So beeinflussen die atlantischen Ozeanzyklen das europäische Klima

Das Klima wandelt sich ständig. Ein wichtiger Treiber sind die Ozeanzyklen, die lang Jahre von der Wissenschaft und dem IPCC ignoriert wurden. Mittlerweile haben die Forscher jedoch gut aufgeholt und publizieren fleissig zum Klimafaktor Ozeanzyklen. Nun wird es höchste Zeit, dass die Modellierer Nachtschichten einlegen und die Zusammenhänge in ihren Simulationen robust nachvollziehen. Im Folgenden wollen wir eine kleinen Streifzug durch die neuere Literatur der atlantischen Ozeanzyklen unternehmen.

Im Dezember 2015 veröffentlichten Luca Lanci und Ann Hirt in Palaeo3 eine Klimarekonstruktion von Alpenseen, die 2500 Jahre zurückreicht. In den magnetischen Komponenten der Seesedimente fanden die Forscher einen deutlichen Zyklus von 50-60 Jahren, den sie als AMO-Zyklus, als Atlantische Multidekadenoszillation, identifizerten Interessanter weise pulste sich die AMO durch den gesamten Datensatz durch, woraufhin die Forscher schlussfolgerten, dass die AMO das Alpenklima bereits seit mehreren Jahrtausenden beeinflusste. Hier der Abstract:

Evidence of Atlantic Multidecadal Oscillation in the magnetic properties of Alpine lakes during the last 2500 years
We re-analyzed rock-magnetic data of lake sediments from the Alpine region, which were previously recognized as sensitive climate proxy records, in search of decadal climate variability. Different methods of spectral analysis applied to the rock-magnetic data from two independent lake records show the presence of a coherent and statistically significant periodicity with a period of about 50–60 years. The frequency-filtered signal of this component co-varies with the instrumental record of the Atlantic Multidecadal Oscillation (AMO) over the last 110 years with a negligible phase shift. After having tested and ruled out the possible influence of solar irradiance in the putative AMO frequency band, based on spectral properties, we suggest that a significant influence of AMO in the Alpine region extends back in time for at least two millennia. Comparison of the amplitude of the AMO record with that of the Younger Dryas/Holocene transition suggests that AMO fluctuations played a significant role in pacing the past variability of the Alpine climate.

Ein ähnliches Bild bietet sich aus Südsibirien, wo der AMO-Zyklus offenbar die Niederschläge steuert, wie eine Studie von Steffi Hildebrandt und Kollegen aus dem Mai 2015 in Palaeo3 zeigte:

Tracing the North Atlantic decadal-scale climate variability in a late Holocene pollen record from southern Siberia
This paper presents a new palynological record from a 146 cm long finely laminated sediment core obtained in 2009 from the deep-water meromictic Lake Shira (54°30′38″N, 90°12″09′E; ca. 353 m a.s.l.) situated in the Khakassian steppe region of southern Siberia between the rivers Ob’ and Yenisei. The area is rich in lakes and represents an exceptionally well preserved sequence of Bronze and Iron Age archeological cultures. Little is known about the changes in vegetation and climate of the region during the Holocene. The palynological analysis of the core allows us to partly fill up this gap in current knowledge. The record of pollen and non-pollen palynomorphs presented here covers the past 2450 year interval with an average resolution of 22 years. The results obtained support the interpretation that the late Holocene vegetation changes around Lake Shira are mainly associated with large-scale atmospheric circulation processes controlling the regional water balance rather than with human activities. An attempt to trace human impact in the pollen assemblages provides no clear evidence for anthropogenic activity, except for the last few decades since ca. 1955, though the region has a long history of mobile pastoralists. For explanation of decadal-scale changes in the regional vegetation cover, the Artemisia/Chenopodiaceae (A/C) pollen ratio proved to be a reliable indicator of effective moisture availability. Using available fossil and published instrumental data our study suggests a link between the North Atlantic warmer/colder temperatures and higher/lower atmospheric precipitation (or moisture availability) in southern Siberia at multi-decadal to centennial scales.

Highlights:

  • Meromictic Lake Shira discloses high-resolution climate archive from the Khakassian steppe.
  • Artemisia/Chenopodiaceae pollen ratio reflects regional moisture variability around Lake Shira.
  • The Atlantic Multidecadal Oscillation may affect moisture variability in southern Siberia.

