Canceling the AMO

Reposted from Dr Judith Curry’s Climate Etc.

Posted on March 6, 2021 by curryja | 

by Judith Curry

Conclusion from Michael Mann’s new paper:  “We conclude that there is no compelling evidence for internal multidecadal oscillations in the climate system.”

Michael Mann’s most recent paper:

Multidecadal climate oscilliations during the past millennium driven by volcanic forcing

Abstract. Past research argues for an internal multidecadal (40- to 60-year) oscillation distinct from climate noise. Recent studies have claimed that this so-termed Atlantic Multidecadal Oscillation is instead a manifestation of competing time-varying effects of anthropogenic greenhouse gases and sulfate aerosols. That conclusion is bolstered by the absence of robust multidecadal climate oscillations in control simulations of current-generation models. Paleoclimate data, however, do demonstrate multidecadal oscillatory behavior during the preindustrial era. By comparing control and forced “Last Millennium” simulations, we show that these apparent multidecadal oscillations are an artifact of pulses of volcanic activity during the preindustrial era that project markedly onto the multidecadal (50- to 70-year) frequency band. We conclude that there is no compelling evidence for internal multidecadal oscillations in the climate system.

From the Penn State press release Apparent Atlantic Warming Cycle an Artifact of Climate Forcing:

<begin quote>

“It is somewhat ironic, I suppose,” said Michael E. Mann, distinguished professor of atmospheric science and director, Earth System Science Center, Penn State. “Two decades ago, we brought the AMO into the conversation, arguing that there was a long-term natural, internal climate oscillation centered in the North Atlantic based on the limited observations and simulations that were available then, and coining the term ‘AMO.’ Many other scientists ran with the concept, but now we’ve come full circle. My co-authors and I have shown that the AMO is very likely an artifact of climate change driven by human forcing in the modern era and natural forcing in pre-industrial times.”

The researchers previously showed that the apparent AMO cycle in the modern era was an artifact of industrialization-driven climate change, specifically the competition between warming over the past century from carbon pollution and an offsetting cooling factor, industrial sulphur pollution, that was strongest from the 1950s through the passage of the Clean Air Acts in the 1970s and 1980s. But they then asked, why do we still see it in pre-industrial records?

Their conclusion, reported today (Mar. 5) in Science, is that the early signal was caused by large volcanic eruptions in past centuries that caused initial cooling and a slow recovery, with an average spacing of just over half a century. The result resembles an irregular, roughly 60-year AMO-like oscillation.

“Some hurricane scientists have claimed that the increase in Atlantic hurricanes in recent decades is due to the uptick of an internal AMO cycle,” said Mann. “Our latest study appears to be the final nail in the coffin of that theory. What has in the past been attributed to an internal AMO oscillation is instead the result of external drivers, including human forcing during the industrial era and natural volcanic forcing during the pre-industrial era.”

<end quote>

Mann has a blog post on the paper at RealClimate 

Wow.  In one fell swoop, the pesky problems of the ‘grand hiatus’ in the mid 20th century,  debates over the attribution of 20th century warming and the role of multidecadal internal variability, and the difficulty of attributing the recent increase in Atlantic hurricane activity to AGW, all go away.  Brilliant!  Almost as ‘brilliant’ as the Hockey Stick.

As it happens, I have a draft chapter in my pocket from a report I’m writing, I’ve excerpted the relevant text below (apologies for not having links to the references):

9.Atlantic Multidecadal Variability

The Atlantic Ocean is particularly important to the global ocean circulation due to the existence of North Atlantic Deep Water (NADW) formation in the northern North Atlantic, a vital component of the Atlantic Meridional Overturning Circulation (AMOC). The AMOC includes the northward flow of warm salty water in the upper Atlantic and the southward flow of the transformed cold fresh NADW in the deep Atlantic, which is a major driver of the substantial northward Atlantic heat transport across the equator.

Basin‐scale multidecadal fluctuations have been observed in the Atlantic sea surface temperature (SST). The large‐scale multidecadal variability observed in the Atlantic has been referred to as the Atlantic Multidecadal Oscillation (AMO). The multidecadal character of the AMO is distinguished from interannual ocean variability forced by the leading mode of atmospheric circulation variability over the North Atlantic, the North Atlantic Oscillation (NAO). The term Atlantic Multidecadal Variability (AMV) is often used, because the observed multidecadal fluctuations in the Atlantic may not be an oscillation at a single frequency but consist of a broader band of low‐frequency signals.

