The Role of Anthropogenic Aerosol Forcing in the 1850–1985 Strengthening of the AMOC in CMIP6 Historical Simulations

Robson, Jon; Menary, Matthew; Sutton, Rowan; Mecking, Jenny; Gregory, Jonathan M.; Jones, Colin; Sinha, Bablu; Stevens, David P.; Wilcox, Laura

doi:10.1175/jcli-d-22-0124.1

Cited by 26 publications

(52 citation statements)

References 67 publications

(50 reference statements)

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“…Following this, ocean circulation is far from constant on timescales longer than a century or so. Indeed consideration of the Gulf Stream which keeps northern Europe warmer than other similar-latitude locations make it clear that changes to ocean circulation could potentially have enormous repercussions [8]. All of this is without any discussion at all of the longer-term implications gleaned from paleoclimate studies, e.g.…”

Section: Feedbacksmentioning

confidence: 99%

Is equilibrium climate sensitivity clouding our judgement?

Williams¹

2023

Preprint

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Equilibrium climate sensitivity - ECS - is easily-understood, has been studied for over 150 years and is therefore appealing as a metric for communication of climate model results. In this work I argue that ECS is not a good metric for comparing different climate models. Via brief examples concerning the Pliocene epoch and the Paleocene-Eocene Thermal Maximum , I further argue that models which produce temperatures towards the higher end of model intercomparisons are useful in spite of recent studies concluding that these models are ‘too hot’. I hope that this brief manuscript generates discussion on how to prioritise the consideration of more useful, and potentially novel, ways of comparing climate models going forward.

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Section: Feedbacksmentioning

confidence: 99%

Is equilibrium climate sensitivity clouding our judgement?

Williams¹

2023

Preprint

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Section: The Role Of Sulphate Aerosolsmentioning

confidence: 99%

“…The question of how aerosols affect North Atlantic SSTs is still actively debated, and could include the role of both direct radiative adjustments local to the region, as well as more indirect impacts from upstream emissions (Booth et al, 2012;Robson et al, 2022). Here we will only consider the hypothesis put forth by Robson et al (2022), due to it being based on a large multimodel ensemble, unlike most other studies. In their study, they argued that sulphate aerosol emissions over the eastern part of North America led to a cooling of surface temperatures there, and that this cool air was then advected over the North Atlantic, thereby cooling the SSTs and ultimately modulating the AMOC.…”

Section: The Role Of Sulphate Aerosolsmentioning

confidence: 99%

Predictable Decadal Forcing of the North Atlantic Jet Stream by Sub-Polar North Atlantic Sea Surface Temperatures

Strommen¹,

Woollings²,

Davini³

et al. 2023

Preprint

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Abstract. It has been demonstrated that decadal variations in the North Atlantic Oscillation (NAO) can be predicted by current forecast models. While Atlantic Multidecadal Variability (AMV) in sea surface temperatures (SSTs) has been hypothesised as the source of this skill, the validity of this hypothesis and the pathways involved remain unclear. We show, using reanalysis and data from two forecast models, that the decadal predictability of the NAO can be entirely accounted for by the predictability of decadal variations in the speed of the North Atlantic eddy-driven jet, with no predictability of decadal variations in the jet latitude. The sub-polar North Atlantic (SPNA) is identified as the only potential source of an SST-based signal across the models and reanalysis, and the predictability of the jet speed is shown to be consistent with a forcing from the SPNA visible already within a single season. The pathway is argued to be tropospheric in nature, with the SPNA-induced heating extending up to the mid-troposphere, which alters the meridional temperature gradient around the climatological jet position. The link between SSTs and heatfluxes, which mediates the predictability, is shown to be underestimated in the forecast models by approximately a factor of two, with potential implications for the `signal-to-noise paradox'. The relative roles of anthropogenic aerosol emissions and the AMOC at generating predictable SPNA variability are also discussed. The analysis is extensively supported by the novel use of a set of seasonal hindcasts spanning the 20th century and forced with prescribed SSTs.

