Wellennummer-5 und Ananas-Express beenden langjährige Dürrezeit in Kalifornien

Gute Nachrichten aus Kalifornien. Nach einigen trockenen Jahren ist nun der Regen zurückgekehrt, wie Zeit Online am 8. April 2017 meldete:

Klimawandel: Kaliforniens Dürre ist offiziell vorbei

Fünf Jahre lang litt Kalifornien unter Waldbränden, Wasserknappheit und trockenen Äckern. Regen erlöste den US-Bundesstaat schließlich. Doch die nächste Dürre droht.

Nach heftigen Regenfällen in den vergangenen Monaten hat der kalifornische Gouverneur Jerry Brown die Dürre in seinem US-Bundesstaat für beendet erklärt. „Der Dürre-Notstand ist vorbei“, sagte Brown. Eine Karte der Klimatologen vom United States Drought Monitor zeigt, dass kein Gebiet in Kalifornien mehr von außergewöhnlicher oder extremer Dürre betroffen ist. Noch vor einem Jahr war mehr als die Hälfte der Fläche Kaliforniens diesen besonders schweren Formen der Dürre ausgesetzt.

Weiterlesen auf Zeit Online.

Was Die Zeit unerwähnt lässt: Das Winterhalbjahr 2016/17 war sogar das zweitfeuchteste der letzten 122 Jahre. Die NASA gab hierzu eine eigene Meldung heraus und erklärte die Entwicklung mit einem atmosphärischen Fluss, dem „Ananas-Express“. Schön wäre es zudem gewesen, wenn Die Zeit hier hochaktuelle Forschungsergebnisse erwähnt hätte, nach denen die kalifornische Dürreserie mit einem natürlichen Phänomen zusammenhängt, nämlich den atmosphärischen Wellen. Pressemitteilung der University Corporation for Atmospheric Research vom 6. April 2017, also zwei Tage vor dem Zeit-Artikel:

Scientists link California droughts and floods to distinctive atmospheric waves

The crippling wintertime droughts that struck California from 2013 to 2015, as well as this year’s unusually wet California winter, appear to be associated with the same phenomenon: a distinctive wave pattern that emerges in the upper atmosphere and circles the globe.

Scientists at the National Center for Atmospheric Research (NCAR) found in a recent study that the persistent high-pressure ridge off the west coast of North America that blocked storms from coming onshore during the winters of 2013-14 and 2014-15 was associated with the wave pattern, which they call wavenumber-5. Follow-up work showed that wavenumber-5 emerged again this winter but with its high- and low-pressure features in a different position, allowing drenching storms from the Pacific to make landfall.  „This wave pattern is a global dynamic system that sometimes makes droughts or floods in California more likely to occur,“ said NCAR scientist Haiyan Teng, lead author of the California paper. „As we learn more, this may eventually open a new window to long-term predictability.“

The finding is part of an emerging body of research into the wave pattern that holds the promise of better understanding seasonal weather patterns in California and elsewhere. Another new paper, led by NCAR scientist Grant Branstator, examines the powerful wave pattern in more depth, analyzing the physical processes that help lead to its formation as well as its seasonal variations and how it varies in strength and location. The California study was published in the Journal of Climate while the comprehensive study into the wave patterns is appearing in the Journal of the Atmospheric Sciences. Both papers were funded by the National Science Foundation, which is NCAR’s sponsor, as well as by the Department of Energy, the National Oceanic and Atmospheric Administration, and NASA. The new papers follow a 2013 study by Teng and Branstator showing that a pattern related to wavenumber-5 tended to emerge about 15-20 days before major summertime heat waves in the United States.

Strong impacts on local weather systems

Wavenumber-5 consists of five pairs of alternating high- and low-pressure features that encircle the globe about six miles (10 kilometers) above the ground. It is a type of atmospheric phenomenon known as a Rossby wave, a very large-scale planetary wave that can have strong impacts on local weather systems by moving heat and moisture between the tropics and higher latitudes as well as between oceanic and inland areas and by influencing where storms occur. The slow-moving Rossby waves at times become almost stationary. When they do, the result can be persistent weather patterns that often lead to droughts, floods, and heat waves. Wavenumber-5 often has this stationary quality when it emerges during the northern winter, and, as a result, is associated with a greater likelihood of persistent extreme events.