9.1 Index definition and climatology

The Atlantic Multidecadal Oscillation (AMO) is associated with basin-wide SST and sea level pressure (SLP) fluctuations. For the positive AMO phase, this is sometimes presented as an almost uniform warming of the North Atlantic. The traditional AMO index associates the positive AMO phase with a pattern of horseshoe-shaped SST anomalies in the North Atlantic with pronounced warming in the tropical and parts of the eastern subtropical North Atlantic, an anomalously cool area off the U.S. East Coast, and warm anomalies surrounding the southern tip of Greenland.

Figure 9.1. Atlantic Multidecadal Oscillation Index (1880-2018).

The past 100 to 150 years of Atlantic SSTs are characterized by a net century-long rise and periods of multidecadal warming and cooling. However, the rationale for the ‘Trend+AMO’ separation is confounded by lack of linearity in the global warming trend, and hence detrending aliases the AMO index. The nonlinearity is particularly pronounced during the period 1945-1975, when global SSTs showed a slight cooling trend.

To address the ambiguities associated with detrending in formulating the AMO index, Johnstone (2017) has formulated an Atlantic ‘Arc’ Index, based on the leading Principal Component of north Atlantic SST variability (60°N to 0°, 70°W-0°W). The Atlantic ‘Arc’ SST index reflects coherent variability within a basin-scale arc-shaped pattern (sometimes referred to as a ‘horseshoe’), a signature of the AMO that encompasses the tropical North Atlantic, the midlatitude eastern boundary and much of the subpolar north (Fig. 9.2).

Figure 9.2. The Arc pattern is delimited by the bold black line, which encompasses the tropical North Atlantic, the midlatitude eastern boundary and much of the subpolar north Atlantic.  From Johnstone.

The Arc pattern is recognized as a spatial signature of the AMO, identified with coupled ocean-atmosphere variability, and is closely related to a ‘tripole’ pattern of SST response to the NAO. The Arc Index displays a net warming in addition to multidecadal period recognized as a cool phase of the AMO (Figure 9.3). Since the Arc Index combines both the AMO variability and the overall warming trend, it is more usefully interpreted as defining multidecadal regimes and shifts (see section 9.4).

The Arc Index (Figure 9.3) shows abrupt shifts to the warm phase in 1926 and 1995, consistent with the conventional AMO analysis in Figure 9.1. Johnstone’s analysis indicates a shift to the cold phase in 1971, which differs from the analysis shown in Figure 9.1 that indicates the shift to the cold phase in 1964. The AMO index of Klotzbach and Gray (2008) also indicates a shift to the cold phase in 1970.

Figure 9.3. Time series of the Atlantic Arc Index from 1880 through early 2018. From Johnstone.

9.2 Paleoclimate reconstructions

The brevity of available instrumental data limits our understanding of the Atlantic Multidecadal Variability (AMV). Paleoclimate proxy‐derived reconstructions of AMV‐related signals that extend beyond the instrumental era provide an important basis for understanding the nature and stationarity in time of the AMV.

The recent Wang et al. (2017) AMV reconstruction using terrestrial proxy records (tree rings, ice cores, etc.) over the past 1,200 years has both broad spatial coverage and high temporal (annual) resolution. Wang et al. (2017) found that large volcanic eruptions and solar irradiance minima induce cool phases of Atlantic multidecadal variability and collectively explain about 30% of the variance in the reconstruction on timescales greater than 30 years. They isolated the internally-generated component of Atlantic multidecadal variability, which they define as the AMO. They found that the AMO is the largest contributor to Atlantic multidecadal variability over the past 1,200 years.

Zhang et al. (2019) provides a summary of studies that have analyzed paleoclimate data to investigate whether AMV is internally or externally driven. Over the past 12 centuries, the reconstructed solar and volcanic forcing do correlate with the Wang et al. (2017) AMV reconstruction, but their combined contribution explains less than one third (28%) of the total AMV variance; the reconstructed AMV is dominated by internal variability. The internal variability component of the Wang et al. (2017) AMV reconstruction also reveals significant signals at multidecadal timescales above a red noise background and its amplitude during the preindustrial period, especially before the Little Ice Age, is on the same order as that found in the instrumental AMV index. Paleo proxies also supportthe existence of an AMOC‐AMV linkage over the past several centuries.

