Forschern dämmert: Atlantische Ozeanzyklen machen Klima

Die Ozeane stellen ein großes Wärmereservoir dar. Im Takte von 60 jährigen Zyklen verschlucken sie drei Jahrzehnte lang Wärme, die sie dann in den folgenden 30 Jahren wieder ausspucken. Im Atlantik läuft dieses Phänomen unter dem Begriff Atlantische Multidekadenoszillation (AMO). Klimamodelle können die Zyklik nicht richtig nachvollziehen, was die daraus abgeleiteten Klimaprognosen zweifelhaft erscheinen lässt. In den letzten Jahren hat sich auf dem Gebiet der Ozeanzyklik kräftig etwas getan. Die systematische Klimabeeinflussung ist jetzt von der Fachwelt endlich akzeptiert. Ein gutes Beispiel ist ein Paper von Dan Seidov und Kollegen im Mai 2017 in den Geophysical Research Letters:

Multidecadal variability and climate shift in the North Atlantic Ocean
Decadal variability of ocean heat content (OHC) and temperature trends over ~60 years in the North Atlantic Ocean were analyzed using a new high-resolution ocean climatology based on quality-controlled historic in situ observations. Тwo ~30 year ocean climates of 1955–1984 and 1985–2012 were compared to evaluate the climate shift in this region. The spatial distribution of the OHC climate shift is highly inhomogeneous, with the climate shift being the strongest southeast of the Gulf Stream Extension. This may be caused by the Atlantic Meridional Overturning Circulation slowdown in conjunction with heaving of warm subtropical water. The 30 year climate shift shows higher OHC gain in the Gulf Stream region than reported in shorter timescale estimates. The OHC change is generally coherent with the Atlantic Multidecadal Oscillation index. This coherence suggests that quasi-cyclicity of the OHC may exist, with a period of 60 to 80 years, superimposed on the slow basin-wide warming trend.

Endlich können auch klimatische Mittelfristprognosen von der Ozeanzyklik profitieren, die vor 5 Jahren – als unser Buch Die kalte Sonne erschien – noch heftig vom peinlich berührten Klimaestablishment abgestritten wurden. Heute ist man klüger. In Irland hängen Temperaturen und Niederschläge zu über 90% an der AMO, wie McCarthy et al. 2015 im Juli 2015 im Fachblatt ‚Weather‘ dokumentierten:

The influence of ocean variations on the climate of Ireland
The influence of the ocean circulation on the climate of Ireland is more subtle than it first appears. Temperatures in Ireland are warmer than similar Pacific maritime climates. It is heat – carried primarily in the Atlantic overturning circulation – released over the Atlantic that provides this additional warmth. We investigate variations in Irish climate using long-term station-based time series. The Atlantic multidecadal oscillation (AMO) explains over 90% of the pronounced decadal temperature and summer precipitation variation. Understanding the impact of these ocean variations when interpreting long climate records, particularly in the context of a changing climate, is crucial.

Die natürliche dekadische Variablität des irischen Klimas ist zwei mal so stark wie der Erwärmungstrend sagt die Arbeit. In den Jahren 1990-2002 trug also die Variabilität zum Anstieg bei. Das wird so nicht gesagt, jedoch vor der anderen “Flanke” des Signals gewarnt:

“Otherwise, decades of cooling can be seen as a contradiction to increased surface temperature trends (in response to continually increasing greenhouse gas emission) when natural ocean variability may be the cause.”