Aber auch ein anderer atlantischer Ozeanzyklus zeigt sich in den Klimarekonstruktionen: Die Nordatlantische Oszillation (NAO). Eine Gruppe um Sndy Baker fand die NAO in Tropfsteinen einer schottischen Höhle und konnte das NAO-Pochen 3000 Jahre weit zurückverfolgen. Während der Mittelalterlichen Wärmeperiode verharrte die NAO ungewöhnlich lange in der positiven Phase. Pressemeldung der University of New South Wales vom 15. Juni 2015 (via Science Daily):

Remote cave study reveals 3000 years of European climate variation
University of New South Wales Australia-led research on limestone formations in a remote Scottish cave has produced a unique 3000-year-long record of climatic variations that may have influenced historical events including the fall of the Roman Empire and the Viking Age of expansion. The study of five stalagmites in Roaring Cave north of Ullapool in north-west Scotland is the first to use a compilation of cave measurements to track changes in a climate phenomenon called the North Atlantic Oscillation.’Our results also provide the longest annual record of this important phenomenon, which has a big impact on the climate in Europe,’ says study leader, UNSW Professor Andy Baker. ‘It confirms that the during the Medieval Warm Period between 1080 and 1430 the oscillation index was in an unusually prolonged positive phase, which brings increased rain to Scotland and drier conditions in the western Mediterranean,’ says Baker, of the UNSW Connected Waters Initiative Research Centre.

‘Our results also reveal there was another persistent positive phase between 290 and 550, which coincides with the decline of Rome and a period of intensified human migration in southern Europe during the Dark Ages.’This was followed by a persistent negative phase between 600 and 900 which may have provided warm and dry conditions in northwestern Europe that made it suitable for westward expansion by the Vikings, although the precise timing of this event is contested.’The study is published in the journal Scientific Reports.

The North Atlantic Oscillation climate index measures the air pressure difference between Iceland and the Azores islands off the Portuguese coast, and is a record of the strength of the westerly winds in the North Atlantic. Roaring Cave, or Uamh an Tartair, in northwest Scotland, is a shallow cave beneath a blanket of peat that has accumulated during the past 4000 years. Rainfall levels in this region closely correspond with the strength of the oscillation index in winter, with higher precipitation when it is positive. And the upward rate of growth of stalagmites in the cave is very sensitive to rainfall — the more water in the peat, the more slowly the stalagmites grow.

‘We painstakingly measured the thickness of each annual growth ring in five stalagmites taken from the cave, including one that provides a continuous annual record spanning more than 1800 years,’ says Baker. By overlapping the five stalagmites they obtained a proxy record of the climate at the cave during a 3000-year period from about 1000 BC to 2000 AD. ‘Our research provides a climate context for some of the big human migration events in Europe and allows us to start building hypotheses about the impact of environment on societal change,’ says Baker. The team includes researcher from UNSW, the University of Lausanne in Switzerland and the University of Arizona in the U.S.

Paper: Andy Baker, John C. Hellstrom, Bryce F. J. Kelly, Gregoire Mariethoz, Valerie Trouet. A composite annual-resolution stalagmite record of North Atlantic climate over the last three millennia. Scientific Reports, 2015; 5: 10307 DOI: 10.1038/srep10307

Eine Gruppe um Pablo Ortega wollte die positive NAO während der Mittelalterlichen Wärmeperiode nicht wahr haben und schrieb im Juli 2015 in Nature kräftig dagegen an, bezugnehmend auf eine Arbeit von Trouet et al. 2009. Ob sie die neuen Ergebnisse von Baker et al. 2015 noch nicht kannten? Abstract von Ortega et al. 2015:

A model-tested North Atlantic Oscillation reconstruction for the past millennium
The North Atlantic Oscillation (NAO) is the major source of variability in winter atmospheric circulation in the Northern Hemisphere, with large impacts on temperature, precipitation and storm tracks1, and therefore also on strategic sectors such as insurance2, renewable energy production3, crop yields4 and water management5. Recent developments in dynamical methods offer promise to improve seasonal NAO predictions6, but assessing potential predictability on multi-annual timescales requires documentation of past low-frequency variability in the NAO. A recent bi-proxy NAO reconstruction7 spanning the past millennium suggested that long-lasting positive NAO conditions were established during medieval times, explaining the particularly warm conditions in Europe during this period; however, these conclusions are debated. Here, we present a yearly NAO reconstruction for the past millennium, based on an initial selection of 48 annually resolved proxy records distributed around the Atlantic Ocean and built through an ensemble of multivariate regressions. We validate the approach in six past-millennium climate simulations, and show that our reconstruction outperforms the bi‐proxy index. The final reconstruction shows no persistent positive NAO during the medieval period, but suggests that positive phases were dominant during the thirteenth and fourteenth centuries. The reconstruction also reveals that a positive NAO emerges two years after strong volcanic eruptions, consistent with results obtained from models and satellite observations for the Mt Pinatubo eruption in the Philippines8, 9.