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“…Cooling would have the opposite effect, leading to increases in F SW in the North Atlantic region. It has also been suggested that aerosols may have a local influence on the Atlantic Meridional Overturning Circulation (AMOC) that is more direct than the effect of aerosols on large scale temperatures (Yu and Pritchard, 2019;Robson et al, 2022). Menary et al (2020) shows that the AMOC speeds up in the DAMIP-hist-aer run as a result of aerosol emissions and it is feasible that changes in the AMOC could also lead to changes in cloud cover or properties and hence changes in F SW .…”

Section: Aerosol Forcing Vs Cloud-climate Feedbacksmentioning

confidence: 99%

“…Here we can only speculate about possible reasons for this; further work would be needed to draw conclusions. One possibility is that the dedicated aerosol-only DAMIP-hist-aer simulation allows the AMOC to increase in strength until 1970 due to the increase in aerosols over that period, which is a proven effect of aerosol forcing in many models (Menary et al, 2020;Robson et al, 2022). This may prevent a rapid response of the climate in the NA to the post-1970 reduction in aerosols due to inertia in the AMOC perhaps related to ocean heat storage, sea-ice changes, etc.…”

Section: A6 Summarymentioning

confidence: 99%

What controls the historical timeseries of shortwave fluxes in the North Atlantic?

Grosvenor

Carslaw

2022

Preprint

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Abstract. Both aerosol radiative forcing and cloud-climate feedbacks have large effects on climate, mainly through modification of solar shortwave radiative fluxes. Here we determine what causes the long-term trends in the shortwave (SW) top-of-the-atmosphere (TOA) fluxes (FSW) over the North Atlantic region. The UK Earth System Model (UKESM1) and the Hadley Centre General Environment Model (HadGEM) simulate a positive FSW trend between 1850 and 1970 (increasing SW reflection) and a negative trend between 1970 and 2014. We find that the pre-1970 positive FSW trend is mainly driven by an increase in cloud droplet number concentrations due to increases in aerosol and the 1970–2014 trend is mainly driven by a decrease in cloud fraction, which we attribute mainly to cloud feedbacks caused by greenhouse gas-induced warming. Using nudged simulations where the meteorology can be controlled we show that in the pre-1970 period aerosol-induced cooling and greenhouse gas warming in coupled atmosphere-ocean simulations roughly counteract each other so that aerosol forcing is the dominant effect on FSW, with only a weak temperature-driven cloud feedback effect. However, in the post-1970 period the warming from greenhouse gases intensifies and aerosol radiative forcing falls, leading to a large overall warming and a reduction in FSW that is mainly driven by cloud feedbacks. Our results show that it is difficult to use satellite observations in the post-1970 period to evaluate and constrain the magnitude of the aerosol-cloud interaction forcing, but that cloud feedbacks might be evaluated. Comparisons to observations between 1985 and 2014 show that the simulated reduction in FSW and the increase in temperature are too strong. However, analysis shows that this temperature discrepancy can account for only part of the FSW discrepancy given the estimated model feedback strength (λ = ∂FSW/∂T). This suggests a model bias in either λ or in the strength of the aerosol forcing (aerosols are reducing during this time period) is required to explain the too-strong decrease in FSW. Both of these biases would also tend to cause temperature increases over the 1985–2014 period that are too large, which would be consistent with the sign of the model temperature bias reported here. Either of these model biases would have important implications for future climate projections using these models.

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The Role of Anthropogenic Aerosol Forcing in the 1850–1985 Strengthening of the AMOC in CMIP6 Historical Simulations

Cited by 26 publications

References 67 publications

Is equilibrium climate sensitivity clouding our judgement?

Is equilibrium climate sensitivity clouding our judgement?

Predictable Decadal Forcing of the North Atlantic Jet Stream by Sub-Polar North Atlantic Sea Surface Temperatures

What controls the historical timeseries of shortwave fluxes in the North Atlantic?

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