Robust and Nonrobust Aspects of Atlantic Meridional Overturning Circulation Variability and Mechanisms in the Community Earth System Model

2019 ◽  
Vol 32 (21) ◽  
pp. 7349-7368
Author(s):  
Gokhan Danabasoglu ◽  
Laura Landrum ◽  
Stephen G. Yeager ◽  
Peter R. Gent
Keyword(s):  
Heat Budget ◽  
Atlantic Meridional Overturning Circulation ◽  
Earth System Model ◽  
Meridional Overturning Circulation ◽  
System Model ◽  
Earth System ◽  
Overturning Circulation ◽  
Budget Analysis ◽  
Community Earth System Model ◽  
Meridional Overturning

Abstract Robust and nonrobust aspects of Atlantic meridional overturning circulation (AMOC) variability and mechanisms are analyzed in several 600-yr simulations with the Community Earth System Model. The simulations consist of a set of cases where a few loosely constrained ocean model parameter values are changed, a pair of cases where round-off level perturbations are applied to the initial atmospheric temperature field, and a millennium-scale integration. The time scales of variability differ among the cases with the dominant periods ranging from decadal to centennial. These dominant periods are not stationary in time, indicating that a robust characterization of AMOC temporal variability requires long, multimillennium-scale simulations. A robust aspect is that positive anomalies of the Labrador Sea (LS) upper-ocean density and boundary layer depth and the positive phase of the North Atlantic Oscillation lead AMOC strengthening by 2–3 years. Respective contributions of temperature and salinity to these density anomalies vary across the simulations, but in a majority of the cases temperature contributions dominate. Following an AMOC intensification, all cases show that advection of warm and salty waters into the LS region results in near-neutral density anomalies. Analysis of the LS heat budget indicates that temperature acts to increase density in all cases prior to an AMOC intensification, primarily due to losses by sensible and latent heat fluxes. The accompanying salt budget analysis reveals that the salt contribution to density anomalies varies across the cases, taking both positive and negative values.

A Note on the Stability Indicator of the Atlantic Meridional Overturning Circulation

2014 ◽  
Vol 27 (2) ◽  
pp. 969-975 ◽  
Author(s):  
Wei Liu ◽  
Zhengyu Liu
Keyword(s):  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
System Model ◽  
Overturning Circulation ◽  
Climate System Model ◽  
Freshwater Transport ◽  
Meridional Overturning ◽  
Freshwater Export ◽  
The Stability ◽  
Generalized Indicator

Abstract This study examines the validity of the net freshwater transport ΔMov as a stability indicator of the Atlantic meridional overturning circulation (AMOC) in a low-resolution version of the NCAR Community Climate System Model, version 3 (CCSM3). It is shown that the sign of ΔMov indicates the monostability or bistability of the AMOC, which is based on a hypothesis that a collapsed AMOC induces a zero net freshwater transport. In CCSM3, this hypothesis is satisfied in that the collapsed AMOC, with a nonzero strength, induces a zero net freshwater transport ΔMov across the Atlantic basin by generating equivalent freshwater export MovS and freshwater import MovN at the southern and northern boundaries, respectively. Because of the satisfaction of the hypothesis, ΔMov is consistent with a generalized indicator L for a slowly evolving AMOC, both of which correctly monitor the AMOC stability.

scholarly journals The Atlantic's freshwater budget under climate change in the Community Earth System Model with strongly eddying oceans

Ocean Science ◽  
2021 ◽  
Vol 17 (3) ◽  
pp. 729-754
Author(s):  
André Jüling ◽  
Xun Zhang ◽  
Daniele Castellana ◽  
Anna S. von der Heydt ◽  
Henk A. Dijkstra
Keyword(s):  
Climate Change ◽  
Earth System Model ◽  
Meridional Overturning Circulation ◽  
Multiple Equilibrium ◽  
System Model ◽  
Earth System ◽  
Basin Scale ◽  
Community Earth System Model ◽  
Freshwater Budget ◽  
Resolution Simulation

Abstract. We investigate the freshwater budget of the Atlantic and Arctic oceans in coupled climate change simulations with the Community Earth System Model and compare a strongly eddying setup with 0.1∘ ocean grid spacing to a non-eddying 1∘ configuration typical of Coupled Model Intercomparison Project phase 6 (CMIP6) models. Details of this budget are important to understand the evolution of the Atlantic Meridional Overturning Circulation (AMOC) under climate change. We find that the slowdown of the AMOC in the year 2100 under the increasing CO2 concentrations of the Representative Concentration Pathway 8.5 (RCP8.5) scenario is almost identical between both simulations. Also, the surface freshwater fluxes are similar in their mean and trend under climate change in both simulations. While the basin-scale total freshwater transport is similar between the simulations, significant local differences exist. The high-ocean-resolution simulation exhibits significantly reduced ocean state biases, notably in the salt distribution, due to an improved circulation. Mesoscale eddies contribute considerably to the freshwater and salt transport, in particular at the boundaries of the subtropical and subpolar gyres. Both simulations start in the single equilibrium AMOC regime according to a commonly used AMOC stability indicator and evolve towards the multiple equilibrium regime under climate change, but only the high-resolution simulation enters it due to the reduced biases in the freshwater budget.

