Multiple drivers of the North Atlantic warming hole

Keil, Paul; Mauritsen, Thorsten; Jungclaus, Johann; Hedemann, Christopher; Olonscheck, Dirk; Ghosh, Rohit

doi:10.1038/s41558-020-0819-8

Cited by 155 publications

(163 citation statements)

References 29 publications

(65 reference statements)

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“…However, there are potentially important differences that need further investigation: The weakening of the Atlantic meridional overturning circulation is less pronounced compared to the CMIP5 ensemble mean. Since weakening of the Atlantic meridional overturning circulation has been linked to the emergence of a warming hole over the North Atlantic subpolar gyre (e.g., Keil et al, 2020), it is consistent that the warming hole is only weak according to our model results.…”

Section: Discussionsupporting

confidence: 87%

Simulations for CMIP6 With the AWI Climate Model AWI‐CM‐1‐1

Semmler

Danilov

Gierz

et al. 2020

J Adv Model Earth Syst

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The Alfred Wegener Institute Climate Model (AWI‐CM) participates for the first time in the Coupled Model Intercomparison Project (CMIP), CMIP6. The sea ice‐ocean component, FESOM, runs on an unstructured mesh with horizontal resolutions ranging from 8 to 80 km. FESOM is coupled to the Max Planck Institute atmospheric model ECHAM 6.3 at a horizontal resolution of about 100 km. Using objective performance indices, it is shown that AWI‐CM performs better than the average of CMIP5 models. AWI‐CM shows an equilibrium climate sensitivity of 3.2°C, which is similar to the CMIP5 average, and a transient climate response of 2.1°C which is slightly higher than the CMIP5 average. The negative trend of Arctic sea‐ice extent in September over the past 30 years is 20–30% weaker in our simulations compared to observations. With the strongest emission scenario, the AMOC decreases by 25% until the end of the century which is less than the CMIP5 average of 40%. Patterns and even magnitude of simulated temperature and precipitation changes at the end of this century compared to present‐day climate under the strong emission scenario SSP585 are similar to the multi‐model CMIP5 mean. The simulations show a 11°C warming north of the Barents Sea and around 2°C to 3°C over most parts of the ocean as well as a wetting of the Arctic, subpolar, tropical, and Southern Ocean. Furthermore, in the northern middle latitudes in boreal summer and autumn as well as in the southern middle latitudes, a more zonal atmospheric flow is projected throughout the year.

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Section: Discussionsupporting

confidence: 87%

Simulations for CMIP6 With the AWI Climate Model AWI‐CM‐1‐1

Semmler

Danilov

Gierz

et al. 2020

J Adv Model Earth Syst

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“…A recent study suggests that the North Atlantic cooling is not only related to a weaker AMOC, but also northward heat transport. So, inferred AMOC estimates from sea surface temperature are prone to error, and they are not solely a measure of the AMOC (Keil et al, 2020). We do note, however, that multiple proxy observations, support AMOC weakening during 1950-1990 (Chen and Tung, 2018).…”

Section: Cmip6 Anthropogenic Aerosol Simulationsmentioning

confidence: 56%

Anthropogenic aerosol forcing of the AMOC and the associated mechanisms in CMIP6 models