To determine the degree to which the wave pattern influenced the California drought, Teng and Branstator used three specialized computer models, as well as California rainfall records and 20th century data about global atmospheric circulation patterns. The different windows into the atmosphere and precipitation patterns revealed that the formation of a ridge by the California coast is associated with the emergence of the distinctive wavenumber-5 pattern, which guides rain-producing low-pressure systems so that they travel well north of California.

Over the past winter, as California was lashed by a series of intense storms, wavenumber-5 was also present, the scientists said. But the pattern had shifted over North America, replacing the high-pressure ridge off the coast with a low-pressure trough. The result was that the storms that were forced north during the drought winters were, instead, allowed to make landfall.

Clues to seasonal weather patterns

Forecasters who predict seasonal weather patterns have largely looked to shifting sea surface temperatures in the tropical Pacific, especially changes associated with El Niño and La Niña. But during the dry winters of 2013-14 and 2014-15, those conditions varied markedly: one featured the beginning of an El Niño while the sea surface temperatures during the other were not characteristic of either El Niño or La Niña. The new research indicates that the wave pattern may provide an additional source of predictability that sometimes may be more important than the impacts of sea surface temperature changes. First, however, scientists need to better understand why and when the wave pattern emerges.

In the paper published in Journal of the Atmospheric Sciences, Branstator and Teng explored the physics of the wave pattern. Using a simplified computer model of the climate system to identify the essential physical processes, the pair found that wavenumber-5 forms when strong jet streams act as wave guides, tightening the otherwise meandering Rossby wave into the signature configuration of five highs and five lows. „The jets act to focus the energy,“ Branstator said. „When the jets are present, the energy is trapped and cannot escape.“ But even when the jets are present, the wavenumber-5 pattern does not always form, indicating that other forces requiring study are also at play.

The scientists also searched specifically for what might have caused the wave pattern linked to the severe California drought to form. In the paper published in the Journal of Climate, the pair found that extremely heavy rainfall from December to February in certain regions of the tropical Pacific could double the probability that the extreme ridge associated with wavenumber-5 will form. The reason may have to do with the tropical rain heating parts of the upper atmosphere in such a way that favors the formation of the wavenumber-5 pattern. But the scientists cautioned that many questions remain. „We need to search globally for factors that cause this wavenumber-5 behavior,“ Teng said, „Our studies are just the beginning of that search.“

About the articles:
Title: Causes of Extreme Ridges That Induce California Drought

Authors: Haiyan Teng and Grant Branstator
Journal: Journal of Climate, DOI: 10.1175/JCLI-D-16-0524.1

Title: Tropospheric Waveguide Teleconnections and Their Seasonality
Authors: Grant Branstator and Haiyan Teng
Journal: Journal of the Atmospheric Sciences, DOI: 10.1175/JAS-D-16-0305.1

Ein Paper von Luo und Kollegen in den Geophysical Research Letters vom 9. April 2017 stellte fest, dass die kalifornischen Dürren auf Niederschlagsmangel und weniger auf Temperaturextreme zurückgeht:

Contribution of temperature and precipitation anomalies to the California drought during 2012–2015
The recent multiyear drought over California was characterized by large precipitation deficits and abnormally high temperatures during both wet and dry seasons. This study investigates and quantifies the contributions of precipitation and temperature anomalies to the development of the multiyear drought with a set of modeling experiments where the anomalies are either removed or randomly replaced with other historical observations. The study reveals that precipitation deficits have been largely responsible for producing the extreme agricultural drought (i.e., large soil moisture deficits) while warmer temperatures have only marginally intensified the drought. However, the warmer temperatures over the high-elevation areas during the wet season have contributed equally or more than the precipitation deficits to the reduction of snowpack. The interplay between temperature and precipitation anomalies in space and time also appears to be important for the drought development.

