7) Climate Dynamics and ENSO

Overview of Climate Dynamics and Ocean-Atmosphere Interactions
Northern Hemisphere Temperature and Monsoon Patterns

  • The Northern Hemisphere is colder than the Southern Hemisphere, leading to weaker summer monsoons due to less land warming.

  • A halt in the North Atlantic current correlates with the lowest summertime rainfall in China in 10,000 years, indicating a strong link between North Atlantic temperatures and East Asian monsoons/precipitation.

Walker Circulation in the Equatorial Pacific

  • Definition: A dynamic atmospheric and oceanic circulation in the Equatorial Pacific, characterized by rising air in the west and sinking air in the east.

  • Air Movement:

    • Rising air is prominent in the western Pacific (e.g., Indonesia, The Philippines), a "Maritime Continent" with significant land-water interaction (30%30\% land, 70%70\% water) and a global center for rising air due to warm, moist conditions.

    • Sinking air occurs in the eastern Pacific, with trade winds blowing west and a high-altitude return flow completing the circulation.

ENSO: El Niño-Southern Oscillation

  • Definition: A coupled ocean-atmosphere phenomenon (El Niño for ocean, Southern Oscillation for atmosphere).

El Niño Mechanics

  • Ocean Dynamics:

    • Trade winds push warm water west, accumulating in the western Pacific.

    • Off Peru, upwelling of cold, nutrient-rich water (vital for fisheries) is displaced by warm water moving east as the thermocline shifts, leading to significant biological activity.

Air Pressure and Convection

  • Rising air (low surface pressure) is over warm water; sinking air (high surface pressure) is over cool water.

  • During El Niño, the atmospheric cell inverts: warm central Pacific temperatures cause rising air and excessive rainfall there, while Indonesia experiences reduced rainfall.

Feedback Loop in ENSO

  • The Bjerknes feedback is a positive loop: stronger trade winds enhance upwelling and temperature gradients, maintaining Walker circulation. Weaker trade winds lessen gradients and upwelling, potentially triggering El Niño.

La Niña

  • Definition: Stronger-than-average Pacific trade winds.

  • Conditions: Trade winds push warm water further west, enhancing cold water upwelling off South America. This leads to colder central/eastern Pacific sea temperatures, increased rainfall/cooler temperatures in the Western Pacific, and dry conditions in the Eastern Pacific, often causing drier Southern U.S. and wetter Western U.S. weather.

Seasonal Impacts of ENSO

  • ENSO-induced ocean temperature changes impact the jet stream, influencing global weather patterns. El Niño typically brings increased rainfall to the southern U.S., while La Niña causes drier conditions.

Quantifying ENSO

  • Southern Oscillation Index (SOI): Measures sea level pressure differences between Tahiti and Darwin, Australia. Increased Tahiti pressure implies cooler sea temperatures; lower pressure implies warmer (El Niño) conditions.

  • Oceanic Niño Index (ONI): Monitors sea surface temperature anomalies in Niño regions 3 and 4 to identify El Niño and La Niña.

Sources of Ongoing Research

  • Research continues on the origins and triggers of El Niño and La Niña, focusing on atmospheric influence on ocean dynamics, anomalies, and feedback mechanisms.