To Be or Not to Be, La Niña?

May 18, 2000 -- The cold-water cousin of El Niño appears to be nearing an end, according to the latest spacecraft and ocean buoy data reported by the NASA Goddard Space Flight Center. The return of warmer surface water temperatures in the Pacific Ocean around the equator may mean changes in winter weather patterns all over the world and a milder hurricane season in the U.S. this fall.

It may also mean that those sled-loving huskies in an earlier Science@NASA article will finally get the snow they've been wishing for this winter.

Above: This map shows the difference between the current sea surface temperature and the average temperature for this time of year (red is 5 degrees C above normal, blue is 5 degrees C below normal). The expanse of cold water spanning the tropical Pacific is a signature of the La Niña phenomenon. By comparing this map with similar maps obtained earlier this year, scientists have concluded that the La Nina effect is declining. These data were recorded by the TMI instrument on board NASA's Tropical Rainfall Measuring Mission satellite. Image credit: Gregory W. Shirah, NASA/Goddard Space Flight Center. [more images]

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Although the declining influence of La Niña won't affect worldwide weather overnight, winter temperatures in the Southwest U.S., where the huskies live, as well as in the Southeast could be cooler this year, while the North Central and Northwest regions could be warmer. Also, the increased hurricane activity that accompanies La Niña may eventually abate.

How soon these changes will take hold is difficult to say.

"I don't think I would revise (the hurricane) forecast yet," said Pete Robertson, group leader for the Climate Diagnostics and Modeling Group at the Global Hydrology and Climate Center in Huntsville, Ala. "Most observers would agree that the ocean temperature data show some warming in the eastern equatorial Pacific. The question is, 'how fast is the abnormally cold water across the central Pacific going to weaken; what's actually going to happen?'

Follow the Bouncing Thermometer

Sea surface temperatures in the tropical Pacific have been bouncing back and forth between "El Niño" and "La Niña" phases for a long time. Historical records show this oscillation stretching at least as far back as 1925, and it's likely to continue far into the future.

Below: The top graphic shows that during normal or La Niña conditions, winds push the water toward southeast Asia, forcing cold water from the deep to well up in the eastern Pacific. The bottom graphic shows conditions during El Niño, when winds over the Pacific fade, and the eastern Pacific becomes warmer than usual near the equator.

see caption
Above: normal or La Niña conditions

Below: El Niño conditions

During a La Niña phase, strong winds blowing east to west along the equator push the water at the surface toward Southeast Asia. Deep, cold water wells up in the eastern Pacific near South America to replace the leaving water, causing a streak of cold water along the eastern half of the Pacific's "waistline." When the cycle switches to an El Niño phase, the winds die down, the upwelling weakens, and the eastern Pacific becomes warmer than usual near the equator.

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These and other changes in the temperature of the ocean's surface affect the humidity and temperature of the air over it, which influences the direction of winds and the overall weather.

Unfortunately, says Robertson, "we're not very good at forecasting transitions from El Niño to La Niña or vice versa."

One model predicts that La Niña will peter out by August and then normal water temperatures will persist through April 2001. Yet another indicates that La Niña will be gone by August and El Niño will return by October.

The conservative estimate is that La Niña will fade out over the next 6 months.

see caption Left: The upper false-color globe shows El Niño (white represents the warmest water), while the lower globe shows evidence of La Niña (purple represents cooler water).

The cycles of the El Niño-La Niña oscillation are inherently erratic. Since 1985, El Niño phases have occurred in 1986-87, 1991-92, 1993, 1994 and 1997-98. The time between El Niño phases is sometimes a year, sometimes five years. La Niña phases come between El Niño phases sometimes, but not always.

As a rule of thumb, though, La Niña phases come roughly every three to five years, Robertson said.

Knowing when a La Niña phase will come and go could be very valuable information, since data show that hurricanes striking the eastern coast of the U.S. -- and the damage that results -- are more frequent and severe during La Niña years.

One survey showed that the chance of at least two hurricanes striking the U.S. is 66 percent in La Niña years. In normal years, the chance is only 48 percent. Damage from the storms is also greater. The chance of at least $1 billion in damage from hurricanes during a La Niña year is 77 percent, compared to only 48 percent in a normal year.

Right: Hurricane Bonnie impacts the Eastern coast of the US on August 26, 1998. For a QuickTime movie of a 3 dimensional fly-by of Bonnie, click the image.

Before La Niña phases can be accurately predicted, the mathematical models used by scientists will have to be further refined.

Just last month, scientists were predicting that this La Niña phase would persist, and now data show it beginning to wane.

"I would just say that it shows us how far we have to go in our modeling," Robertson said.

The Global Hydrology and Climate Center is a joint venture between government and academia to study the global water cycle and its effect on Earth's climate. Jointly funded by NASA and its academic partners, and jointly operated by NASA's Marshall Space Flight Center in Huntsville, Ala., and the University of Alabama in Huntsville, the Center conducts research in a number of critical areas.

see caption Left: This map shows the difference between the current sea level and the average sea level for this time of year (red is 15 cm above normal, blue is 15 cm below normal). Higher than normal sea levels in the western Pacific are a signature of the La Niña phenomenon, which scientists believe is declining. These data were recorded by the TMI instrument on board NASA's Tropical Rainfall Measuring Mission satellite. Image credit: Gregory W. Shirah, NASA/Goddard Space Flight Center. [more images]

Web Links

Global Hydrology and Climate Center -- a joint venture between government and academia to study the global water cycle and its effect on Earth's climate

El Niño Tutorial -- from the NASA/Goddard Space Flight Center

The Evolution of 1997/98/99/2000 El Niño/La Niña -- an image archive at the NASA/Goddard Space Flight Center

Howling for Snow -- Science@NASA headline about La Nina.

Adios, La Niña -- NASA/Goddard press release

La Niña, A Cool Problem Child -- NASA/JPL press release

El Niño/La Niña Watch -- from JPL: images and news releases based on observations of the El Niño/La Niña phenomenon in the Pacific Ocean by the U.S./French TOPEX/Poseidon and other NASA/JPL satellites and instruments.

To Be or Not to Be, La Niña?

Just last month, scientists were predicting that current La Niña conditions would persist, but now data from Earth-orbiting satellites show that it may be on the decline.