Modelle die die ansteigende Flanke der AMO gut replizieren und allein auf CO2 zurückführen, überschätzen es demnach. Auch wenn die volle Erkenntnis aus verschiedenen Gründen nicht immer vollständig genannt werden kann, setzt sich die überragende Bedeutung der Ozeanzyklen und ihr Beitrag zu den Erwärmungsphasen immer mehr durch. So erschien im Mai 2017 in PNAS eine Arbeit von Bowene und Kollegen. Dort traute man sich nicht ganz an die Heutezeit heran und beschäftigte sich lieber mit einem Zyklus früher. Ihre Botschaft: Die Erwärmung des frühen 20. Jahrhunderts in der Arktis wurde durch die Ozeanzyklen verstärkt. Hier die Pressemiteilung der Kyoto University (via Science Daily):

Scientists uncover a cause for early 20th century Arctic warming

Is a warmer Arctic a canary of global warming?

Since the 1970s the northern polar region has warmed faster than global averages by a factor or two or more, in a process of ‘Arctic amplification’ which is linked to a drastic reduction in sea ice. But then how to explain a similar rapid warming that occurred during the early 20th century, when the effects of greenhouse gases were considerably weaker than today? And what can we prove about the period, given the scarcity of usable data and observations prior to the 1950s? Now scientists from Kyoto University and UC San Diego have discovered that this phenomenon occurred when the warming phase — ‘interdecadal variability mode’ — of both the Pacific and Atlantic Oceans coincided. The team’s findings appeared recently in the journal PNAS.

“We found that early 20th century sea surface temperatures in the tropical Pacific and North Atlantic had warmed much more than previously thought,” explains lead author Hiroki Tokinaga of Kyoto. “Using observations and model simulations, we’ve demonstrated that rising Pacific-Atlantic temperatures were the major driver of rapid Arctic warming in the early 20th century.” Previous explanations for early Arctic warming have including decreased volcanic aerosols and increased solar radiation, but none of these have been able to simulate observed conditions from the period.

Tokinaga’s team found that when the interdecadal rise in sea surface temperatures was included in simulation calculations, the results properly reflected early Arctic conditions. “Coupled ocean-atmosphere simulations also support the intensification of Arctic warming,” continues Shang-Ping Xie of UCSD, “which was caused by a concurrent, cold-to-warm phase shift of Pacific and Atlantic interdecadal modes.” The researchers explain that these new findings can help constrain model climate projections over the Arctic region.

“It is likely that temperatures in the Arctic will continue to rise due to anthropogenic global warming,” concludes Tokinaga. “Our study does not deny this. We are rather suggesting that Arctic warming could accelerate or decelerate due to internal variability of the Pacific and the Atlantic.” “It is a challenge to accurately predict when the next big swing of multidecadal variability will occur. Careful monitoring is essential, given the enormous impact on the Arctic climate.”

Gabriel J. Bowene et al. Early 20th-century Arctic warming intensified by Pacific and Atlantic multidecadal variability. PNAS, May 2017 DOI: 10.1073/pnas.1615880114

Im September 2015 diskutierte Judith Curry eine Nature-Arbeit von McCarthy et al., in der der bevorstehende Umschwung der AMO in die negative, kühlende Phase prognostizert wird:

Ocean impact on decadal Atlantic climate variability revealed by sea-level observations
Decadal variability is a notable feature of the Atlantic Ocean and the climate of the regions it influences. Prominently, this is manifested in the Atlantic Multidecadal Oscillation (AMO) in sea surface temperatures. Positive (negative) phases of the AMO coincide with warmer (colder) North Atlantic sea surface temperatures. The AMO is linked with decadal climate fluctuations, such as Indian and Sahel rainfall1, European summer precipitation2, Atlantic hurricanes3 and variations in global temperatures4. It is widely believed that ocean circulation drives the phase changes of the AMO by controlling ocean heat content5. However, there are no direct observations of ocean circulation of sufficient length to support this, leading to questions about whether the AMO is controlled from another source6. Here we provide observational evidence of the widely hypothesized link between ocean circulation and the AMO. We take a new approach, using sea level along the east coast of the United States to estimate ocean circulation on decadal timescales. We show that ocean circulation responds to the first mode of Atlantic atmospheric forcing, the North Atlantic Oscillation, through circulation changes between the subtropical and subpolar gyres—the intergyre region7. These circulation changes affect the decadal evolution of North Atlantic heat content and, consequently, the phases of the AMO. The Atlantic overturning circulation is declining8 and the AMO is moving to a negative phase. This may offer a brief respite from the persistent rise of global temperatures4, but in the coupled system we describe, there are compensating effects. In this case, the negative AMO is associated with a continued acceleration of sea-level rise along the northeast coast of the United States9, 10.

