The name "El Niño" has become familiar to many people as the cause of unpleasant, even extreme, weather. To Californians, it means heavy rains, floods, and mudslides. To Australians, it brings thoughts of drought and catastrophic wildfires. But to meteorologists and oceanographers, El Niño is a fascinating glimpse into the intricate dance of the ocean and the atmosphere and the swirling patterns of heat and moisture that describe the global climate.
Peruvian fishermen have long noted that in some winters, coastal waters heat up and fish stocks dwindle, a phenomenon they named El Niño, (a reference to the birth of the Christ child, which also came in winter). Scientifically, El Niño refers to unusual sea surface temperatures throughout the equatorial Pacific, and to the resulting worldwide weather effects. An El Niño condition is officially declared by NOAA (the National Oceanographic and Atmospheric Administration) when a 3-month average of sea surface temperatures in the east central equatorial Pacific Ocean is 0.5° C or more above normal. Historically, El Niño occurred about once or twice per decade, but there is some indication that their frequency is increasing.
Severe El Niños cause death and destruction across countries, even continents, and yet the cause and consequences of these events is still not well understood. Only recently has the technology been available to allow scientists to study, model, and begin to predict global weather and climate.
Instructions: In this activity, you will explore the links between ocean currents, El Niño, and weather. Examine the images and graphics below, and then answer the questions that follow.
Sea Surface Conditions
- Compare sea temperatures and surface topography in the eastern and western equatorial Pacific in normal, non- El Niño conditions.
The seawater on the surface in the eastern Pacific is much colder, and cools off with depth much faster. On the west side, there is an enormous blob of warm water that is right on the surface and also extends hundreds of meters down.
The surface of the sea is irregular all across the Pacific, but there is a broad mound on the western side.
- Explain how winds and currents cause these patterns.
The trade winds blow most of the time in the tropics, and start the Equatorial Currents moving just north and south of the equator. These currents push water from east to west across the Pacific. The land masses on the west side of the sea (Indonesia, the Philippines, New Guinea, etc.) block the currents, and water piles up into a low mound against the land.
Seawater in the Equatorial Currents is heated by intense sunlight during the journey across the Pacific, and so is quite warm by the time it reaches the west side. In the east, cold water is flowing toward the equator in the Eastern Boundary Currents, so sea surface temperatures are much cooler on that side of the Pacific.
- Now compare sea temperatures and surface topography in the eastern and western equatorial Pacific during an El Niño.
During El Niño, the surface of the Pacific slopes up toward the east. The temperature of the sea is much warmer all across the equator, both right at the surface and down several hundred meters. The hottest water is no longer on the west side of the Pacific, but between the center and east edge of the ocean.
- Explain how winds and currents might cause these patterns.
It looks as if during El Niño, warm water is flowing east across the Pacific instead of west. This could only happen if the Trade Winds were weaker or less steady than normal, which would in turn diminish the strength of the Equatorial Currents. If the forces of wind and water moving west decline, that big mound of hot water that is usually near Indonesia would collapse, and a lot of it would flow back east in the Equatorial Countercurrents. Warm water would then build up in the eastern Pacific.
- What is the "normal" weather around the equatorial Pacific, and how do currents influence this pattern?
Most of the time, currents concentrate warm water in the western Pacific, and that leads to lots of evaporation, cloud formation, and rainfall in that area. Because of the currents, the normal condition in Indonesia, Australia, and Southeast Asia is wet.
In the eastern Pacific, the water and air are colder, so there would be less evaporation and precipitation. California, Central America, Ecuador, and Peru will generally be dry.
- What is El Niño weather around the equatorial Pacific, and how do currents influence this pattern?
When the countercurrents start to flow more strongly out of the west, they sweep warm water and the associated heavy rainfall with them. This can cause increased precipitation in the eastern Pacific, and on the west coasts of North and South America.
In contrast, cooler water means less evaporation and rain, and unusually dry weather in the countries of the western Pacific Ocean.
Tracking El Niño
- What use could this array serve that justifies the time and taxes spent upon it?
This array measures ocean surface and near-surface temperatures and wind speed, direction, and temperature, the principal factors involved in El Niño. If these conditions are constantly monitored, they could provide an early warning that an El Niño was on the way and allow people and governments to take precautions. For example, in western Pacific countries, water supplies could be stockpiled and drought-tolerant crops could be planted. On the eastern side of the Pacific, reservoirs could be lowered to make room for storm waters, and vaccine supplies for diseases that flourish in warm, wet weather could be increased.
If the TAO/TRITON Array is effective in providing an early warning of an El Niño, it would help save many lives and ease the financial and emotional tolls caused by powerful, unforeseen weather events.