Aerosol-forced AMOC changes in CMIP6 historical simulations.

Menary, Matthew; Allan, Richard P.; Robson, Jon; Booth, Ben B. B.; Cassou, Christophe; Gregory, Jonathan M.; Hodson, Daniel L. R.; Jones, Colin; Mignot, Juliette; Ringer, Mark A.; Sutton, Rowan; Wilcox, Laura; Zhang, Rong; Gastineau, Guillaume

doi:10.5194/egusphere-egu21-8693

Cited by 4 publications

(14 citation statements)

References 44 publications

(47 reference statements)

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“…However, the similarity between the temporal evolution in basin mean AOD and many variables does suggest that anthropogenic aerosol emissions play an important role in decadal‐to‐multidecadal time scale change across the North Atlantic climate system in UKESM1. This role of anthropogenic aerosols is consistent with previous modeling studies (Bellomo et al., 2018; Booth et al., 2012; Menary et al., 2013, 2020; Watanabe & Tatebe, 2019). However, it is important to recognize that there continues to be debate on the relative role of aerosols in driving recent changes in the North Atlantic, and in AMV (Kim, Yeager & Danabasoglu, 2018; Robson et al., 2016; Yan et al., 2019; Zhang et al., 2013).…”

Section: Discussionsupporting

confidence: 92%

“…It is also interesting to note that these low‐frequency changes in the AMOC are similar to those seen in AOD (Figure 9) and N d (Figure 12), albeit with a lag. Therefore, the changes in AMOC are consistent with the aerosol forcing (Menary et al., 2013, 2020) and are also consistent with the forced AMOC changes in GC3.1 (Andrews et al., 2020). However, we note that the rising AMOC is also accompanied by some multidecadal variability about the long‐term AMOC trend with similar time scales to AMV (Figure 16a).…”

Section: Resultssupporting

confidence: 80%

“…However, the long‐term changes in SPNA surface salinity appear to be opposite to long‐term trends suggested by observations (i.e., Friedman et al., 2017) The AMOC and northward heat transport at 26°N shows a forced upward trend for much of the twentieth century in UKESM1, before starting to decline around 1990. The increase in AMOC is also consistent with a forced response due to changes in anthropogenic aerosols (Menary et al., 2013, 2020). By the end of the twentieth century the AMOC is comparable to the observed AMOC strength (as measured by RAPID), but the northward heat transport is too low.…”

Section: Discussionsupporting

confidence: 77%

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The Evaluation of the North Atlantic Climate System in UKESM1 Historical Simulations for CMIP6

Robson

Aksenov

Bracegirdle

et al. 2020

J Adv Model Earth Syst

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Earth system models enable a broad range of climate interactions that physical climate models are unable to simulate. However, the extent to which adding Earth system components changes or improves the simulation of the physical climate is not well understood. Here we present a broad multivariate evaluation of the North Atlantic climate system in historical simulations of the UK Earth System Model (UKESM1) performed for CMIP6. In particular, we focus on the mean state and the decadal time scale evolution of important variables that span the North Atlantic climate system. In general, UKESM1 performs well and realistically simulates many aspects of the North Atlantic climate system. Like the physical version of the model, we find that changes in external forcing, and particularly aerosol forcing, are an important driver of multidecadal change in UKESM1, especially for Atlantic Multidecadal Variability and the Atlantic Meridional Overturning Circulation. However, many of the shortcomings identified are similar to common biases found in physical climate models, including the physical climate model that underpins UKESM1. For example, the summer jet is too weak and too far poleward; decadal variability in the winter jet is underestimated; intraseasonal stratospheric polar vortex variability is poorly represented; and Arctic sea ice is too thick. Forced shortwave changes may be also too strong in UKESM1, which, given the important role of historical aerosol forcing in shaping the evolution of the North Atlantic in UKESM1, motivates further investigation. Therefore, physical model development, alongside Earth system development, remains crucial in order to improve climate simulations.

