Study Notes on Ocean-Atmosphere Interactions: Walker Circulation, El Nino, and La Nina

Overview of Ocean-Atmosphere Interactions

  • Understanding the roles of Walker circulation, El Nino, and La Nina in ocean-atmosphere interactions.

Walker Circulation

  • Description:

    • A continuous circulation pattern in the atmosphere over the equatorial regions.

    • Characterized by sinking air towards the surface, leading to varying pressure distributions.

    • Connected to sea surface temperature patterns in the ocean.

  • Features:

    • Fast circulation at the ocean surface, where warm water sinks in the west and cold water rises in the east.

    • Balancing water motions; slow motion towards the east.

El Nino Phenomenon

Definition and Characteristics

  • Definition:

    • An anomaly, where warm water accumulates in the eastern Pacific compared to the average conditions.

  • Oceanic and Atmospheric Effects:

    • Warmer sea surface temperatures in the Eastern Pacific than usual, exceeding average levels.

    • Anomalous high pressure in the west and low pressure in the east resulting from altered convection processes.

      • Warmer water leads to rising air masses (lower pressure), resulting in convection.

      • Sinking air due to compression results in higher pressure near the top of the troposphere.

  • Changes in Trade Winds:

    • Shift from easterly trade winds to anomalous westerly flow in the western Pacific, influencing ocean currents.

    • Slow down of westward flow allows surface waters to warm up more, resulting in higher sea surface temperatures.

Impacts on Weather Patterns

  • Central Pacific Convection:

    • The main region of convection moves from the western Pacific to the Central Pacific, causing increased rainfall and cloud formation there.

    • Formation of a "heat dome" generates high atmospheric pressure over warm ocean waters, amplifying upward air motion.

  • Cyclonic Activity and Rainfall:

    • Enhanced convection leads to heavy rains in the Central Pacific, while the Caribbean experiences a stable and dry atmosphere.

      • Higher air pressure results in subsidence, causing drier conditions in areas east of the warm dome towards the Caribbean.

      • Reduction in storm activity over the Caribbean due to increased vertical wind shear between surface easterly winds and westerly upper winds.

La Nina Phenomenon

Definition and Characteristics

  • Definition:

    • An amplified version of neutral conditions characterized by stronger easterly trade winds leading to more warm water amassing in the western Pacific.

  • Oceanic and Atmospheric Effects:

    • Increased downwelling of warm water in the west and further cooling, leading to intensified upwelling in the east.

    • Steeper thermocline and strengthened Walker circulation improve the cold phase.

    • High pressure remains prominent in the eastern Pacific whereas low pressure dominates in the west, intensifying circulation.

Weather Impacts

  • Dry Conditions:

    • Very dry air occurs in the Eastern Pacific due to the intense high pressure, affecting weather patterns and precipitation.

    • Enhanced trade winds contribute to increased cooling in the West while maintaining drier conditions in the east.

ENSO Phase Monitoring

  • ENSO Phenomena Mapping:

    • Red colors represent positive (warm) phases (El Nino), while blue depicts negative (cold) phases (La Nina).

    • Warm phases, when surpassing 1 standard deviation, indicate strong El Nino events that cause significant weather changes.

  • Historical Events:

    • Strong El Nino events recorded in 1982-83, 1997-98, and 2015-16, with La Nina events that can last multiple years.

    • La Nina is inherently more stable and can result in climate effects for a longer duration compared to El Nino, which typically lasts months to a year.

Implications of El Nino

Caribbean Weather and Climate Effects

  • Effects of Warm Dome:

    • Warm dome influences air movement, resulting in descending and stable air in the Caribbean, leading to hotter, sunnier conditions.

    • Increased temperatures at higher elevations lower pressure at surface, creating dry conditions during the summer months.

  • Changes in Hurricane Activity:

    • Increased vertical wind shear disrupts storm development, leading to fewer hurricanes during El Nino years.

    • Stable atmosphere and higher surface pressure inhibit storm intensification, changing hurricane activity patterns.

  • Seasonal Differences:

    • Early summer leads to dryness due to increased atmospheric stability and heat dome effects.

    • In the second half of the year, the vertical wind shear continues to suppress hurricane development and maintain drier than normal conditions.

  • Climate Summary:

    • Long-term impacts lead to heat waves during early stages and persistent dryness even into the cool season, with variations across the Caribbean.