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1. Role of AMOC in Transient Climate Response to Greenhouse Gas Forcing in Two Coupled Models

   https://doi.org/10.1175/JCLI-D-19-1027.1  

   Hu, Aixue ; Van Roekel, Luke ; Weijer, Wilbert ; Garuba, Oluwayemi A. ; Cheng, Wei ; Nadiga, Balu T. ( July 2020 , Journal of Climate)

   null (Ed.)

   Abstract As the greenhouse gas concentrations increase, a warmer climate is expected. However, numerous internal climate processes can modulate the primary radiative warming response of the climate system to rising greenhouse gas forcing. Here the particular internal climate process that we focus on is the Atlantic meridional overturning circulation (AMOC), an important global-scale feature of ocean circulation that serves to transport heat and other scalars, and we address the question of how the mean strength of AMOC can modulate the transient climate response. While the Community Earth System Model version 2 (CESM2) and the Energy Exascale Earth System Model version 1 (E3SM1) have very similar equilibrium/effective climate sensitivity, our analysis suggests that a weaker AMOC contributes in part to the higher transient climate response to a rising greenhouse gas forcing seen in E3SM1 by permitting a faster warming of the upper ocean and a concomitant slower warming of the subsurface ocean. Likewise the stronger AMOC in CESM2 by permitting a slower warming of the upper ocean leads in part to a smaller transient climate response. Thus, while the mean strength of AMOC does not affect the equilibrium/effective climate sensitivity, it is likely to play an important role in determining the transient climate response on the centennial time scale. 

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2. Greenhouse-gas forced changes in the Atlantic meridional overturning circulation and related worldwide sea-level change

   https://doi.org/10.1007/s00382-022-06386-y  

   Couldrey, Matthew P. ; Gregory, Jonathan M. ; Dong, Xiao ; Garuba, Oluwayemi ; Haak, Helmuth ; Hu, Aixue ; Hurlin, William J. ; Jin, Jiangbo ; Jungclaus, Johann ; Köhl, Armin ; et al ( August 2022 , Climate Dynamics)

   The effect of anthropogenic climate change in the ocean is challenging to project because atmosphere-ocean general circulation models (AOGCMs) respond differently to forcing. This study focuses on changes in the Atlantic Meridional Overturning Circulation (AMOC), ocean heat content ($$\Delta$$$\Delta$OHC), and the spatial pattern of ocean dynamic sea level ($$\Delta \zeta$$$\Delta \zeta$). We analyse experiments following the FAFMIP protocol, in which AOGCMs are forced at the ocean surface with standardised heat, freshwater and momentum flux perturbations, typical of those produced by doubling$$\hbox {CO}_{{2}}$$${\text{CO}}_2$. Using two new heat-flux-forced experiments, we find that the AMOC weakening is mainly caused by and linearly related to the North Atlantic heat flux perturbation, and further weakened by a positive coupled heat flux feedback. The quantitative relationships are model-dependent, but few models show significant AMOC change due to freshwater or momentum forcing, or to heat flux forcing outside the North Atlantic. AMOC decline causes warming at the South Atlantic-Southern Ocean interface. It does not strongly affect the global-mean vertical distribution of$$\Delta$$$\Delta$OHC, which is dominated by the Southern Ocean. AMOC decline strongly affects$$\Delta \zeta$$$\Delta \zeta$in the North Atlantic, with smaller effects in the Southern Ocean and North Pacific. The ensemble-mean$$\Delta \zeta$$$\Delta \zeta$and$$\Delta$$$\Delta$OHC patterns are mostly attributable to the heat added by the flux perturbation, with smaller effects from ocean heat and salinity redistribution. The ensemble spread, on the other hand, is largely due to redistribution, with pronounced disagreement among the AOGCMs.

    

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3. Resolution Dependence of Atmosphere–Ocean Interactions and Water Mass Transformation in the North Atlantic

   https://doi.org/10.1029/2021JC018102  

   Oldenburg, Dylan ; Wills, Robert C. J. ; Armour, Kyle C. ; Thompson, LuAnne ( April 2022 , Journal of Geophysical Research: Oceans)