Knudsen et al (2010) used paleoclimatic data to show that distinct ~55-70 year oscillations characterized the North Atlantic ocean-atmosphere variability over the past 8,000 years (the Holocene). The Holocene AMO signal appears to have been quasi-periodic and the associated climate response to have been of highly variable intensity, both in time and space. In the tropical Atlantic, the AMO response signal was generally relatively weak during the Northern Hemisphere warming of the Holocene thermal maximum (HTM) between 5,500 and 9,000 BP, after which it picked up in intensity. Through the past 8,000 years, minor shifts appear to have occurred in the dominating period within the 55- to 70-year band. The dominant oscillation period in the interval of 5,500–8,000 BP was ~65 years, whereas it shortened somewhat between 5,500 BP (before present) and ~2,700 BP (55–60 years). The period of the dominant oscillations increased slightly again after ~2,700 BP (65–70 years), but the oscillations were generally not as well defined as during the early Holocene, when the AMO period bandwidth appears to have been narrower.

Knudsen et al (2010) provides the following additional insights. The AMO response signal exhibits a general shift in its pattern within the last 8,000 years, as the signal was most pronounced in the Arctic during the HTM, whereas in the tropics its maximum was generally reached after the HTM. Between 2,000 and 3,500 BP, there was a statistically significant recurrence of multidecadal oscillations in the Arctic. This interval overlaps with a part of the neo-glaciation between 2,000 and 3,000 BP, which was characterized by relatively high SST and generally warmer, and particularly unstable, climate conditions in parts of the northern North Atlantic region. Such conditions meant that the Arctic sites temporarily became more sensitive to multidecadal SST oscillations, possibly due to an associated reduction in Arctic sea-ice cover. However, the major changes in North Atlantic circulation patterns that followed the neo-glaciation in both hemispheres led to a generally weakened AMO response signal after ~2,000 BP. This change was accompanied by a distinct SST decline between 2,000 and 500 BP in some parts of the northern North Atlantic.

9.3 Climate dynamics

Despite ongoing debates about the climate dynamics of the AMV, it is generally accepted that the AMV represents a complex conflation of natural internal variability of the Atlantic Meridional Overturning Circulation (AMOC), natural red-noise stochastic forcing of the ocean by the atmosphere (primarily the NAO), and external forcing from volcanic events, aerosol particles and greenhouse gases.

9.3.1 AMO

The Atlantic Multidecadal Oscillation (AMO) is the most prominent mode of multi-decadal Atlantic variability; however the AMO’s physical origins remain a topic of ongoing debates. Observed AMO SST changes have long been attributed to slow variations in northward upper-ocean heat transport by the AMOC. Ocean processes offer a plausible mechanism for large multidecadal climate variations; such inferences are based largely on climate model simulations due to the short record of AMOC circulation that begins only recently in 2004.

Several additional hypotheses for AMV mechanisms have been proposed. Anthropogenic aerosols have been hypothesized to be a prime driver of the observed AMV. The argument is that an increase in the linearly detrended AMV SST index is forced by the increased downward shortwave radiative heat flux induced by the decreased anthropogenic aerosols through their interaction with clouds. However, the observed decline in the subpolar AMV SST signal over the most recent decade is inconsistent with the recently observed change (a slight decrease) in anthropogenic aerosols over the North Atlantic region. As summarized by Zhang et al. (2019), the hypothesis that changes in external radiative forcing is a prime driver of AMV disagrees with many observed key elements of AMV.

