Feature

La Niña Returns, NASA Watches Sea Surface Indicators
01.26.09
 
Image showing the most recent 10 day average of Sea Surface Temperature (SST) anomalies The most recent 10 day average of Sea Surface Temperature (SST) anomalies derived from NASA's Aqua satellite AMSR-E instrument SST measurements. The annotated date indicates the last (most recent) day of the 10-day moving average. This data is used by scientists for studying El Niño and La Niña. The legend shows sea surface temperature anomalies. The color scale ranges from dark blue for areas 5 degrees Celsius (9 degrees Fahrenheit) below normal to dark red for regions 5 degrees Celsius (9 degrees Fahrenheit) above normal. Credit: NASA GSFC SVS
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Diagram showing the polar and pacific jet streams during La Niña in the winter General positions of the polar and pacific jet streams during La Niña in the winter, although this year the blue (Northern) jet stream dipped further south in the eastern U.S. Credit: NOAA
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Jason-1 satellite image of sea surface height from Dec 15, 2008 showing La Niña The Jason-1 satellite image of sea surface height from Dec 15, 2008 also shows the La Niña, as well as a negative PDO signal. The blue area depicts cooler water and lower sea surface heights. Credit: NASA JPL
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La Niña conditions are likely to continue in the Northern Hemisphere during the spring of 2009. That's the forecast from the National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center, National Center for Environmental Prediction and National Weather Service in their official statement on January 8, 2009.

Knowing La Niña conditions is an important factor to hurricane researchers at NASA and hurricane forecasters at NOAA's agencies. La Niñas do have an influence on the Atlantic Ocean hurricane season.

NASA watches the sea surface temperatures (SSTs) in the eastern Pacific (where the cooler La Niña ocean temperature conditions happen) with two satellites that provide global SST measurements. NASA’s Aqua satellite’s Advanced Microwave Scanning Radiometer (AMSR-E) instrument and the Tropical Rainfall Measuring Mission (TRMM) satellite’s Microwave Imager can detect sea surface temperatures through clouds. NASA also has a 10-day average SST data product on-line that has been showing the cooler-than-normal La Niña SSTs during December 2008 and early January, 2009.

During a La Niña, trade winds are stronger than normal, and the cold water that normally exists along the coast of South America extends to the central equatorial Pacific. A La Niña changes global weather patterns and is associated with less moisture in the air, resulting in less rain along the coasts of North and South America, and in the central Pacific.

"La Niña shifts the high-altitude weather highway known as the 'jet stream,'" said Bill Patzert, a climatologist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "It funnels storm tracks to the Pacific Northwest, which has resulted in heavy rainfall and lots of snow in that region so far, as well as in the upper Midwest. La Niña often gives the upper Midwest and the Northeast a fierce winter. Much of the Southwest, by contrast, will be shielded from stormy weather and, as a result, is expected to receive significantly less precipitation than normal." Patzert calls La Niña "the diva of drought" for the U.S. Southwest.

La Niña is signaled by wind shifts and cooler waters in the eastern Pacific. NOAA found that the negative (cooler) equatorial SST anomalies strengthened across the central and east-central Pacific Ocean during December 2008.

The NOAA report further noted that "The subsurface oceanic heat content anomalies (average temperatures in the upper 300 meters (about two-tenths of a mile) of the ocean) became increasingly negative as below-average (cooler) temperatures at thermocline depth strengthened in the central and eastern Pacific."

The thermocline is a thin but distinct layer in a large body of water, like an ocean, where temperature changes more rapidly with depth than it does in the layers above or below. In the ocean, the thermocline is like a layer that separates the upper mixed layer of the ocean's surface from the calm deep water below.

During La Niña events thermocline depths get more shallow and that's what NASA's Jason-1 and Ocean Surface Topography Mission/Jason-2 satellite altimeters observe. They provide data on sea surface height, a key measurement of ocean energy available to encourage and sustain hurricanes. "A Jason-1 satellite image of sea surface height from Dec 15, 2008 also shows the La Niña, as well as a negative Pacific Decadal Oscillation signal," Patzert said.

Sea surface temperatures are monitored by the AMSR-E instrument onboard NASA Aqua satellite. That data is used to create a daily SST update for researchers and forecasters. NASA's Scientific Visualization Studio at the Goddard Space Flight Center, Greenbelt, Md. uses the AMSR-E data to create a 10-day average of SST anomalies. On January 26, the 10-day average clearly showed cooler-than-normal ocean surface water temperatures (depicted in blue). To see NASA's updated 10-day sea surface temperatures, visit: http://svs.gsfc.nasa.gov/vis/a000000/a003300/a003376/

A La Niña event also includes wind changes. "Convection remained suppressed near the International Date Line, and became more persistent near Indonesia during December," the NOAA report indicated. "Low-level easterly winds and upper-level westerly winds also strengthened across the equatorial Pacific Ocean. Collectively, these oceanic and atmospheric anomalies reflect La Niña conditions."

While the magnitude of cooling remains uncertain, NOAA’s official La Niña threshold will likely be met during January-March 2009 (3-month average). The report concludes that based on current observations, recent trends, and model forecasts, La Niña conditions are likely to continue into the Northern Hemisphere Spring 2009.

"In realizing the effect of La Niña on hurricanes, you're actually removing an impediment for hurricane formation," said David Adamec, Oceanographer at NASA's Goddard Space Flight Center, Greenbelt, Md. "During these events, the jet streams have a harder time making it south into the hurricane formation regions, so the wind shear (winds that tear a storm apart) is often weaker and allow hurricanes a better chance of formation."

To see NASA's updated 10-day sea surface temperatures, visit:
http://svs.gsfc.nasa.gov/vis/a000000/a003300/a003376/

For weekly updates on oceanic and atmospheric conditions, visit the Climate Prediction Center web site:
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml

 
 
Rob Gutro
Goddard Space Flight Center