By Dr. Sebastian Lüning and Prof. Fritz Vahrenholt
(German text translated/edited by P. Gosselin)
The oceans are the world’s largest water reservoirs, and over 60-year cycles they swallow heat three decades long, and release over the 30 years or so that follow. In the Atlantic this phenomenon is called the Atlantic Multidecadal Oscillation (AMO).
Climate models have not been able to correctly account for them, and thus the climate prognoses are fraught with uncertainty.
However, much has been done in the area of ocean cycles research over the past years. The systematic climate impact has finally been accepted by the scientific community. A good example is a paper authored by Dan Seidov et al appearing in the Geophysical Research Letters in May.
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.”
Finally climatic mid-term prognoses are able to benefit from the ocean cycles, which some five years ago – when our book “The Neglected Sun” was published – we were viciously attacked by the somewhat humiliated climate establishment.
Today we are far better informed. In Ireland temperatures and precipitation are 90% related to the AMO, as McCarthy et al. documented in the journal ‘Weather‘ in July 2015:
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.”
The natural decadal variability of the Irish climate is two times more than the warming trend the study shows. In the years of 1990-2002 variability contributed to the trend. It is not stated, but it is warned of the other downward side of the temperature signal flank:
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.”
Models that well reproduce the rising flank of the AMO, and attribute it solely to CO2, overestimate CO2. Even if the full knowledge of the different reasons cannot be fully named, the profound importance of the ocean cycles and their contribution to the warming phases is emerging more and more.
A paper Bowene et al appeared in the PNAS in May, 2017. In it the authors did not dare to examine the current cycle and instead chose to look at an earlier cycle. Their message: The warming of the early 20th century in the Arctic was enhanced by ocean cycles. What follows is the press release from 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“
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.”
Let’s take a look at the current AMO curve from the NOAA:
Fig. 1: The AMO ocean cycle curve. Status: 1 September 2017. Source: NOAA.
When one looks at the previous AMO cycle, a peak in the AMO could last another decade, just as we projected in our “The Neglected Sun” book. However the PDO (Pacific Decadal Oscillation) is already falling, which will mean cooling globally over the coming years.
Climate publicity seeker Mojib Latif is by the way also a co-author of a study by Klöwer et al. from 2014. In the paper the authors projected similarly as we did in our “The Neglected Sun” book, namely that the AMO plateau would continue with a slight downward trend, i.e. slight cooling:
Fig. 2: AMO prognosis by the Latif group (from: Klöwer et al. 2014)
Fig. 3: AMO prognosis from our “The Neglected Sun” (book 2012).
So why doesn’t Latif mention this when speaking before the next industry group? Here’s the paper’s 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.”
The authors write in the paper’s highlights:
North Atlantic sea surface temperature will stay anomalously warm until about 2030.”
But they could have just as well written:
The North Atlantic will cool considerably by 2030.”
Just how the AMO works is a fundamental question, similar to the “hen and the egg”, and remains to be explained. The modelers have not been able to robustly duplicate the cycle. That’s been embarrassing.
In the scientific community a controversial discussion has since broken out. Example: Amy Clement et al in October 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.
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.”
What follows is the corresponding press release from the University of Miami Rosenstiel School of Marine & Atmospheric Science, dated 15 October 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.”