Using observations and models, Delworth et al. (2017) examined the relationship between the North Atlantic Oscillation (NAO) and Atlantic decadal SST variations. Consistent with many previous studies, on short time scales NAO-related surface heat flux anomalies drive a tripole pattern of SST anomalies in the Atlantic. On decadal and longer time scales, there is a lagged response of the ocean to the NAO fluxes, with the AMOC playing a prime role in modulating meridional oceanic heat transport and generating an AMO-like SST response. A prolonged positive phase of the NAO enhances the AMOC after a decadal-scale delay. Delworth et al. (2017) found that decadal-scale SST variability in the subpolar and tropical North Atlantic are well correlated. While ocean dynamics plays a crucial role for decadal-scale SST variability in the extratropical North Atlantic, the results of this study suggest that its direct influence in the tropical North Atlantic appears to be smaller, with local air–sea fluxes playing a larger role.

Lin et al. (2019) argues for two different sources for AMO variability, identifying 50–80 year and 10–30 year AMOs that are associated with different underlying dynamics. Associated with a positive AMO at 50–80 year period is enhanced westerlies north of 60N but weakened between 40-60N, which is dynamically consistent with an enhanced polar vortex and linked to variability in the Pacific. The atmospheric variability associated with the 10–30 year AMO is a zonally asymmetric pattern with blockings prevailing over high latitude North Atlantic and cyclonic anomaly over subtropical North Atlantic, which is independent from the variability over Pacific sector. While the 10–30 year AMO may be linked directly to the dynamics over the tropical Atlantic, the 50– 80 year AMO is heavily related to the cross-basin interaction between the North Atlantic and the Greenland-Iceland-Norwegian Seas. (Note: consistent with the stadium wave.)

Willis et al. (2019) identified a tripolar SST anomaly between the Gulf Stream, the subpolar gyre, and the Norwegian seas that varies on 8–20 yr time scales. Their results suggest that the AMO is confined to the subpolar North Atlantic, while the tropical Atlantic varies primarily on shorter (intradecadal) time scales. In another study, Muller, Curry et al. (2013) identified a strong narrow peak in the AMO with period of ~ 9 yrs.

Nigam et al. (2018) showed that the decadal component of the AMO is closely related to the Gulf Stream variability: the northward shift of the Gulf Stream (GS) path coincides with the cold AMO phase with cold SST anomalies in the subpolar gyre. The GS’s northward shift is preceded by the positive phase of the low-frequency NAO and followed by a positive AMO tendency by 1.25 and 2.5 years, respectively. The temporal phasing is such that the GS’s northward shift is nearly concurrent with the AMO’s cold decadal phase (cold, fresh subpolar gyre).

Kwon et al. (2019) found that the evolution of SST anomalies is very different in the warm versus the cold phase of the AMV. For the AMV warm phase, the warm SST anomalies in the western subpolar gyre are damped by the surface heat flux, and thus pose anomalous heating in the lower troposphere and reduce the overall meridional gradient of the atmospheric temperature. Consequently, the storm track activity weakens. As the blocking substantially influences the seasonal mean atmospheric circulation, the negative phase of NAO dominates at the same time.

RuprichRobert and Cassou (2014) found that the full life cycle of AMOC/AMV events relies on a complex time-evolving relationship with both North Atlantic Oscillation (NAO) and East Atlantic Pattern (EAP) (Figure 9.8). The AMOC rise leading to a warm phase of AMV is statistically preceded by wintertime NAO+ and EAP+ from lag -40/-20 yrs. Associated wind stress anomalies induce an acceleration of the subpolar gyre (SPG) and enhanced northward transport of warm and saline sub- tropical water. Concurrent positive salinity anomalies occur in the Greenland–Iceland–Norwegian Seas in link to local sea-ice decline; those are advected by the Eastern Greenland Current to the Labrador Sea participating to the progressive densification of the SPG and the intensification of ocean deep convection leading to AMOC strengthening. From lag -10 yrs prior to an AMOC maximum, the opposite relationship is found with the NAO for both summer and winter seasons. NAO- acts as a positive feedback for the full development of the AMV through surface fluxes but, at the same time, prepares its termination through negative retroaction on AMOC. Relationship between EAP- and AMOC is also present in summer from lags -30/+10 yrs, while winter EAP- is favored around the AMV peak.

All together, the combined effect of NAO and EAP are responsible for an irregular and damped mode of variability of AMOC/AMV that takes about 35–40 years to build up and about 15–20 years to dissipate. In addition to the direct NAO-/EAP- action, the termination of AMOC/AMV events is also induced by the advection of anomalous fresh water from the subtropical North Atlantic basin along the mean western boundary ocean circulation, and also from the Arctic due to considerable ice volume loss associated with overall atmospheric warmer conditions when AMOC is enhanced.