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Section: Discussionsupporting

confidence: 92%

Section: Resultssupporting

confidence: 80%

Section: Discussionsupporting

confidence: 77%

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The Evaluation of the North Atlantic Climate System in UKESM1 Historical Simulations for CMIP6

Robson

Aksenov

Bracegirdle

et al. 2020

J Adv Model Earth Syst

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“…However, despite differences in forcing structure, previous studies demonstrated a similar spatial pattern of response of surface temperature to GHGs and aerosol forcing (with an opposite sign; Xie et al, 2013). Specifically, aerosol forcing was shown to strengthen the AMOC and hence to oppose the GHGs effect (Cai et al, 2006;Delworth & Dixon, 2006;Menary et al, 2020). The aerosol strengthening of the AMOC was explained by a combination of temperature and salinity effects on the sea water density in the North Atlantic (Delworth & Dixon, 2006).…”

Section: Introductionmentioning

confidence: 80%

Aerosol Forcing Masks and Delays the Formation of the North Atlantic Warming Hole by Three Decades

Dagan

Stier

Watson-Parris

2020

Geophysical Research Letters

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The North Atlantic warming hole (NAWH) is referred to as a reduced warming, or even cooling, of the North Atlantic during an anthropogenic-driven global warming. A NAWH is predicted by climate models during the 21st century, and its pattern is already emerging in observations. Despite the known key role of the North Atlantic surface temperatures in setting the Northern Hemisphere climate, the mechanisms behind the NAWH are still not fully understood. Using state-of-the-art climate models, we show that anthropogenic aerosol forcing opposes the formation of the NAWH (by leading to a local warming) and delays its emergence by about 30 years. In agreement with previous studies, we also demonstrate that the relative warming of the North Atlantic under aerosol forcing is due to changes in ocean heat fluxes, rather than air-sea fluxes. These results suggest that the predicted reduction in aerosol forcing during the 21st century may accelerate the formation of the NAWH. Plain Language Summary Anthropogenic aerosols are particles suspended in the atmosphere, which were released due to anthropogenic activity. These particles have a general cooling effect on the Earth due to their interactions with radiation and with clouds. Here we show that the surface temperature in the North Atlantic Ocean is predicted to increase due to aerosol forcing (despite the global cooling). This trend is the opposite of the surface temperature trend predicted due to increase in greenhouse gases (global warming with a warming "hole" in the North Atlantic, trend known as the North Atlantic warming hole-NAWH). Using state-of-the-art climate models, we show that aerosol forcing delays the formation of the NAWH by about 30 years. This trend could have important climatic impacts due to the key role of the North Atlantic surface temperatures in setting the Northern Hemisphere's climate and due to the predicted reduction in aerosol forcing in the next few decades.

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“…However, there is a notable statistically significant region of warming over parts of the North Atlantic and Southern Ocean (Figure 2b). These areas of warming may be connected to a strengthened Atlantic Meridional Overturning Circulation (Dagan et al., 2020; Keil et al., 2020; Menary et al., 2020). The global signature of cooling prior to 2000 is associated with an increase in industrial aerosol emissions.…”

Section: Resultsmentioning

confidence: 99%

Detecting Climate Signals Using Explainable AI With Single‐Forcing Large Ensembles

Labe

Barnes

2021

J Adv Model Earth Syst

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It remains difficult to disentangle the relative influences of aerosols and greenhouse gases on regional surface temperature trends in the context of global climate change. To address this issue, we use a new collection of initial‐condition large ensembles from the Community Earth System Model version 1 that are prescribed with different combinations of industrial aerosol and greenhouse gas forcing. To compare the climate response to these external forcings, we adopt an artificial neural network (ANN) architecture from previous work that predicts the year by training on maps of near‐surface temperature. We then utilize layer‐wise relevance propagation (LRP) to visualize the regional temperature signals that are important for the ANN's prediction in each climate model experiment. To mask noise when extracting only the most robust climate patterns from LRP, we introduce a simple uncertainty metric that can be adopted to other explainable artificial intelligence (AI) problems. We find that the North Atlantic, Southern Ocean, and Southeast Asia are key regions of importance for the neural network to make its prediction, especially prior to the early‐21st century. Notably, we also find that the ANN predictions based on maps of observations correlate higher to the actual year after training on the large ensemble experiment with industrial aerosols held fixed to 1920 levels. This work illustrates the sensitivity of regional temperature signals to changes in aerosol forcing in historical simulations. By using explainable AI methods, we have the opportunity to improve our understanding of (non)linear combinations of anthropogenic forcings in state‐of‐the‐art global climate models.

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Aerosol-forced AMOC changes in CMIP6 historical simulations.

Cited by 4 publications

References 44 publications

The Evaluation of the North Atlantic Climate System in UKESM1 Historical Simulations for CMIP6

The Evaluation of the North Atlantic Climate System in UKESM1 Historical Simulations for CMIP6

Aerosol Forcing Masks and Delays the Formation of the North Atlantic Warming Hole by Three Decades

Detecting Climate Signals Using Explainable AI With Single‐Forcing Large Ensembles

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