Bereits ein Jahr zuvor hatte die University of Texas in Austin den Wind als wichtigen Auslöser der Regenarmut und kalifornischen Dürren ausgemacht. In einer Pressemitteilung gab die Uni am 5. Juli 2016 bekannt:

California Droughts Caused Mainly by Changes in Wind, Not Moisture

Droughts in California are mainly controlled by wind, not by the amount of evaporated moisture in the air, new research has found. The findings were published in Geophysical Research Letters, a journal of the American Geophysical Union, on June 30. The research increases the understanding of how the water cycle is related to extreme events and could eventually help in predicting droughts and floods, said lead author Jiangfeng Wei, a research scientist at The University of Texas at Austin’s Jackson School of Geosciences. “Ocean evaporation provides moisture for California precipitation but is not the reason for droughts there, although the ocean evaporation is slightly lower during droughts,” Wei said.

The researchers analyzed 30-year data sets that recorded precipitation, ocean evaporation, surface wind speed and atmospheric pressure on and near the west coast of the United States. These are all factors that influence the water cycle in California. One of the difficulties of studying the water cycle, Wei said, is that the water sources for precipitation cannot be directly observed, so the team also used a mathematical moisture-tracking method and high-resolution model simulations. Their analysis showed that although moisture evaporated from the Pacific Ocean is the major source for California precipitation, the amount of water evaporated did not strongly influence precipitation in California, except in the cases of very heavy flooding. That’s because the amount of water evaporated from this ocean region does not change much year by year, researchers found, and did not cause rain to occur more or less often. “Ocean evaporation has little direct influence on California precipitation because of its relatively weak variability,” Wei said.

Instead, the researchers found that disturbances in atmospheric circulation, the large-scale movement of air, have the most effect on drought because they can affect factors that will cause it to rain more or less. The study co-authors are Qinjian Jin, a postdoctoral researcher at the Massachusetts Institute of Technology who earned his Ph.D. at the Jackson School; Zong-Liang Yang, a professor in the Jackson School’s Department of Geological Sciences; and Paul Dirmeyer, a professor at George Mason University.

Most of California has been in a severe drought since 2011, although a strong El Niño in the winter of 2015 helped diminish the drought. The current drought is caused by a high-pressure system that disturbs the atmospheric circulation. The development of the high-pressure system is related to a sea surface temperature pattern in the Pacific Ocean, according to research cited by the study. “Although this is a very rare event, the probability of this kind of high-pressure system is likely increasing with global warming,” the authors said. Yang said that the research could aid in the prediction of droughts and floods by improving scientific understanding of the intricate factors that influence rainfall the most. “The topic is extremely timely as current and future climate change would mean more changes in extreme events such as droughts and floods,” Yang said. “Understanding this asymmetric contribution of ocean evaporation to drought and flooding in California will ultimately help us make better predictions.”

Welche Schuld trägt nun der Mensch an den Dürren? Laut den oben genannten Studien spielt hier vor allem die natürliche Variabilität eine Rolle, auch wenn Stefan Rahmstorf und Michael Mann dies nicht wahrhaben wollen. Trotzdem ist der Mensch am Ausmaß der kalifornischen Dürren offenbar nicht ganz unschuldig. Eine Studie von Xiaogang He und Kollegen vom 18. Februar 2017 in den Geophysical Research Letters fand, dass unangepasstes Wassermanagement die Dürren künstlich verlängert hat. Anbei der Abstract:

Intensification of hydrological drought in California by human water management
We analyze the contribution of human water management to the intensification or mitigation of hydrological drought over California using the PCR-GLOBWB hydrological model at 0.5° resolution for the period 1979–2014. We demonstrate that including water management in the modeling framework results in more accurate discharge representation. During the severe 2014 drought, water management alleviated the drought deficit by ∼50% in Southern California through reservoir operation during low-flow periods. However, human water consumption (mostly irrigation) in the Central Valley increased drought duration and deficit by 50% and 50–100%, respectively. Return level analysis indicates that there is more than 50% chance that the probability of occurrence of an extreme 2014 magnitude drought event was at least doubled under the influence of human activities compared to natural variability. This impact is most significant over the San Joaquin Drainage basin with a 50% and 75% likelihood that the return period is more than 3.5 and 1.5 times larger, respectively, because of human activities.