Q&A with a Climatologist: Part I

Lissa Gregg

Dr. John Nielsen-Gammon answered our questions about El Niño, La Niña and their effects on weather patterns and drought in the United States.

Dr. Nielsen-Gammon is a Professor of Meteorology at Texas A&M University, and the Texas State Climatologist, and has participated in Climate Change and Drought Seminars hosted by the City of Dallas and SAWS.

FNI Water: What are El Niño and La Niña?

Dr. Nielsen-Gammon: They are the names for the opposite phases of a combined atmosphere-ocean persistent weather pattern in the tropical Pacific Ocean. Scientists call the whole thing ENSO, which stands for El Niño-Southern Oscillation. The Southern Oscillation was the name given to the fact that when air pressure on one side of the Pacific Ocean went up, air pressure on the other side went down. Later, it was learned that low air pressure toward the east corresponds to unusually warm sea surface temperatures in the central and eastern tropical Pacific Ocean. Other things over the tropical Pacific associated with the warm phase, which is called El Niño, are weakened trade winds and more widespread thunderstorm activity.

FNI Water: What do El Niño and La Niña do to Texas weather conditions? To broader conditions in the United States?

Dr. Nielsen-Gammon: In the wintertime, ENSO alters the location of the Northern Hemisphere jet stream. It’s farther south than usual during El Niño and farther north during La Niña. There are also characteristic wave patterns in the jet stream that appear. Because Texas is near the southern edge of the jet stream on average, storminess increases during El Niño and decreases during La Niña. In general, the chances of wet weather in the winter increase across the southern United States and decrease a bit in the northern United States. Temperatures in the southern United States tend to be a bit cooler than normal during El Niño, and warmer than normal in the northern United States. In the summertime, with the jet stream far away from the tropical Pacific, the main impact is on hurricane activity: it’s enhanced in the eastern Pacific and suppressed in the Atlantic.

FNI Water: How certain is the effect of El Niño and La Niña on U.S. weather?

Dr. Nielsen-Gammon: In some places the effect is more reliable than others. In Texas the effect is especially reliable, but even so, it only accounts for about a third of the year-to-year weather variations we experience. Since at least 1950, Texas as a whole has never had a November through March significantly drier than normal during an El Niño.

FNI Water: Is climate always considered either El Niño or La Niña? Are there other classifications?

Dr. Nielsen-Gammon: The most common state is called Neutral. Goldilocks would be happy with neutral conditions. To be an El Niño, sea surface temperatures in the key region have to be at least 1°F or so above normal for at least several months, and La Niña’s threshold is -1°F. Neutral conditions are present about 40 percent of the time.

FNI Water: How will El Niño impact the average citizen? Should we all be buying rain boots?

Dr. Nielsen-Gammon: An El Niño winter is typically cold, wet, and cloudy. The best overall adjective would be “yucky.” But think of all the food that’s going to be grown next spring that will have a good solid start thanks to all the wet weather over the winter.

FNI Water: How do scientists determine when we are in El Niño and La Niña conditions?

Dr. Nielsen-Gammon: ENSO is the result of a positive feedback between the atmosphere and the ocean. During an El Niño, winds are weak because the air pressure variations are weak, because the tropical thunderstorm activity is widespread. The tropical thunderstorm activity is widespread because warm water temperatures extend across most of the tropical Pacific. Warm water temperatures extend across most of the tropical Pacific because there’s very little upwelling to bring colder water to the surface. There’s very little upwelling because winds are weak.

About the only thing that prevents the system from getting permanently locked into an El Niño or La Niña situation is that the warm water can only pile up for so long at one side of the ocean or the other before it sloshes back. It takes about a year or two for this to happen.

FNI Water: Is there a pattern to El Niño and La Niña?

Dr. Nielsen-Gammon: There are two patterns. The classical El Niño has the largest change in tropical sea surface temperatures right along the coast of South America, with unusually warm water extending westward along the equator. The other El Niño pattern, called El Niño Modoki after a Japanese word that loosely translated means “the same, but not really”, has its largest temperature change out in the open ocean, near the Date Line. These seem to have somewhat different impacts on the weather in the Northern Hemisphere.

FNI Water: Are El Niño and La Niña seasonal?

Dr. Nielsen-Gammon: An El Niño or La Niña can form any time of year, but typically one will get going in late spring or summer and peak during winter.

Read Part II for resources and information about predicting El Niño and La Niña.