Apr 13, 2000

Howling for Snow


TOPEX/Poseidon satellite imagery
April 14, 2000 -- Tony Phillips' Huskies are not happy.

For the past two years, the Huskies have eagerly awaited winter and their favorite pastime of dog sledding. But two years of La Niña conditions have left the Eastern Sierra mountain range in California with less than its usual supply of snow. No snow means no sledding. And the latest U.S.-French TOPEX/Poseidon satellite imagery shows the La Niña pattern holding on in the Pacific, in a similar manner to this time last year.

So the question on every dog's lips is "When is the snow coming back?"

Right: These TOPEX/Poseidon data, collected during the interval March 1 to 11, 2000 show the La Niña condition still exists. The image of sea surface heights reflects unusual patterns of heat storage in the ocean. Sea-surface height is shown relative to normal height (green). The cooler water (blue and purple) measures between 8 and 24 centimeters (3 and 9 inches) lower than normal. The giant horseshoe of warmer water (red and white) continues to dominate the western Pacific with higher than normal sea-surface heights between 8 and 24 centimeters (3 and 9 inches). [more data from JPL]

Unfortunately, this is a very hard question to answer, according to Pete Robertson, group leader for the Climate Diagnostics and Modeling Group at the Global Hydrology and Climate Center in Huntsville, Alabama.


"Many people in this business are trying to understand the 'teleconnections' between the La Niña and El Niño conditions in the tropics and how they affect the United States' climate," says Robertson. "But what is important to remember is that there is so much variability in what affects weather conditions, while you can hedge your bets about weather during a La Niña or El Niño episode, we cannot directly predict the weather from just looking at these tropical systems only."

La Niña and El Niño are opposite and extreme phases of natural oscillation in a climate system. Of current interest is the La Niña pattern, now influencing the worldwide climate system. La Niña is commonly defined as a lower than normal sea level or cold pool of water along the equator. This low sea level is accompanied by higher than average sea levels in a giant horseshoe around the equator, creating warm seas around the coast of Asia and Australia. Sea levels and temperatures during El Niño and La Niña are alike but have opposite characteristics.

With La Niña's lower sea surface temperatures come lower than normal surface precipitation in the tropics, says Robertson. These changes in the tropics cause changes in the jet stream, which is a river of the fastest moving air on the planet. La Niña conditions generally give the expectation that storm tracks will be more northern, resulting in drier than normal conditions for much of the southern half of the United States, and above-normal rainfall in the Pacific northwest and Canada.

The past two years have seen exactly that, with heavy rainfall and lots of snow in the Pacific Northwest and upper Midwest, but relatively mild weather and significantly less precipitation than normal to date in Southern California and the Southwest.


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Left: The question on every dog's lips is "When is the snow coming back?" Pictured here, Tovic (left) and Ruby (right) enjoy a run during the El Niño winter of 1997-98. Recently, their sledding season has been cut short thanks to the influence of La Niña.

The rainfall associated with the La Niña system is one thing that the GHCC and Robertson are particularly interested in studying, as rainfall and the corresponding levels of evaporation have a great impact on the atmosphere.

The world's oceans are giant reservoirs of heat that influence global climate because they can cool or heat the atmosphere above. This transfer of heat impacts weather patterns across land and sea.

"We try and estimate precipitation from space. We get data collected from the Microwave Sounding Unit, which flies on the NOAA satellites, to study," says Robertson. "We are particularly interested in seeing how precipitation changes in location and intensity during a La Niña event."

During a La Niña event, less precipitation is seen over the tropics, and vice versa during an El Niño event, says Robertson. Ultimately, studying these patterns allows them tostudy the hydrologic and energy cycles over the tropics, he continues.

Energy from the sun comes in through the tropics by heating the oceans. The energy then principally finds its way into the atmosphere through evaporation.

"When we look at precipitation, we are looking at the smoking gun of how energy gets into the atmosphere," says Robertson. "We can then study the year to year changes in how energy is added to the earth's atmosphere."

Scientists at the GHCC have gathered tropical precipitation data over the past 20 years. This large data set allows them to analyze the most recent precipitation data from the La Niña/El Niño patterns and to see how they are similar or different from earlier events. However, as Robertson points out, there is much more to be studied before the La Niña/El Niño pattern can be understood to the stage of being able to draw conclusions about all its effects on climate.

"For now, we know it is a natural oscillation of a climate system. And it's the biggest we know of," Robertson says.


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Above: These data show the "index of precipitating ice" in tropical rain systems (Winter '97/'98 minus Winter '98/'99) inferred from the Microwave Sounding Unit flying aboard NOAA Polar Orbiting satellites. Multiplying the index values by a factor of 10 gives a rough estimate of the percentage departure from normal rainfall amounts. Note that precipitation across the equatorial central Pacific is anti-correlated with that in the western Pacific, much of Australia, and Amazonia. It is positively correlated in much of the Caribbean and southern U.S.

There continues to be much debate among scientists about how long this climate system will last. Some scientists, such as Dr. William Patzert, an oceanographer at NASA's Jet Propulsion Laboratory in Pasadena, Calif., believe that the persistence of La Niña events indicates the presence of a longer-lasting climate pattern known as the "Pacific decadal oscillation". In a statement released by JPL, he comments that the unusual La Niña conditions, which have persisted for two and a half years, tells us there is much more going on in the Pacific Ocean than a single isolated event and that the La Niña conditions are a part of this decade-long pattern.

However, Robertson says much more research is needed.

It is certainly a definite possibility," he says. "But there is a lot of debate about this issue. The two oscillations have the same sort of structure, but the La Niña/El Niño pattern is concentrated at the equator. The Pacific oscillation involves higher latitudes. The two may look similar or may be working together, but they are not the same thing."



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According to the JPL press release, the Pacific decadal oscillation waxes and wanes approximately every 20 to 30 years, alternating between its present phase, with a warm horseshoe pattern of higher than normal sea-surface levels connecting the north, west, and southern Pacific, in contrast to a cool wedge of lower than normal sea-surface levels along the equator. After that the Pacific switches to the opposite phase, showing a reversal of the warm and cool regions.

Both the JPL and the GHCC plan to continue monitoring the development of the La Niña/El Niño conditions and their implications for climate over the next several years.

"We are at the stage of seeing how much good we can do with studying La Niña/El Niño patterns," says Robertson. "You can't predict climate like you predict weather. There is just so much variability."

As for the Huskies' hopes for snow in Southern California next winter, it seems only time will tell.

"Typically in the past, we have seen these events last about a year," says Robertson. "In the 1980s we saw two separate episodes of La Niña that lasted over two years. This La Niña, therefore, should begin to wane later this fall. However, if it persists longer, then it lends credence to the Pacific decadal oscillation theory, and that we could be looking at a much longer period of La Niña conditions."

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.

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

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

GHCC Water Cycle Research Overview -- The transport of water and the energy exchanged as it is converted from one state to another are important drivers in our weather and climate. One of the key missions of the GHCC is to develop a better understanding of the global water cycle at a variety of scales so that we can improve model forecasts of climate trends, predictions of short-term and regional weather events, and even their impacts on society's regional and global activities.

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.