Figure 9.8 Schematic diagram for an AMOC/AMV positive event. RuprichRobert and Cassou (2014)

Update:  An excellent new publication was pointed out to me on twitter that supports the general conclusions of my write-up https://journals.ametsoc.org/view/journals/clim/32/22/jcli-d-19-0177.1.xml#.YEO-7oO1x98.twitter

9.4 Recent shifts

As summarized by Robson et al. (2012), in the mid-1990s the subpolar gyre of the North Atlantic underwent a remarkable rapid warming, with sea surface temperatures increasing by around 1.8^oC in just 2 years. This rapid warming followed a prolonged positive phase of the North Atlantic Oscillation (NAO), but also coincided with an unusually negative NAO index in the winter of 1995/96. By comparing ocean analyses and carefully designed model experiments, they showed that this rapid warming can be understood as a delayed response to the prolonged positive phase of the NAO and not simply an instantaneous response to the negative NAO index of 1995/96. Furthermore, they inferred that the warming was partly caused by a surge and subsequent decline in the meridional overturning circulation and northward heat transport of the Atlantic Ocean.

Robson et al. (2016) showed that since 2005, a large volume of the subpolar 
North Atlantic Ocean has cooled significantly, reversing the previous warming trend. By analyzing observations 
and a state-of-the-art climate model, they showed that this cooling is consistent 
with a reduction in the strength of the ocean circulation and heat transport, 
linked to record low densities in the deep Labrador Sea. The low density in the deep Labrador Sea is primarily due to deep ocean warming since 1995, but a long-term freshening also played a role. They inferred that the observed cooling of a large region of the upper North Atlantic Ocean since 2005 cannot be explained as a direct response to changes in atmospheric circulation over the same period.

Johnstone (2017) describes a ‘coupled shift model’ of low-frequency North Atlantic climate 
change, based on abrupt transitions between quasi-stable sea surface temperatures and coupled 
atmospheric circulations. This hypothesis describes recurrent step-like changes in North Atlantic SST, wherein high-amplitude SST perturbations are occasionally maintained as anomalous multidecadal climate states by positive atmosphere-ocean feedbacks. Statistical evidence is presented that low-frequency SST changes were not gradual processes as commonly described, but through a series of short, discrete events, characterized by abrupt ~1 year step-like shifts that separate longer multidecadal periods of relatively little change.

The strong Atlantic warming of the mid-1990s (Figure 9.1), which is represented by filtered AMO indices as a gradual process lasting a decade or more, can be traced to an abrupt and remarkably continuous rise in basin-scale SST, beginning in October-November 1994, and essentially accomplished as a +0.8°C SST change across most of the North Atlantic by July 1995 (Figure 9.3). The abrupt warming of 1994-95 went undamped in successive months, during the next few years, and fully through the subsequent two decades up to the present,  rapidly introducing a new warmer climate state. The basin-scale expanse of the 1995 shift can be seen in the abrupt shift in monthly SST anomalies over both the subpolar North Atlantic (50-60^oN) and the subtropical margins of NW Africa, which warming together in nearly simultaneous fashion (Arc Index, Figure 9.2).

Shifts appear in the annual Arc SST record (Fig. 9.3) as pronounced year-to-year jumps in 1925-26 (+0.5°C), 1970-71 (-0.3°), and 1994-95 (+0.6°) that were followed by multidecadal persistence of similar 
anomalies with respect to prior years (1926-1970: +0.5°C, 1971-1994: -0.2°C, and 1995-2014: +0.5°). Each of these intervals lacks a significant linear Arc SST trend, suggesting that large 
transient climate changes were followed by restabilization of the upper-ocean heat balance and persistence of new anomalous conditions over years to multiple decades. Arc SST changes 1926, 1971 and 1995 occurred with moderate same-signed anomalies of winter (October-March) Niño 3.4 SST, suggesting a systematic role for 
ENSO in the generation of low-frequency North Atlantic climate changes.

