The latest satellite imagery from June 11, 2010 shows that the tropical portion of the Pacific Ocean has started cooling down over the past few months. This may be the beginning of a transition from El Niño, to La Niña.

El Niño is a warming of the waters in the tropical eastern Pacific Ocean. El Niño is said to occur when the waters are 0.5 °C warmer than average. La Niña is its opposite, and is when the waters in the tropical eastern Pacific Ocean become cooler by an average of 0.5 °C. The El Niño/La Niña-Southern Oscillation is important because it affects weather in many parts of the world. From NASA’s Image of the Day article:

The blue area in the center of the image depicts the recent appearance of cold water hugging the equator, which the satellite measures as a region of lower-than-normal sea level. (Water contracts slightly when it cools and expands when it warms.) Remnants of the El Niño warm water pool, shown here in red and yellow, still linger in pockets to the north and south of the equator in the center of the image.

The image shows sea surface height relative to normal ocean conditions for this time of year. Red (warmer) areas are about 10 centimeters (4 inches) above normal. Green areas indicate near-normal conditions. Purple (cooler) areas are 14 to 18 centimeters (6 to 7 inches) below normal. Blue areas are 5 to 13 centimeters (2 to 5 inches) below normal

via Adios El Nino, Hello La Nina? : Image of the Day.