scholarly journals The Atlantic's Freshwater Budget under Climate Change in the Community Earth System Model with Strongly Eddying Oceans

2020 ◽  
Author(s):  
André Jüling ◽  
Xun Zhang ◽  
Daniele Castellana ◽  
Anna S. von der Heydt ◽  
Henk A. Dijkstra
Keyword(s):  
Climate Change ◽  
Earth System Model ◽  
Meridional Overturning Circulation ◽  
Multiple Equilibrium ◽  
System Model ◽  
Earth System ◽  
Basin Scale ◽  
Community Earth System Model ◽  
Freshwater Budget ◽  
Resolution Simulation

Abstract. We investigate the freshwater and salinity budget of the Atlantic and Arctic oceans in a strongly eddying coupled climate change simulation with the Community Earth System Model (CESM) and compare it to a simulation with a coarse ocean resolution CESM configuration, typical of CMIP6 models. Details of these budgets are important to understand the evolution of the Atlantic Meridional Overturning Circulation (AMOC) under climate change. We find that the slowdown of the AMOC in 2100 under the increasing CO2 concentrations of the RCP8.5 scenario is almost identical between both simulations. Also, the surface freshwater fluxes are similar in their mean and trend under climate change in both simulations. While the basin-scale total freshwater transport is similar between the simulations, significant local differences exist. The high ocean resolution simulation exhibits significantly reduced ocean state biases, notably in the salt distribution, due to an improved circulation. Mesoscale eddies contribute considerably to the freshwater and salt transport, in particular at the boundary of the subtropical and subpolar gyres. Both simulations start in the single equilibrium AMOC regime according to a commonly used AMOC stability indicator and evolve towards the multiple equilibrium regime under climate change, but only the high resolution simulation enters it due to the reduced biases in the freshwater budget.

scholarly journals Internally generated millennial-scale climate variability in an earth system model of intermediate complexity: sensitivity to ocean bathymetry and orbital forcing

2010 ◽  
Vol 3 (1) ◽  
pp. 273-307
Author(s):  
T. Friedrich ◽  
A. Timmermann ◽  
L. Menviel ◽  
O. Timm ◽  
A. Mouchet ◽  
...  
Keyword(s):  
Earth System Model ◽  
Meridional Overturning Circulation ◽  
System Model ◽  
Hudson Bay ◽  
Earth System ◽  
Overturning Circulation ◽  
Intermediate Complexity ◽  
The North ◽  
Stochastic Excitations ◽  
Millennial Scale

Abstract. The effect of orbital variations on simulated millennial-scale variability of the Atlantic Meridional Overturning Circulation (AMOC) is studied using the earth system model of intermediate complexity LOVECLIM. It is found that for present-day topographic boundary conditions low obliquity values (~22.1°) favor the triggering of internally generated millennial-scale variability in the North Atlantic region. Reducing the obliquity leads to changes of the pause-pulse ratio of the corresponding AMOC oscillations. Stochastic excitations of the density-driven overturning circulation in the Nordic Seas can create regional sea-ice anomalies and a subsequent reorganization of the atmospheric circulation. The resulting remote atmospheric anomalies over the Hudson Bay can release freshwater pulses into the Labrador Sea leading to a subsequent reduction of convective activity. The millennial-scale AMOC oscillations disappear if LGM bathymetry (with closed Hudson Bay) or Hudson Bay salinity is prescribed. Furthermore, our study documents the marine and terrestrial carbon cycle response to millennial-scale AMOC variability.

scholarly journals The mechanism behind internally generated centennial-to-millennial scale climate variability in an earth system model of intermediate complexity

2010 ◽  
Vol 3 (2) ◽  
pp. 377-389 ◽  
Author(s):  
T. Friedrich ◽  
A. Timmermann ◽  
L. Menviel ◽  
O. Elison Timm ◽  
A. Mouchet ◽  
...  
Keyword(s):  
Earth System Model ◽  
Meridional Overturning Circulation ◽  
System Model ◽  
Hudson Bay ◽  
Earth System ◽  
Overturning Circulation ◽  
Intermediate Complexity ◽  
The North ◽  
Stochastic Excitations ◽  
Millennial Scale

Abstract. The mechanism triggering centennial-to-millennial-scale variability of the Atlantic Meridional Overturning Circulation (AMOC) in the earth system model of intermediate complexity LOVECLIM is investigated. It is found that for several climate boundary conditions such as low obliquity values (~22.1°) or LGM-albedo, internally generated centennial-to-millennial-scale variability occurs in the North Atlantic region. Stochastic excitations of the density-driven overturning circulation in the Nordic Seas can create regional sea-ice anomalies and a subsequent reorganization of the atmospheric circulation. The resulting remote atmospheric anomalies over the Hudson Bay can release freshwater pulses into the Labrador Sea and significantly increase snow fall in this region leading to a subsequent reduction of convective activity. The millennial-scale AMOC oscillations disappear if LGM bathymetry (with closed Hudson Bay) is prescribed or if freshwater pulses are suppressed artificially. Furthermore, our study documents the process of the AMOC recovery as well as the global marine and terrestrial carbon cycle response to centennial-to-millennial-scale AMOC variability.

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