A more specific regional indicator appears in the correspondence of Arc SST shifts with high-amplitude SST changes off northwest Africa, which peaked September 1925, August 1970 and November 1994. West African 
SSTs are a prominent component of the Atlantic Multidecadal Mode (AMM), which may serve as a bridge across time scales, sustaining SST perturbations as 
sustained climate anomalies.

A physical implication of the shift model is that low-frequency climate changes occur through occasional pulses of upper-ocean heat uptake and release, rather than gradual or cumulative processes.

Atmosphere-ocean conditions leading to 1994-95 warming share notable similarities with the warming of 1925-26 and (oppositely) with the cooling of 1970-1971, suggesting predictability of major North Atlantic climate shifts. All three events were preceded during the prior 2-3 years by uniquely strong sea level pressure (SLP) anomalies of opposite sign around the Norwegian Sea within a broader NAO-like pattern. In each case, the transitional winter featured moderate ENSO conditions favorable to the developing temperature change, and each shift was distinguished by extreme local SST changes off NW Africa.

Historically, Atlantic shifts have been marked by extreme short-term SST changes off NW Africa: behavior that is 
not currently evident, as subtropical and tropical areas of the Arc remain in a warm state begun in 1995. 
However, it is notable that subpolar SSTs from 50-60N show evidence of abrupt cooling since 2015 (Fig. 9.10), behavior suggestive of a ‘partial’ shift that might soon involve the broader North Atlantic, including the tropics. The current divergence between subpolar and tropical North Atlantic SST is potentially analogous to 
behavior seen during the late 1960s-early-1970s, when rapid subpolar cooling in 1969-70 slightly 
preceded the sharp 1971 drop in tropical SST. Based on historical patterns, an abrupt shift to cooler conditions may be imminent, although the unusually long regime from 1926 to 1970 suggests that a substantial delay of up to 10-20 years may also be plausible.

Figure 9.10. Annual SST anomalies for the subpolar and tropical North Atlantic. Subpolar SST (blue, 60°-50°N) 
displays a sharp drop and persistently cool conditions since 2015 (20°N- 0°, red). Similar divergence around 1970 might provide an early indication of tropical and broader North Atlantic 
cooling within the next few years.

To what extent was the 1995 shift in the AMO predictable by climate models? Msadek et al. (2014) summarize the decadal prediction experiments conducted using the GFDL Climate Model. Initializing the model produces high skill in retrospectively predicting the mid-1990s warming, which is not captured by the uninitialized forecasts. All hindcasts initialized in the early 1990s show a warming of the SPG (subpolar gyre); however, only the ensemble-mean hindcasts initialized in 1995 and 1996 are able to reproduce the observed abrupt warming and the associated decrease and contraction of the SPG. The enhanced Atlantic decadal prediction skill is achieved primarily by initializing AMOC anomalies, instead of predicting AMOC anomalies at northern high latitudes.

In contemplating a possible future shift to the cold phase of the AMO, it is instructive to consider the prior shift to the cold phase that occurred in the 1960’s and early 1970’s, when the sea surface temperatures in the North Atlantic Ocean cooled rapidly. Hodson et al. (2014) demonstrated that the cooling proceeded in several distinct stages:

• 1964–68: The initial cooling is largely confined to the Nordic Seas and the Gulf Stream Extension. There are no notable atmospheric circulation anomalies during this period, aside from a small low MSLP anomaly over the Arctic in October–June.
• 1968–72: As the cooling progresses, cool anomalies extend to cover much of the subpolar gyre (SPG) and northern midlatitudes. There is a hint of low SLP anomalies over North Africa, but the most prominent evidence of circulation anomalies is an anti-cyclonic anomaly in July–September, which extends over northern Europe and into Asia.
• 1972–76: The cool anomalies reach their maximum magnitude and spatial extent during this period. The western part of the subtropical North Atlantic does not show a significant cooling, resulting in a tripole (or horseshoe) pattern. The pattern of SLP anomalies projects on the positive phase of the NAO.

9.5 Climate model simulations

Many coupled climate models simulate Atlantic Decadal Variability that is consistent in some respects with the available observations. However, the mechanisms differ strongly from model to model, and the inadequate observational database does not allow a distinction between ‘realistic’ and ‘unrealistic’ simulations (Latif and Keenlyside, 2011). Ruiz-Barradas et al. (2013) examined historical simulations of the AMO in CMIP3 and CMIP5 models. Variability of the AMO in the 10–20/70–80 year ranges is overestimated/ underestimated in the models.

