When the easterly trade winds die down and a pool of warm water builds up in the equatorial Pacific Ocean off the coast of South America, the phenomenon is known as an El Niño. When winds strengthen and the water is unusually cool, the phenomenon is known as La Niña. An August 7, 2007, report issued by the National Oceanic and Atmospheric Administration (NOAA) stated that while current conditions in the Pacific were primarily neutral, they were leaning toward a La Niña event, with sea surface temperatures slightly below normal.
This image shows where sea surface temperatures are above (red) or below (blue) the long-term average between July 28 to August 4, 2007. The Advanced Microwave Scanning Radiometer (AMSR-E) flying on NASA’s Aqua satellite collected these observations. Characteristic of La Niña conditions, this image shows a predominance of blue near the South American coast, particularly off the coast of Peru. At the time of the August 7, 2007 prediction, NOAA had derived mixed results from its models, with some models predicting a transition to a mild La Niña over the next couple months, and other models predicting a continuation of neutral conditions.
A wave pattern with deep blue (cooler-than-normal) crests that stretches across the equator between the central and eastern Pacific. These are tropical instability waves. The waves arise from small instabilities in the wind-driven surface current that moves water from the eastern Pacific to the western Pacific. The flow of the current is not perfectly smooth and even, and small pockets of instability (turbulence) are magnified into large-scale wave patterns in the westward-flowing current. An animation based on 30 days of data shows the waves moving across the eastern Pacific. Research has linked La Niña episodes with greater tropical instability wave activity in the eastern Pacific.
Daily, 8-day, and monthly sea surface temperature anomaly images like the one pictured above are available from the NASA Earth Observations (NEO) Website.
- Yu, J-Y., and Liu, W.T.(2003) A linear relationship between ENSO intensity and tropical instability wave activity in the eastern Pacific Ocean, Geophysical Research Letters, 30 (14), 1735.
- La Niña Factsheet on the Earth Observatory
NASA image by Jesse Allen, using Sea Surface Temperature data from the Advanced Microwave Radiometer for EOS (AMSR-E), courtesy Remote Sensing Systems.