Lecture Notes on ENSO and Hurricanes

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.

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 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).

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).

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.

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.

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.

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.

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.

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.

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.