Der systematische Einfluss der NAO auf das europäische Klima wird immer deutlicher. Lee Drake stellte im April 2017 in Scientific Reports den geschichtlichen Zusammenhang zwischen Ozeanzyklus und Völkerwanderungen in eindrucksvoller Weise dar:

Changes in North Atlantic Oscillation drove Population Migrations and the Collapse of the Western Roman Empire
Shifts in the North Atlantic Oscillation (NAO) from 1–2 to 0–1 in four episodes increased droughts on the Roman Empire’s periphery and created push factors for migrations. These climatic events are associated with the movements of the Cimbri and Teutones from 113–101 B.C., the Marcomanni and Quadi from 164 to 180 A.D., the Goths in 376 A.D., and the broad population movements of the Migration Period from 500 to 600 A.D. Weakening of the NAO in the instrumental record of the NAO have been associated with a shift to drought in the areas of origin for the Cimbri, Quadi, Visigoths, Ostrogoths, Huns, and Slavs. While other climate indices indicate deteriorating climate after 200 A.D. and cooler conditions after 500 A.D., the NAO may indicate a specific cause for the punctuated history of migrations in Late Antiquity. Periodic weakening of the NAO caused drought in the regions of origin for tribes in antiquity, and may have created a powerful push factor for human migration. While climate change is frequently considered as a threat to sustainability, its role as a conflict amplifier in history may be one of its largest impacts on populations.

Ganz frisch aus der Druckerpresse ist eine Studie von Caroline Ummenhofer und Kollegen vom 25. August 2017 aus den Geophysical Research Letters. Dort geht es ebenfalls um die sogenannte multidekadische Variabilität, die in den vergangenen Jahrzehnten in Nordeuropa eine Zunahme und in Südeuropa eine Abnahme des Winterregens brachte:

Emerging European winter precipitation pattern linked to atmospheric circulation changes over the North Atlantic region in recent decades
Dominant European winter precipitation patterns over the past century, along with their associated extratropical North Atlantic circulation changes, are evaluated using cluster analysis. Contrary to the four regimes traditionally identified based on daily wintertime atmospheric circulation patterns, five distinct seasonal precipitation regimes are detected here. Recurrent precipitation patterns in each regime are linked to changes in atmospheric blocking, storm track, and sea surface temperatures across the North Atlantic region. Multidecadal variability in the frequency of the precipitation patterns reveals more (fewer) winters with wet conditions in northern (southern) Europe in recent decades and an emerging distinct pattern of enhanced wintertime precipitation over the northern British Isles. This pattern has become unusually common since the 1980s and is associated with changes in moisture transport and more frequent atmospheric river events. The observed precipitation changes post-1950 coincide with changes in storm track activity over the central/eastern North Atlantic toward the northern British Isles.

Wäre es nicht nützlich, wenn man die NAO besser vorhersagen könnte, anstatt immer nur auf sie zu reagieren. Dazu müsste man sie aber zunächst vollständig verstehen. Nick Dunstone und Kollegen beschrieben im Oktober 2016 in Nature Geoscience den Versuch einer einjährigen Vorhersage. Beachtenswert: Die Autoren berücksichtigen in ihrem verbesserten Modell Veränderungen der Sonnenaktivität.

Skilful predictions of the winter North Atlantic Oscillation one year ahead
The winter North Atlantic Oscillation is the primary mode of atmospheric variability in the North Atlantic region and has a profound influence on European and North American winter climate. Until recently, seasonal variability of the North Atlantic Oscillation was thought to be largely driven by chaotic and inherently unpredictable processes1, 2. However, latest generation seasonal forecasting systems have demonstrated significant skill in predicting the North Atlantic Oscillation when initialized a month before the onset of winter3, 4, 5. Here we extend skilful dynamical model predictions to more than a year ahead. The skill increases greatly with ensemble size due to a spuriously small signal-to-noise ratio in the model, and consequently larger ensembles are projected to further increase the skill in predicting the North Atlantic Oscillation. We identify two sources of skill for second-winter forecasts of the North Atlantic Oscillation: climate variability in the tropical Pacific region and predictable effects of solar forcing on the stratospheric polar vortex strength. We also identify model biases in Arctic sea ice that, if reduced, may further increase skill. Our results open possibilities for a range of new climate services, including for the transport6, 7, energy, water management8 and insurance sectors.