Schauen wir auf die aktuelle AMO-Kurve der NOAA:

Abb. 1: Verlauf des AMO-Ozeanzyklus. Stand: 1. September 2017. Quelle: NOAA.

 

Naja, wenn man sich die bisherige AMO-Zyklik anschaut, könnte das AMO-Hoch noch mehr als ein Jahrzehnt andauern, so wie wir es auch in unserem Buch ‘Die kalte Sonne’ prognostizert haben. Allerdings befindet sich die PDO (Pazifische Dekadenoszillation) bereits im Sinkflug, was auf globale Sicht in den kommenden Jahren bereits wohl zur Abkühlung führen wird.

Der klimatisch umtriebige Mojib Latif ist übrigens auch Co-Autor einer Studie von Klöwer et al. aus dem Jahr 2014. Dort prognostizieren die Autoren doch in der Tat Ähnliches wie wir in unserem Buch ‘Die kalte Sonne’, nämlich die vorübergehende Fortsetzung des AMO-Plateaus, leicht abknicked, also leicht kühlend.

Abb. 2: AMO-Prognose der Latif-Gruppe (aus: Klöwer et al. 2014)

 

Abb. 3: AMO-Prognose aus unserem Buch “Die kalte Sonne” (2012).

 

Weshalb thematisiert Latif dies nicht einmal bei einem seiner nächsten Sparkassenauftritte? Hier der Abstract:

Atlantic meridional overturning circulation and the prediction of NorthAtlantic sea surface temperature
The Atlantic Meridional Overturning Circulation (AMOC), a major current system in the Atlantic Ocean, is thought to be an important driver of climate variability, both regionally and globally and on a large range of time scales from decadal to centennial and even longer. Measurements to monitor the AMOC strength have only started in 2004, which is too short to investigate its link to long-term climate variability. Here the surface heat flux-driven part of the AMOC during 1900–2010 is reconstructed from the history of the North Atlantic Oscillation, the most energetic mode of internal atmospheric variability in the Atlantic sector. The decadal variations of the AMOC obtained in that way are shown to precede the observed decadal variations in basin-wide North Atlantic sea surface temperature (SST), known as the Atlantic Multidecadal Oscillation (AMO) which strongly impacts societally important quantities such as Atlantic hurricane activity and Sahel rainfall. The future evolution of the AMO is forecast using the AMOC reconstructed up to 2010. The present warm phase of the AMO is predicted to continue until the end of the next decade, but with a negative tendency.

In den Highlights der Studie schreiben die Autoren:

North Atlantic sea surface temperature will stay anomalously warm until about 2030.

Sie hätten aber auch schreiben können:

Ab etwa 2030 wird sich der Nordatlantik spürbar abkühlen

Wie die AMO genau funktioniert inklusive grundsätzlicher Fragen wie Henne oder Ei, ist noch immer ungeklärt. Die Modellierer stehen im Regen, können die Zyklik nicht robust nachbilden. Peinlich. In der Fachwelt ist eine kontroverse Diskussion entbrannt. Beispiel Amy Clement und Kollegen im Oktober 2015 in Science:

The Atlantic Multidecadal Oscillation without a role for ocean circulation
The Atlantic Multidecadal Oscillation (AMO) is a major mode of climate variability with important societal impacts. Most previous explanations identify the driver of the AMO as the ocean circulation, specifically the Atlantic Meridional Overturning Circulation (AMOC). Here we show that the main features of the observed AMO are reproduced in models where the ocean heat transport is prescribed and thus cannot be the driver. Allowing the ocean circulation to interact with the atmosphere does not significantly alter the characteristics of the AMO in the current generation of climate models. These results suggest that the AMO is the response to stochastic forcing from the mid-latitude atmospheric circulation, with thermal coupling playing a role in the tropics. In this view, the AMOC and other ocean circulation changes would be largely a response to, not a cause of, the AMO.

