Q&A with a Climatologist: Part II

Lissa Gregg

Our Q&A on El Niño, La Niña and their effects on weather patterns and drought in the United States continues with Dr. John Nielsen-Gammon, a Professor of Meteorology at Texas A&M University, and the Texas State Climatologist. Read Part I

FNI Water: How do forecasters predict El Niño and La Niña?

Dr. Nielsen-Gammon: There are two basic forecasting tools. The first are statistical tools, which basically look at past temperature patterns and changes in ENSO and try to predict future changes based on the current temperature patterns. The second are coupled atmosphere-ocean models, the same models that are used for long-range forecasts everywhere. They try to actually simulate what’s going on in the tropical Pacific and how it will evolve. These dynamical models are getting better and better, and they have the advantage of not relying on memory of past events, so that they can do just as well with an unusual El Niño as with an ordinary one. There are about 15 or so models worldwide that make regular ENSO forecasts, and they all get compiled and are available to the public at the International Research Institute for Climate and Society, or IRI.

FNI Water: Several computer models are used to help predict El Niño and La Niña conditions. When are these models most reliable in their predictions?

Dr. Nielsen-Gammon: During the spring, the future evolution of ENSO is very sensitive to individual short-term weather events that are hard to forecast. Scientists call this the ENSO predictability barrier, but it’s simpler to call it the zone of cluelessness. Later in the summer, the forecasts become more reliable.

FNI Water: Can the models be used reliably to plan for precipitation events? For instance for water suppliers projecting lake levels or farmers trying to gage the upcoming agricultural season?

Dr. Nielsen-Gammon: ENSO changes the odds. Depending on the circumstances, the change in odds is often too small to be actionable. Every once in a while, an El Niño or La Niña is a nearly sure thing, and the predictable change in the odds is large. If it matters to you that you can rule out an extremely wet season, or perhaps an extremely dry one, then the seasonal forecast is worth paying attention to. Unfortunately, what most people would really like is a forecast for late spring, and ENSO doesn’t have a very large impact in late spring most of the time. This year seems to have been an exception, and years in transition seem to have a relatively large springtime signal.

FNI Water: What does El Niño mean for the state’s water supplies? If we are in El Niño now, why has it been so hot and dry the past month? What is “The Blob?” How is different from El Niño? How are they related (or unrelated)? 

Dr. Nielsen-Gammon: ENSO is defined purely by what’s happening in the tropical Pacific Ocean, but there are sea surface temperature patterns in the North Pacific that are also influential. Some patterns tend to go along with El Niño, others with La Niña. The Blob is an area of warm sea surface temperatures in the Northeast Pacific that actually fits fairly well with the El Niño conditions that we typically see. So I’m not expecting any distinct impact from the Blob, except to note that when the North Pacific and tropical Pacific are in sync like they are now, the impacts tend to be largest.

FNI Water: Is there anything noteworthy about the current El Niño vs El Niños that have been recorded in the past?

Dr. Nielsen-Gammon: The current El Niño is unusually strong and is likely to continue strengthening for a few more months. It will, perhaps, rival the two strongest El Niños of the past 120 years: 1982-1983 and 1997-1998. It’s interesting that those two El Niños were much drier than your typical El Niño. It’s as though there is an El Niño sweet spot for wet Texas winters, and the really strong El Niños overshoot it.  Maybe we’ll see this year whether that’s a fluke or a pattern. Even a dry El Niño, though, is not much drier than normal.

FNI Water: How will climate change impact El Niño and La Niña cycles?

Dr. Nielsen-Gammon: Scientists don’t know yet whether the frequency or intensity of El Niño or La Niña will change, or even if it is changing already. (ENSO is so erratic that it’s impossible to detect a small trend.) Recent research seems to be showing that the mid-latitude effects of ENSO are increasing, though, even if ENSO itself is not changing. The increase is probably due to rising global temperatures, which enhances tropical rainfall, and in turn enhances the changes in tropical rainfall that are associated with ENSO.

FNI Water: Texas recently experienced extreme drought conditions, followed by a strong El Niño. Are these cycles getting more extreme? If so, how should water suppliers be preparing for a future of more extreme events?

Dr. Nielsen-Gammon: If the response to ENSO intensifies, then so will the year-to-year swings in Texas weather. The past couple of decades have seen some very wild swings, but the increase in variability is actually too large to be accounted for by climate change. Besides this effect, the increasing temperatures will accelerate evaporation, meaning that soils, streams, and reservoirs will tend to dry out faster between rainfalls. Also, the evidence consistently shows that the heaviest one-day and two-day rainfall events are getting more intense. None of that is going to make life easier for water suppliers. In general, the past is not as reliable a guide to the future as it used to be, and that creates a whole lot of challenges.