Aus der Hurrikanforschung: Antikorrelation von Häufigkeit und Stärke gefunden

Momentan wüten wieder schwere Hurrikane im Karibikraum und den südlichen USA. Den vielen Opfern und Geschädigten gilt unser Mitgefühl. Umso dreister ist die Instrumentalisierung der Wirbelstürme Harvey und Irma durch klimaalarmistische Aktivisten. Wie Springfeder-Teufel schnellen sie aus ihren Schreibtischstühlen und erklären in jedes offene Mikrofon, dass dies ja wohl Beweis genug für die große Gefahr wäre, die der Klimawandel mit sich bringt.

Ein ernstes betroffenes Gesicht ist aber leider kein Garant dafür, dass die Aussage auch seriös wäre. Wie üblich wird der klimahistorische Kontext ausgeblendet bzw. Betrachtungsintervalle nach Gutdünken so verzerrt, dass der gewünschte Trend herauskommt. Vielfach haben wir an dieser Stelle bereits über Hurrikane und ihre klimatische Bedeutung berichtet (siehe Rubrik ‘Nordamerika’ hier). Beispiele:

 

Heute wollen wir neue Resultate aus dem Bereich der Hurrikanforschung vorstellen. Beginnen wollen wir mit einer Studie einer Gruppe um Berenice Rojo-Garibaldi, die im Oktober 2016 im Journal of Atmospheric and Solar-Terrestrial Physics erschien. Die Wissenschaftler nahmen die Hurrikanhäufigkeit der letzten 250 Jahre unter die Lupe und erkannten einen interessanten Zusammenhang mit der Sonnenaktivität: Je größer die Sonnenaktivität, desto geringer die Hurrikanhäufigkeit. Hier der Abstract:

Hurricanes in the Gulf of Mexico and the Caribbean Sea and their relationship with sunspots
We present the results of a time series analysis of hurricanes and sunspots occurring from 1749 to 2010. Exploratory analysis shows that the total number of hurricanes is declining. This decline is related to an increase in sunspot activity. Spectral analysis shows a relationship between hurricane oscillation periods and sunspot activity. Several sunspot cycles were identified from the time series analysis.

Das größte Versäumnis der alarmistischen Klimakommentatoren ist ihre klimageschichtliche Kurzsicht. Gerne wird auf den “stärksten”, “gefährlichsten”, “feuchtesten” Hurrikan der Messgeschichte hingewiesen. Vergessen wird dabei, dass erst seit wenigen Jahrzehnten Satelliten zur flächendeckenden Sturmüberwachung verwendet werden. Davor war man auf Einzelbeobachtungen und historische Berichte angewiesen, wobei etliche Stürme “durch die Lappen gingen”. Michael Chenoweth hat sich durch eine Vielzahl von historischen Quellen gewühlt und im Journal of Climate im Dezember 2014 eine Rekonstruktion der nordatlantischen Hurrikangeschichte 1851-1898 veröffentlicht. Dabei wies er große Lücken im offiziellen Hurrikankatalog nach. Hier der Abstract:

A New Compilation of North Atlantic Tropical Cyclones, 1851–98
A comprehensive new compilation of North Atlantic tropical cyclone activity for the years 1851–98 is presented and compared with the second-generation North Atlantic hurricane database (HURDAT2) for the same years. This new analysis is based on the retrieval of 9072 newspaper marine shipping news reports, 1260 original logbook records, 271 Maury abstract logs, 147 U.S. marine meteorological journals, and 34 Met Office (UKMO) logbooks. Records from throughout North America and the Caribbean region were used along with other primary and secondary references holding unique land and marine data. For the first time, North Atlantic daily weather maps for 1864/65, 1873, and 1881–98 were used in historical tropical cyclone research. Results for the years 1851–98 include the omission of 62 of the 361 HURDAT2 storms, and the further reduction resulting from the merging of storms to a total of 288 unique HURDAT2 tropical cyclones. The new compilation gave a total of 497 tropical cyclones in the 48-yr record, or an average of 10.4 storms per year compared to 6.0 per year in HURDAT2 less the author’s omissions. Of this total, 209 storms are completely new. A total of 90 hurricanes made landfall in the United States during this time. Seven new U.S. landfalling hurricanes are present in the new dataset but not in HURDAT2. Eight U.S. landfalling hurricanes in HURDAT2 are now considered to have only tropical storm impact or were actually extratropical at landfall. Across the North Atlantic, the number of category-4 hurricanes based on the Saffir–Simpson hurricane wind scale, compared with HURDAT2, increased from 11 to 25, 6 of which made U.S. landfall at category-4 level.