Significance:

Ocean circulation changes not needed
What causes the pattern of sea surface temperature change that is seen in the North Atlantic Ocean? This naturally occurring quasi-cyclical variation, known as the Atlantic Multidecadal Oscillation (AMO), affects weather and climate. Some have suggested that the AMO is a consequence of variable large-scale ocean circulation. Clement et al. suggest otherwise. They find that the pattern of AMO variability can be produced in a model that does not include ocean circulation changes, but only the effects of changes in air temperatures and winds.

Hier die dazugehörige Pressemitteilung der University of Miami Rosenstiel School of Marine & Atmospheric Science vom 15. Oktober 2015:

New Study Questions Long-Held Theories of Climate Variability in the North Atlantic

UM Rosenstiel School researchers suggest atmosphere drives decades-long climate variations.

A University of Miami (UM) Rosenstiel School of Marine and Atmospheric-led study challenges the prevailing wisdom by identifying the atmosphere as the driver of a decades-long climate variation known as the Atlantic Multi-decadal Oscillation (AMO). The findings offer new insight on the causes and predictability of natural climate variations, which are known to cause wide-ranging global weather impacts, including increased rainfall, drought, and greater hurricane frequency in many parts of the Atlantic basin.

For decades, research on climate variations in the Atlantic has focused almost exclusively on the role of ocean circulation as the main driver, specifically the Atlantic Meridional Overturning Circulation, which carries warm water north in the upper layers of the ocean and cold water south in lower layers like a large conveyor belt. “The idea of the ocean as the driver has been a powerful one.” said UM Rosenstiel School Professor Amy Clement, the lead author on the study. We used computer models in a new way to test this idea, and find that in fact there is a lot that can be explained without the ocean circulation.”

While the overall rise in average temperature of the Atlantic is caused by greenhouse gases, this study examines the fluctuations occurring within this human-related trend. Identifying the main driver of the AMO is critical to help predict the overall warming of the North Atlantic Ocean in coming decades from both natural and man-made climate change. Recent research suggests that an AMO warm phase has been in effect since the mid-1990s, which has caused changes in rainfall in the southeastern US, and resulted in twice as many tropical storms becoming hurricanes than during cool phases.

Using multiple climate models from around the world, Clement’s research team removed the ocean circulation from the analysis to reveal that variations in the Atlantic climate were generally the same. The AMO results in a horseshoe-shaped pattern of ocean surface temperatures in the North Atlantic Ocean that have been naturally occurring for the last 1000 years on timescales of 60-80 years. This new analysis shows that the pattern of the AMO can be accounted for by atmospheric circulation alone, without any role for the ocean circulation.

“These results force us to rethink our ability to predict decade-scale temperature fluctuations in the Atlantic and their associated impacts on land. It may be that many of the changes have limited predictability, which means that we should be prepared for a range of climate outcomes associated with global warming,” said Clement.

The study, titled “The Atlantic Multidecadal Oscillation Without a Role for Ocean Circulation,” was published in the Oct 16 issue of the journal Science. The co-authors include Clement, Katinka Bellomo and Lisa N. Murphy from the UM Rosenstiel School; Mark A. Cane of Lamont-Doherty Earth Observatory of Columbia University; and Thorsten Mauritsen, Gaby Rädel and Bjorn Stevens from Max Planck Institute for Meteorology in Germany. The work was support by grants from the Department of Energy and the National Oceanographic and Atmospheric Administration.