Hassan

Allen

Liu

et al. 2020

Preprint

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Abstract. By regulating the global transport of heat, freshwater and carbon, the Atlantic Meridional Overturning Circulation (AMOC) serves as an important component of the climate system. During the late 20th and early 21st centuries, indirect observations and models suggest a weakening of the AMOC. Direct AMOC observations also suggest a weakening during the early 21st century, but with substantial interannual variability. Long-term weakening of the AMOC has been associated with increasing greenhouse gases (GHGs), but some modeling studies suggest the build up of anthropogenic aerosols (AAs) may have offset part of the GHG-induced weakening. Here, we quantify 1900–2020 AMOC variations and assess the driving mechanisms in state-of-the-art climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6). The CMIP6 all forcing (GHGs, anthropogenic and volcanic aerosols, solar variability, and land use/land change) multi-model mean shows negligible AMOC changes up to ~1950, followed by robust AMOC strengthening during the second half of the 20th century (~1950–1990), and weakening afterwards (1990–2020). These multi-decadal AMOC variations are related to changes in North Atlantic atmospheric circulation, including an altered sea level pressure gradient, storm track activity, surface winds and heat fluxes, which drive changes in the subpolar North Atlantic surface density flux. Similar to previous studies, CMIP6 GHG simulations yield robust AMOC weakening, particularly during the second half of the 20th century. Changes in natural forcings, including solar variability and volcanic aerosols, yield negligible AMOC changes. In contrast, CMIP6 AA simulations yield robust AMOC strengthening (weakening) in response to increasing (decreasing) anthropogenic aerosols. Moreover, the CMIP6 all-forcing AMOC variations and atmospheric circulation responses also occur in the CMIP6 AA simulations, which suggests these are largely driven by changes in anthropogenic aerosol emissions. Although aspects of the CMIP6 all-forcing multi-model mean response resembles observations, notable differences exist. This includes CMIP6 AMOC strengthening from ~1950–1990, when the indirect estimates suggest AMOC weakening. The CMIP6 multi-model mean also underestimates the observed increase in North Atlantic ocean heat content. And although the CMIP6 North Atlantic atmospheric circulation responses–particularly the overall patterns–are similar to observations, the simulated responses are weaker than those observed, implying they are only partially externally forced. The possible causes of these differences include internal climate variability, observational uncertainties and model shortcomings–including excessive aerosol forcing. A handful of CMIP6 realizations yield AMOC evolution since 1900 similar to the indirect observations, implying the inferred AMOC weakening from 1950–1990 (and even from 1930–1990) may have a significant contribution from internal (i.e., unforced) climate variability. Nonetheless, CMIP6 models yield robust, externally forced AMOC changes, the bulk of which are due to anthropogenic aerosols.

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“…Both simulations show a relative lack of warming in the subpolar North Atlantic. The North Atlantic "warming hole" has been associated with long-term forced AMOC decline (e.g., Drijfhout et al, 2012;Rahmstorf et al, 2015), although the relative role of AMOC compared to other possible drivers remains a topic of ongoing debate (Keil et al, 2020). 10.1029/2020MS002298…”

Section: 1029/2020ms002298mentioning

confidence: 99%

An Unprecedented Set of High‐Resolution Earth System Simulations for Understanding Multiscale Interactions in Climate Variability and Change

Chang

Zhang

Danabasoglu

et al. 2020

J Adv Model Earth Syst

154

186

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We present an unprecedented set of high-resolution climate simulations, consisting of a 500-year pre-industrial control simulation and a 250-year historical and future climate simulation from 1850 to 2100. A high-resolution configuration of the Community Earth System Model version 1.3 (CESM1.3) is used for the simulations with a nominal horizontal resolution of 0.25°for the atmosphere and land models and 0.1°for the ocean and sea-ice models. At these resolutions, the model permits tropical cyclones and ocean mesoscale eddies, allowing interactions between these synoptic and mesoscale phenomena with large-scale circulations. An overview of the results from these simulations is provided with a focus on model drift, mean climate, internal modes of variability, representation of the historical and future climates, and extreme events. Comparisons are made to solutions from an identical set of simulations using the standard resolution (nominal 1°) CESM1.3 and to available observations for the historical period to address some key scientific questions concerning the impact and benefit of increasing model horizontal resolution in climate simulations. An emerging prominent feature of the high-resolution pre-industrial simulation is the intermittent occurrence of polynyas in the Weddell Sea and its interaction with an Interdecadal Pacific Oscillation. Overall, high-resolution simulations show significant improvements in representing global mean temperature changes, seasonal cycle of sea-surface temperature and mixed layer depth, extreme events and in relationships between extreme events and climate modes. Plain Language Summary Although the current generation of climate models has demonstrated high fidelity in simulating and projecting global temperature change, these models show large uncertainties when it comes to questions concerning how rising global temperatures will impact local weather conditions. This is because the resolution (~100 km) at which the majority of climate models simulate the climate is not fine enough to resolve these small-scale regional features. Conducting long-term (multi-centuries) high-resolution (~10 km) climate simulations has been a great challenge for the research community due to the extremely high computational demands. Through international

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Multiple drivers of the North Atlantic warming hole

Cited by 155 publications

References 29 publications

Simulations for CMIP6 With the AWI Climate Model AWI‐CM‐1‐1

Simulations for CMIP6 With the AWI Climate Model AWI‐CM‐1‐1

Anthropogenic aerosol forcing of the AMOC and the associated mechanisms in CMIP6 models

An Unprecedented Set of High‐Resolution Earth System Simulations for Understanding Multiscale Interactions in Climate Variability and Change

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