Allmählich schleicht sich die Sonne in das Allerheiligste der Klimawissenschaften, die Klimamodelle. Offenbar hat man nun doch endlich verstanden, dass man langfristig nicht ohne sie auskommt, die Modelle nicht nachhaltig funktionieren können. Ein Konferenzbeitrag von Tobias Spiegl und Ulrike Langematz aus dem April 2016 erhärtet den Zusammenhang zwischen solaren Aktivitätsschwanungen und Ozeanzyklen. Die Autoren sprechen einen wichtigen Punkt an: Der IPCC verwendet viel Zeit darauf, verschiedene Emissionsszenarien zu modellieren, ignoriert aber viel zu oft, dass auch die unklare Rolle der natürlichen Klimafaktoren – wie z.B. solare Aktivitätsschwankungen – ein breites Möglichkeitsspektrum eröffnet, das es zu berücksichtigen gilt. Dies führt letztendlich zu einer weiten Spanne der CO2-Klimasensitivität von 1,5-4,5°C Erwärmung pro CO2-Verdopplung, was enorm ist. Spiegl und Langematz führen die Kleine Eiszeit als klassisches Beispiel der solaren Beeinflussung an, wobei nichtlineare und schlecht verstandene Rückkopplungen (also Klimaverstärker) zu einer starken Abkühlung während einer Phase extrem geringer Sonnenaktivität geführt hat. Da nun wieder ein starker Abfahl der solaren Aktivität stattfindet, sei es umso wichtiger, die genauen Prozesse des solaren Klimatreibers zu verstehen, auch unter Berücksichtigung der Klimageschichte, was wichtige Hindcast-Möglichkeiten eröffnet. Hier der Abstract. Das pdf des Posters ist auf Researchgate verfügbar.

Potential impacts of a future Grand Solar Minimum on decadal regional climate change and interannual hemispherical climate variability
The political, technical and socio-economic developments of the next decades will determine the magnitude of 21st century climate change, since they are inextricably linked to future anthropogenic greenhouse gas emissions. To assess the range of uncertainty that is related to these developments, it is common to assume different emission scenarios for 21st climate projections. While the uncertainties associated with the anthropogenic greenhouse gas forcing have been studied intensely, the contribution of natural climate drivers (particularly solar variability) to recent and future climate change are subject of intense debate. The past 1,000 years featured at least 5 excursions (lasting 60-100 years) of exceptionally low solar activity, induced by a weak magnetic field of the Sun, so called Grand Solar Minima. While the global temperature response to such a decrease in solar activity is assumed to be rather small, nonlinear mechanisms in the climate system might amplify the regional temperature signal. This hypothesis is supported by the last Grand Solar Minimum (the Maunder Minimum, 1645-1715) which coincides with the Little Ice Age, an epoch which is characterized by severe cold and hardship over Europe, North America and Asia. The long-lasting minimum of Solar Cycle 23 as well as the overall weak maximum of Cycle 24 reveal the possibility for a return to Grand Solar Minimum conditions within the next decades. The quantification of the implications of such a projected decrease in solar forcing is of ultimate importance, given the on-going public discussion of the role of carbon dioxide emissions for global warming, and the possible role a cooling due to decreasing solar activity could be ascribed to. Since there is still no clear consensus about the actual strength of the Maunder Minimum, we used 3 acknowledged solar reconstruction datasets that show significant differences in both, total solar irradiance (TSI) and spectral irradiance (SSI) to simulate a future Grand Solar Minimum under RCP6.0 conditions. The results obtained were compared to a RCP6.0 simulation that was carried out using the CCMI recommendations for a 21st century solar forcing. We used the ECHAM/MESSy Atmospheric Chemistry (EMAC) chemistry-climate model that incorporates interactive ozone chemistry, a high-resolution shortwave radiation scheme, a high model top (0.01 hPa) and is coupled to a 3D ocean general circulation model. We focused on the regional responses to a future Grand Solar Minimum and interannual variability patterns (i.e. the Northern and Southern Annular Mode (NAM/SAM)).

Aus den Poster-Conclusions:

Our analysis of the wintertime NH variability shows that a reduced solar irradiance, comparable to that of the Maunder Minimum, leads to more NAO(-) and weak vortex events. Additionally the coupling between the stratosphere is stronger in both cases, weak but in particular strong vortex events.

Eine geringe Sonnenaktivität führt zu einer negativen NAO. Das passt bestens mit dem Ergebnis von Baker et al. 2015 (siehe oben), die eine positive NAO für die solar starke Phase der Mittelalterlichen Wärmeperiode fanden.