ENSO stands for El Nino-Southern Oscillation. It's a climate pattern that occurs in the Pacific Ocean every 2-7 years and affects weather around the entire world. The pattern has three phases: El Nino (warm), La Nina (cool), and Neutral (normal).
El Nino means "The Little Boy" or "The Christ Child" in Spanish. Peruvian fishermen gave it this name because they often noticed warmer waters appearing around Christmas time. La Nina means "The Little Girl" and represents the opposite pattern with cooler waters.
The temperature of the top layer of ocean water. During El Nino, this is warmer than normal in the eastern Pacific.
Steady winds that blow from east to west across the Pacific. Changes in these winds trigger ENSO events.
1. What does ENSO stand for?
The Pacific Ocean covers about 46% of Earth's water surface. Most ENSO action happens in the tropical Pacific—the band from 30°N to 30°S latitude—where the sun's energy is most intense and ocean-atmosphere interactions are strongest.
The tropical Pacific has distinct regions: - The western Pacific (near Indonesia and the Philippines) is normally warm, with surface temperatures around 28-29°C. This warm water evaporates extensively, creating atmospheric moisture that drives the Asian monsoons. - The central Pacific (around 150°W longitude) is cooler than the west, with temperatures around 26-27°C. - The eastern Pacific (near the coast of South America) is coolest, with temperatures around 23-24°C due to upwelling—cold, deep water rising to the surface.
These temperature differences normally drive wind patterns. Because warm water evaporates extensively, it creates atmospheric low pressure; cool water creates high pressure. Air flows from high to low pressure, generating the trade winds that blow from east to west across the tropical Pacific.
During El Niño, the temperature pattern changes. Warm water spreads eastward from the western Pacific toward the coast of South America. The warmth disrupts trade winds. Temperature contrasts between west and east decrease, reducing the pressure difference that drives trade winds.
During La Niña, the opposite happens. The western Pacific becomes even warmer than usual; the eastern Pacific becomes cooler than usual. Temperature contrasts strengthen. Trade winds intensify. The upwelling that brings cold water to the eastern Pacific surface strengthens.
These patterns might seem regional, but they trigger weather changes thousands of kilometers away. The mechanism involves atmospheric waves generated by the warm/cool anomalies interacting with the jet stream and other global wind patterns.
Sea surface temperature (SST) maps are the primary tool for monitoring El Niño. These maps show temperatures in color—usually red for warm, blue for cool. Each map represents a specific month and shows the entire Pacific Ocean.
To read an SST map:
1. Identify the color scale. Warmer colors (reds, oranges) indicate higher temperatures; cooler colors (blues, purples) indicate lower temperatures.
2. Note the geographic features. Recognize the coastlines of Asia, North America, and South America. This orients you to which regions are warm or cool.
3. Look for patterns. During El Niño, notice warm colors spreading eastward along the equator. During La Niña, notice cool colors concentrated in the central and eastern equator.
4. Compare to normal. Anomaly maps show differences from long-term averages rather than absolute temperatures. Red indicates warmer-than-normal; blue indicates cooler-than-normal.
5. Assess intensity. Strong colors indicate large departures from normal; pale colors indicate weak departures.
Scientists actually prefer anomaly maps over absolute temperature maps because they emphasize the ENSO signal. A tropical Pacific location is warm every summer, so absolute temperature maps just show a seasonal pattern. Anomaly maps remove this seasonal variation, revealing ENSO variations underneath.
A typical El Niño develops gradually. In mid-year, warm anomalies might appear in the western Pacific. By late year, they spread eastward. By early next year, maximum warming appears in the central Pacific. By the following year, anomalies weaken as the event decays.
Using provided monthly SST anomaly maps spanning recent decades:
1. Locate the warmest anomalies. During El Niño years, find where the largest warm anomalies appear (deepest red colors).
2. Track spatial progression. Notice whether warm anomalies remain in the western Pacific or spread eastward toward South America.
3. Measure anomaly strength. Check the color scale to estimate how many degrees above normal the warmest regions are.
4. Identify the phase. Determine whether each month represents El Niño (warm), La Niña (cool), or neutral conditions.
5. Timeline sketch. Create a simple timeline showing which years experienced El Niño, La Niña, and neutral conditions.
Using the ONI (Oceanic Niño Index)—a standardized measurement of El Niño strength—compare your visual identification with official classifications. Did the warmest patterns align with years classified as strong El Niño?
- Sea Surface Temperature (SST): The temperature of ocean water at the surface, measured in degrees Celsius or Fahrenheit.
- Anomaly: The difference between observed conditions and the long-term average. Positive anomalies indicate warmer-than-normal; negative anomalies indicate cooler-than-normal.
- Upwelling: The process where cold, deep ocean water rises to the surface. This occurs naturally along the South American coast and strengthens during La Niña.
- Trade Winds: Winds that blow consistently from east to west in the tropical Pacific, driven by temperature and pressure differences.
- El Niño: A warm phase of ENSO characterized by above-normal tropical Pacific sea surface temperatures.
- La Niña: A cool phase of ENSO characterized by below-normal tropical Pacific sea surface temperatures.
- Teleconnection: A weather pattern shift in one region caused by ENSO conditions in the Pacific Ocean thousands of kilometers away.
- ONI (Oceanic Niño Index): The official index used to define ENSO phases, calculated from SST anomalies in the Niño 3.4 region.
Advance to Level 2 to develop deeper skills in interpreting anomaly maps, understanding how scientists define ENSO officially, and analyzing real datasets.