in Southeast Asia, influence on storm patterns in other regions.
• Weather Implications:
  • El Niño usually associated with warmer winters in Northern U.S., increased precipitation in Southern U.S.
  • La Niña leads to cooler and wetter conditions in the North and drier conditions in the South.

Formation of Hurricanes

• Essential Elements:
  • Warm ocean waters (at least 26.5^{ ext{o}}C), Coriolis force, and atmospheric conditions promoting thunderstorms required for hurricane formation.
• Categorization:
  • Tropical Wave: The initial low-pressure area; can evolve into a tropical depression, then a tropical storm, and finally a hurricane.
  • Conditions for Naming: Wind speeds of at least 74 mph classify systems as hurricanes.

Hurricane Structure and Mechanics

• Cross Section:
  • Low-pressure center dictates rotation; pressure gradients drive wind speeds.
  • Eye of the storm: calm conditions, surrounded by the eye wall with the strongest winds.
• Operational Dynamics:
  • Hurricanes draw energy from warm ocean water; dissipate upon landfall.

Climate Change and Hurricanes

• Potential Influence:
  • Rising sea surface temperatures may lead to stronger hurricanes, increasing both precipitation and storm intensity.
  • Emphasis on population growth in vulnerable areas increases potential damage from storms.

Summary Points to Remember

• Key Topics for Quiz:
  • Definitions of ENSO and its components (El Niño/La Niña).
  • Normal vs El Niño conditions and their global impacts.
  • Understanding hurricane formation and characteristics.
  • The role of climate change in influencing hurricane patterns.