Cheng et al. (2013; 2015) examined the Atlantic Meridional Overturning Circulation (AMOC) simulated by 10 models from CMIP5 for the historical and future climate. The multimodel ensemble mean AMOC exhibits multidecadal variability with a 60-yr period; all individual models project consistently onto this multidecadal mode.

As summarized by the NCA (2017), the simulated AMOC-AMV linkage varies considerably among the coupled global climate models, likely resulting from the spread of mean state model biases in the North Atlantic. The AMOC-AMV linkage depends on the amplitudes of low-frequency AMOC variability, which is much weaker in climate models than in the real world owing to the underestimated low-frequency AMOC variability that amplifies the relative role of external radiative forcing or stochastic atmospheric forcing in AMV.

The timing of a shift to the AMO cold phase is not predictable; it depends to some extent on unpredictable weather variability. However, analysis of historical and paleoclimatic records suggest that a transition to the cold phase is expected prior to 2050. Enfield and Cid-Serrano (2006) used paleoclimate reconstructions of the AMO to develop a probabilistic projection of the next AMO shift. Enfield and Cid-Serrano’s analysis indicates that a shift to the cold phase should occur within the next 13 years, with a 50% probability of the shift occurring in the next 6 years.

Evaluation of the Mann et al. paper

With that context, you can see why I am not accepting the aerosol explanation (pollution and/or volcanoes) for an explanation of what causes the AMO.  There is substantial discussion and disagreement in the climate dynamics community on this topic, which isn’t surprising given the apparent complex interactions between ocean circulations and the AMOC, weather and interannual climate variability, and external forcing from the sun and volcanoes.

So, what exactly is wrong with Mann’s analysis? He relies on global climate models, which are inadequate in simulating the AMO.  This was most recently emphasized by Kravtsov et al. (2018), who concluded that:

“While climate models exhibit various levels of decadal climate variability and some regional similarities to observations, none of the model simulations considered match the observed signal in terms of its magnitude, spatial patterns and their sequential time development. These results highlight a substantial degree of uncertainty in our interpretation of the observed climate change using current generation of climate models.”

Relying on global climate models, which don’t adequately simulate the multi-decadal internal variability, to ‘prove’ that such multi-decadal internal variability doesn’t exist, is circular reasoning (at best).  How does this stuff get published in a journal like Science?  Peer review is sooooo broken.

What Mann is seeing in the climate model simulations is the shorter period tropical expression of the AMO that is limited to the North  Atlantic – not the multi-decadal variability that is linked to the global oscillation.  This tropical expression may very well be forced to some extent by tropical volcanic eruptions, but says little about global multi-decadal variability that is of greatest interest.

The true multi-decadal climate variability is mostly internally generated, although solar variations can help set the tempo and major volcanic eruptions can mask the variations or help trigger a shift.

Why does this matter?  The different phases of the AMO are linked to:  Atlantic hurricane activity, mass balance of Greenland and weather patterns influencing North America and Europe (notably droughts); this is not to mention global temperature change.

From the Penn State press release, a primary motive for cancelling the AMO appears so that Mann can attribute the increase in Atlantic hurricane activity since 1995 to AGW.  Cancelling the AMO isn’t going to help much here.  As discussed in my recent post on the  AIR insurance sector report, the number of major hurricanes (Cat 3+) in the Atlantic during the 1950’s and 1960’s (previous warm phase of the AMO) was at least as large as for the last two decades, when SSTs were significantly cooler.

A further key issue with the AMO is that all of the acceleration in sea level rise in recent decades is coming from Greenland melt, which is heavily influenced by the AMO.

Assuming that nature continues to behave as it has for the past 8 millennia, at some point (possibly in the next decade), we will see a shift to the cold phase of the AMO, with a slow down in Atlantic hurricane activity and Greenland mass loss.

In closing, Mann’s quest to cancel the Medieval Warm Period and now the AMO, in the interests of showing that recent warming is 100% anthropogenic, is not at all convincing to scientists who understand anything about climate dynamics and global climate models.