Es ist seit längerem bekannt, dass die Hurrikanhäufigkeit vom AMO-Ozeanzyklus mitgesteuert wird (siehe S. 202 in unserem Buch “Die kalte Sonne“). Aber auch andere Zusammenhänge werden langsam klarer. Offenbar nimmt die Intensität der Hurrikane in hurrikanarmen Zeiten zu. Und wenn es einmal richtig viele Hurrikane gibt, ist ihre Intensität niedriger. Eine solche Hurrikanwippe beschrieb am 4. Januar 2017 die University of Wisconsin-Madison:

More frequent hurricanes not necessarily stronger on Atlantic coast

Active Atlantic hurricane periods, like the one we are in now, are not necessarily a harbinger of more, rapidly intensifying hurricanes along the U.S. coast, according to new research performed at the University of Wisconsin–Madison. In fact, the research — published Wednesday (Jan. 4, 2017) in the journal Nature by James Kossin, a federal atmospheric research scientist based at the UW — indicates that hurricanes approaching the U.S. are more likely to intensify during less active Atlantic periods. During more active periods, they are more likely to weaken. The relationship between the number of hurricanes that develop in the Atlantic basin and the number of major hurricanes that make landfall is a weak one, says Kossin, and one that has not yet been well explained. The new study accounts for at least part of that relationship.

Historically, notes Kossin, researchers (including himself) have focused primarily on the tropical Atlantic — the main hurricane development region —without distinguishing how hurricane-producing conditions may vary outside of it. They knew a combination of warm ocean temperatures in the tropics and low vertical wind shear (changes in wind speed relative to altitude) results in favorable conditions for hurricane formation, while cooler than average sea surface temperatures work in tandem with higher than average wind shears to produce quieter hurricane seasons. Scientists also knew that environmental conditions, primarily ocean temperatures and wind shear, determine whether Atlantic hurricanes intensify or weaken as their natural track pushes them northwesterly toward the U.S. coast. But Kossin, a National Oceanic and Atmospheric Administration National Centers for Environmental Information scientist working out of UW–Madison’s NOAA Cooperative Institute, wondered “what other patterns there might be.” His study took a step back and looked for related patterns over the entire basin.

Kossin analyzed two datasets gathered over three 23-year periods from 1947 to 2015. The first dataset, from the historical record of hurricane observations maintained by the U.S. National Hurricane Center, supplied observations taken every six hours and included information on location, maximum winds and central pressure. The second, an environmental data set from the National Centers for Environmental Prediction and the National Center for Atmospheric Research, provided a benchmark for sea surface temperatures and wind shear for the period of interest. Overall, when the tropics generate many hurricanes — during periods of low wind shear and high ocean temperatures in the tropical Atlantic — they also create a situation where those hurricanes lose energy if they approach the coast, as they encounter a harsh environment of higher wind shear and cooler ocean temperatures. “They have to track through a gauntlet of high shear to reach the coast and many of them stop intensifying,” Kossin says. “It is a natural mechanism for killing off hurricanes that threaten the U.S. coast.”

What are the implications for U.S. coastal regions? “It is good news,” says Kossin. “Greater activity produces more threats, but at the same time, we increase our protective barrier. It’s pretty amazing that it happens to work that way.” The data suggest we may be moving into another quieter period in the basin, however, where less activity works hand in hand with lower wind shears along the coast, eradicating the protective barrier. As a result, says Kossin, while there may be fewer hurricanes approaching the coast, those that do may be much stronger, in the range of Category 3 to Category 5. The threat of rapid strengthening is highly relevant to society, in particular to those who live along densely populated coastlines where the warning times for evacuation may already be short. “Knowing the relationship between tropical activity and coastal conditions that either protect the coast or make it more vulnerable may help us better prepare for future landfalls,” Kossin says.

Like any research study, the results raise more questions. For instance, how might climate change affect this relationship? Other studies, says Kossin, have documented a rise in sea surface temperatures — a shift attributed to anthropogenic climate change. But the sea surface trend does not seem to be having a large effect on hurricane activity in the coastal region, at least over the past 70 years or so. Kossin says this could fall under the heading of a “climate surprise” if the environmental conditions responsible for the protective barrier during active periods are compromised by climate change. “There is no reason to think that this is a stationary mechanism,” says Kossin. “It’s entirely possible that changes in climate could affect the natural barrier and thus significantly increase coastal hazard and risk.”