Tornados seit 2010 immer seltener: Pazifische Ozeanzyklen steuern Sturmhäufigkeit

Man hört so wenig von den Tornados. Früher waren sie die Lieblinge der Klimaaktivisten. Wie kam es zum Bruch? Dazu werfen wir einen Blick auf die offizielle NOAA-Tornadostatistik:

Abb. 1: Kumulative Entwicklung der Tornado-Anzahl, nach Jahren aufgeschlüsselt. Graphik: NOAA.


Wir sehen: Das Jahr 2018 war (zum Glück) nur Durchschnitt. Die Tornadoentwicklung der letzten 60 Jahre zeigt die Verhältnisse eindrücklich. In der Zeit 2005-2010 gab es eine Häufung der Tornados in den USA. Seitdem werden sie wieder seltener. Kein guter Stoff für Klimakatastrophengeschichten.

Abb. 2: Anzahl der Tornados in den USA seit 1950. Quelle: NOAA.


Angesichts der latenten Tornadogefahr fragte sich Hannes Stein 2013 in der Welt, weshalb die Amerikaner nicht nachhaltiger bauen, zum Beispiel Häuser aus Stein anstatt aus Holz.

Die Tornadoschäden sind schlimm – doch warum bauen die Amis keine besseren Häuser? Das fragt sich der Europäer und beweist damit so viel Anmaßung wie Ignoranz

Weiterlesen in der Welt

Eine schöne Visualisierung der globalen Winde gibt es übrigens auf

Weshalb schwankt die Häufigkeit der Tornados eigentlich so stark im Laufe der Jahrzehnte? Forscher der University of Missouri fanden die Antwort: Die Tornados werden vom pazifischen PDO-Ozeanzyklus beeinflusst, wie sie in einer Pressemitteilung vom 10. Oktober 2013 bekanntgaben:

Pacific Ocean Temperature Influences Tornado Activity in U.S., MU Study Finds
Meteorologists often use information about warm and cold fronts to determine whether a tornado will occur in a particular area. Now, a University of Missouri researcher has found that the temperature of the Pacific Ocean could help scientists predict the type and location of tornado activity in the U.S.

Laurel McCoy, an atmospheric science graduate student at the MU School of Natural Resources, and Tony Lupo, professor and chair of atmospheric science in the College of Agriculture, Food and Natural Resources, surveyed 56,457 tornado-like events from 1950 to 2011. They found that when surface sea temperatures were warmer than average, the U.S. experienced 20.3 percent more tornados that were rated EF-2 to EF-5 on the Enhanced Fuijta (EF) scale. (The EF scale rates the strength of tornados based on the damage they cause. The scale has six category rankings from zero to five.). McCoy and Lupo found that the tornados that occurred when surface sea temperatures were above average were usually located to the west and north of tornado alley, an area in the Midwestern part of the U.S. that experiences more tornados than any other area. McCoy also found that when sea surface temperatures were cooler, more tornadoes tracked from southern states, like Alabama, into Tennessee, Illinois and Indiana.

“Differences in sea temperatures influence the route of the jet stream as it passes over the Pacific and, eventually, to the United States,” McCoy said. “Tornado-producing storms usually are triggered by, and will follow, the jet stream. This helps explain why we found a rise in the number of tornados and a change in their location when sea temperatures fluctuated.” In the study, McCoy and Lupo examined the relationship between tornadoes and a climate phenomenon called the Pacific Decadal Oscillation (PDO). PDO phases, which were discovered in the mid-1990s, are long-term temperature trends that can last up to 30 years. According to NASA scientists, the current PDO phase has just entered into a “cool” state. “PDO cool phases are characterized by a cool wedge of lower than normal sea-surface ocean temperatures in the eastern Pacific and a warm horseshoe pattern of higher than normal sea-surface temperatures extending into the north, west and southern Pacific,” McCoy said. “In the warm phase, which lasted from 1977 to 1999, the west Pacific Ocean became cool and the wedge in the east was warm.”

In 2011, more than 550 deaths occurred as a result of tornadoes, resulting in more than $28 billion in property damage, according to the U.S. National Oceanic and Atmospheric Administration. McCoy says that with her findings, officials may be able to save lives in the future. “Now that we know the effects of PDO cool and warm phases, weather forecasters have another tool to predict dangerous storms and inform the public of impending weather conditions,” McCoy said.

Auch El Nino-La Nina (ENSO) beeinflussen die Tornados, wie Lepore et al. 2017 dokumentierten:

ENSO-based probabilistic forecasts of March–May U.S. tornado and hail activity
Extended logistic regression is used to predict March–May severe convective storm (SCS) activity based on the preceding December–February (DJF) El Niño–Southern Oscillation (ENSO) state. The spatially resolved probabilistic forecasts are verified against U.S. tornado counts, hail events, and two environmental indices for severe convection. The cross-validated skill is positive for roughly a quarter of the U.S. Overall, indices are predicted with more skill than are storm reports, and hail events are predicted with more skill than tornado counts. Skill is higher in the cool phase of ENSO (La Niña like) when overall SCS activity is higher. SCS forecasts based on the predicted DJF ENSO state from coupled dynamical models initialized in October of the previous year extend the lead time with only a modest reduction in skill compared to forecasts based on the observed DJF ENSO state.

Auch in Deutschland gibt es übrigens ab und zu Tornados. Einen Häufigkeitstrend für die letzten 15 Jahre gibt es allerdings nicht, wie man aus Abbildung 7 aus diesem DWD-Bericht ersehen kann.