   Water mass transformation (WMT) in the North Atlantic plays a key role in driving the Atlantic Meridional Overturning Circulation (AMOC) and its variability. Here, we analyze subpolar North Atlantic WMT in high‐ and low‐resolution versions of the Community Earth System Model version 1 (CESM1) and investigate whether differences in resolution and climatological WMT impact low‐frequency AMOC variability and the atmospheric response to this variability. We find that high‐resolution simulations reproduce the WMT found in a reanalysis‐forced high‐resolution ocean simulation more accurately than low‐resolution simulations. We also find that the low‐resolution simulations, including one forced with the same atmospheric reanalysis data, have larger biases in surface heat fluxes, sea‐surface temperatures, and salinities compared to the high‐resolution simulations. Despite these major climatological differences, the mechanisms of low‐frequency AMOC variability are similar in the high‐ and low‐resolution versions of CESM1. The Labrador Sea WMT plays a major role in driving AMOC variability, and a similar North Atlantic Oscillation‐like sea‐level pressure pattern leads AMOC changes. However, the high‐resolution simulation shows a pronounced atmospheric response to the AMOC variability not found in the low‐resolution version. The consistent role of Labrador Sea WMT in low‐frequency AMOC variability across high‐ and low‐resolution coupled simulations, including a simulation which accurately reproduces the WMT found in an atmospheric‐reanalysis‐forced high‐resolution ocean simulation, suggests that the mechanisms may be similar in nature.

    

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4. A Strong Role for the AMOC in Partitioning Global Energy Transport and Shifting ITCZ Position in Response to Latitudinally Discrete Solar Forcing in CESM1.2

   https://doi.org/10.1175/JCLI-D-18-0360.1  

   Yu, Sungduk ; Pritchard, Michael S. ( April 2019 , Journal of Climate)

   Ocean circulation responses to interhemispheric radiative imbalance can damp north–south migrations of the intertropical convergence zone (ITCZ) by reducing the burden on atmospheric energy transport. The role of the Atlantic meridional overturning circulation (AMOC) in such dynamics has not received much attention. Here, we present coupled climate modeling results that suggest AMOC responses are of first-order importance to muting ITCZ shift magnitudes as a pair of hemispherically asymmetric solar forcing bands is moved from equatorial to polar latitudes. The cross-equatorial energy transport response to the same amount of interhemispheric forcing becomes systematically more ocean-centric when higher latitudes are perturbed in association with strengthening AMOC responses. In contrast, the responses of the Pacific subtropical cell are not monotonic and cannot predict this variance in the ITCZ’s equilibrium position. Overall, these results highlight the importance of the meridional distribution of interhemispheric radiative imbalance and the rich buffering of internal feedbacks that occurs in dynamic versus thermodynamic (slab) ocean modeling experiments. Mostly, the results imply that the problem of developing a theory of ITCZ migration is entangled with that of understanding the AMOC’s response to hemispherically asymmetric radiative forcing—a difficult topic deserving of focused analysis across more climate models.

    

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5. The Role of Atlantic Basin Geometry in Meridional Overturning Circulation

   https://doi.org/10.1175/JPO-D-21-0036.1  

   Ragen, Sarah ; Armour, Kyle C. ; Thompson, LuAnne ; Shao, Andrew ; Darr, David ( March 2022 , Journal of Physical Oceanography)

   We present idealized simulations to explore how the shape of eastern and western continental boundaries along the Atlantic Ocean influences the Atlantic meridional overturning circulation (AMOC). We use a state-of-the art ocean–sea ice model (MOM6 and SIS2) with idealized, zonally symmetric surface forcing and a range of idealized continental configurations with a large, Pacific-like basin and a small, Atlantic-like basin. We perform simulations with five coastline geometries along the Atlantic-like basin that range from coastlines that are straight to coastlines that are shaped like the coasts of the American and African continents. Changing the Atlantic basin coastline shape influences AMOC strength in a manner distinct from simply increasing basin width: widening the basin while maintaining straight coastlines leads to a 10-Sv (1 Sv ≡ 10⁶m³s⁻¹) increase in AMOC strength, whereas widening the basin with the geometry of the American and African continents leads to a 6-Sv increase in AMOC strength, despite both cases representing the same average basin-width increase relative to a control case. The structure of AMOC changes are different between these two cases as well: a more realistic basin geometry results in a shoaled AMOC while widening the basin with straight boundaries deepens AMOC. We test the influence of the shape of the both boundaries independently and find that AMOC is more sensitive to the American coastline while the African coastline impacts the abyssal circulation. We also find that AMOC strength and depth scales well with basin-scale meridional density difference, even with different Atlantic basin geometries, illuminating a robust physical link between AMOC and the North Atlantic western boundary density gradient.

    

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