A La Nina event occurs when cooler than normal sea surface temperatures form along the equator in the Pacific Ocean, specifically in the eastern to central Pacific. The La Nina we are experiencing now has a significant presence in the eastern part of the ocean.

The cooler water temperatures associated with La Nina are caused by an increase in easterly sea surface winds. Under normal conditions these winds force cooler water from below up to the surface of the ocean. When the winds increase in speed, more cold water from below is forced up, cooling the ocean surface.

"With this La Nina, the sea-surface temperatures are about two degrees colder than normal in the eastern Pacific and that's a pretty significant difference," says David Adamec of NASA's Goddard Space Flight Center, Greenbelt, Md. "I know it doesn't sound like much, but remember this is water that probably covers an area the size of the United States. It's like you put this big air conditioner out there -- and the atmosphere is going to feel it."

While this "air conditioner" may be located in the equatorial Pacific Ocean, it has a great influence on the weather here in the United States and across the globe.

The cool water temperatures of a La Nina slow down cloud growth overhead, causing changes to the rainfall patterns from South American to Indonesia. These changes in rainfall affect the strength and location of the jet stream -- the strong winds that guide weather patterns over the United States. Since the jet stream regulates weather patterns, any changes to it will have a great impact on the United States.

Those changes can be felt throughout the country. The Northwest generally experiences cooler, wetter weather during a La Nina. On the Great Plains, residents normally see a colder than normal winter and southeastern states traditionally experience below average rainfall.

The cooler waters of a La Nina event also increase the growth of living organisms in this part of the ocean. La Ninas amplify the normal conditions in the Pacific. These typically cool and abundant waters experience an increase in phytoplankton growth when the water temperature drops even further.

The increased circulation that brings up cold water from below also brings up with it nutrients from the deeper waters. These nutrients feed the organisms at the bottom of the food chain, starting a reaction that increases life in the ocean. NASA's SeaWiFS satellite documented this increase in phytoplankton during the last La Nina period in 1998.

La Nina and El Nino episodes tend to occur every three to five years. La Ninas are often preceded by an El Nino, however this cycle is not guaranteed.

The lengths of La Nina events vary as well. "We need to watch to see if this La Nina diminishes, because they can last for multiple years. And if it does last for multiple years, the southern tier of the United States, especially the Southeast, can expect dryer weather. That is not a good situation. If this La Nina behaves.

So far this La Nina is behaving like a textbook case: following the predicted weather patterns, strengthening throughout the winter, and peaking toward January. According to NOAA's Climate Prediction Center, this La Nina episode is expected to continue until the spring of 2008, with a gradual weakening starting in February.

NASA will continue to monitor this phenomenon with several of its key Earth observing satellites.

Instruments on NASA's Terra and Aqua satellites measure sea surface temperature and observe changes to life in the ocean, changes of great importance to the fishing industry. The MODIS instruments on these satellites detected the temperature drop that signaled this La Nina period, and SeaWiFS continues to monitor ocean life.

Scientists also look at sea surface height to understand La Nina. The cooler ocean water associated with a La Nina contracts, lowering sea-surface heights. Over the past year, NASA's Jason satellite has observed a lower than normal sea level along the equatorial Pacific where this current La Nina episode is taking place.

NASA also looks at changes in wind and rain patterns to study La Nina. The QuikSCAT satellite measures changes in oceanic surface winds, while the Tropical Rainfall Measuring Mission satellite observes changes in rainfall. These observations add to a fuller understanding of this phenomenon.

The current La Nina episode has far many reaching effects. What some may see as just a small change in sea surface temperature has a much greater impact on our climate here in the U.S. and across the globe, as well as implications for the fishing industry and the global economy. With the help of NASA's earth observing fleet, scientists are becoming better equipped to observe and understand this phenomenon.

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