Positive ENSO, also known as the La Niña phase of the El Niño–Southern Oscillation, is a significant climate pattern characterized by distinct atmospheric and oceanic conditions in the Pacific Ocean. During this phase, there are specific deviations in atmospheric pressure across the equatorial Pacific.
Understanding the Characteristics of Positive ENSO
The defining feature of a positive ENSO phase is its atmospheric pressure anomaly. Atmospheric pressure is abnormally low over Indonesia and the western Pacific Ocean, while simultaneously being unusually high over the eastern Pacific Ocean. This setup is the inverse of the conditions observed during a negative ENSO phase, or El Niño.
The El Niño–Southern Oscillation (ENSO) is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean and the accompanying shifts in the atmosphere above. The "positive" phase specifically refers to conditions associated with La Niña.
Key Features of La Niña (Positive ENSO)
When positive ENSO conditions prevail, several interconnected phenomena occur:
- Oceanic Temperatures: The central and eastern equatorial Pacific Ocean experiences cooler-than-average sea surface temperatures, while the western Pacific generally sees warmer-than-average temperatures.
- Trade Winds: The pressure differences intensify the typical trade winds, making them stronger than average as they blow from east to west across the tropical Pacific.
- Ocean Circulation: This strengthening of trade winds pushes more warm surface water towards the western Pacific and causes an increase in upwelling of cold, nutrient-rich water from the deep ocean along the coast of South America.
- Convection and Rainfall: The lower pressure and warmer waters in the western Pacific lead to increased convection and rainfall over regions like Indonesia, Malaysia, and parts of Australia. Conversely, the higher pressure and cooler waters in the eastern Pacific contribute to drier conditions in parts of the Americas.
Contrasting Positive and Negative ENSO Phases
Understanding the positive ENSO phase is often clarified by comparing it to its counterpart, the negative ENSO (El Niño).
| Characteristic | Positive ENSO (La Niña) | Negative ENSO (El Niño) |
|---|---|---|
| Associated Climate Phase | La Niña | El Niño |
| Pressure over Indonesia/West Pacific | Abnormally Low | Abnormally High |
| Pressure over East Pacific | Abnormally High | Abnormally Low |
| Equatorial Pacific SSTs | Cooler than average (East/Central) | Warmer than average (East/Central) |
| Trade Winds | Stronger than average | Weaker than average |
Global Climatic Implications
The positive ENSO phase has widespread impacts on global weather and climate patterns. For instance, La Niña often leads to:
- Increased rainfall in parts of Southeast Asia and Australia.
- Drier conditions in parts of the southern United States and northern Mexico.
- Altered hurricane activity: Typically, it contributes to an active hurricane season in the Atlantic Basin due to reduced wind shear.
- Changes in temperature patterns across different continents.
These shifts in weather patterns can influence agriculture, water resources, and disaster preparedness worldwide. For more detailed information, you can refer to resources on the El Niño–Southern Oscillation, La Niña, and El Niño.
