scholarly journals Surface Flux Drivers for the Slowdown of the Atlantic Meridional Overturning Circulation in a High‐Resolution Global Coupled Climate Model

2019 ◽  
Vol 11 (5) ◽  
pp. 1349-1363 ◽  
Author(s):  
D. A. Putrasahan ◽  
K. Lohmann ◽  
J.‐S. Storch ◽  
J. H. Jungclaus ◽  
O. Gutjahr ◽  
...  
Keyword(s):  
High Resolution ◽  
Climate Model ◽  
Surface Flux ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Coupled Climate Model ◽  
Overturning Circulation ◽  
Meridional Overturning

scholarly journals Variability of the Atlantic Meridional Overturning Circulation in CCSM4

2012 ◽  
Vol 25 (15) ◽  
pp. 5153-5172 ◽  
Author(s):  
Gokhan Danabasoglu ◽  
Steve G. Yeager ◽  
Young-Oh Kwon ◽  
Joseph J. Tribbia ◽  
Adam S. Phillips ◽  
...  
Keyword(s):  
Low Frequency ◽  
Surface Flux ◽  
Atlantic Meridional Overturning Circulation ◽  
Horizontal Resolution ◽  
Meridional Overturning Circulation ◽  
Positive Density ◽  
Overturning Circulation ◽  
Climate System Model ◽  
Control Simulation ◽  
Meridional Overturning

Abstract Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Climate System Model, version 4 (CCSM4) preindustrial control simulation that uses nominal 1° horizontal resolution in all its components. AMOC shows a broad spectrum of low-frequency variability covering the 50–200-yr range, contrasting sharply with the multidecadal variability seen in the T85 × 1 resolution CCSM3 present-day control simulation. Furthermore, the amplitude of variability is much reduced in CCSM4 compared to that of CCSM3. Similarities as well as differences in AMOC variability mechanisms between CCSM3 and CCSM4 are discussed. As in CCSM3, the CCSM4 AMOC variability is primarily driven by the positive density anomalies at the Labrador Sea (LS) deep-water formation site, peaking 2 yr prior to an AMOC maximum. All processes, including parameterized mesoscale and submesoscale eddies, play a role in the creation of salinity anomalies that dominate these density anomalies. High Nordic Sea densities do not necessarily lead to increased overflow transports because the overflow physics is governed by source and interior region density differences. Increased overflow transports do not lead to a higher AMOC either but instead appear to be a precursor to lower AMOC transports through enhanced stratification in LS. This has important implications for decadal prediction studies. The North Atlantic Oscillation (NAO) is significantly correlated with the positive boundary layer depth and density anomalies prior to an AMOC maximum. This suggests a role for NAO through setting the surface flux anomalies in LS and affecting the subpolar gyre circulation strength.

scholarly journals Climate impacts of a weakened Atlantic Meridional Overturning Circulation in a warming climate

2020 ◽  
Vol 6 (26) ◽  
pp. eaaz4876 ◽  
Author(s):  
Wei Liu ◽  
Alexey V. Fedorov ◽  
Shang-Ping Xie ◽  
Shineng Hu
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Arctic Sea Ice ◽  
Climate Impacts ◽  
Meridional Overturning Circulation ◽  
Future Climate Change ◽  
Boreal Summer ◽  
Overturning Circulation ◽  
Meridional Overturning

While the Atlantic Meridional Overturning Circulation (AMOC) is projected to slow down under anthropogenic warming, the exact role of the AMOC in future climate change has not been fully quantified. Here, we present a method to stabilize the AMOC intensity in anthropogenic warming experiments by removing fresh water from the subpolar North Atlantic. This method enables us to isolate the AMOC climatic impacts in experiments with a full-physics climate model. Our results show that a weakened AMOC can explain ocean cooling south of Greenland that resembles the North Atlantic warming hole and a reduced Arctic sea ice loss in all seasons with a delay of about 6 years in the emergence of an ice-free Arctic in boreal summer. In the troposphere, a weakened AMOC causes an anomalous cooling band stretching from the lower levels in high latitudes to the upper levels in the tropics and displaces the Northern Hemisphere midlatitude jets poleward.

scholarly journals The Atlantic Meridional Overturning Circulation in High‐Resolution Models

2020 ◽  
Vol 125 (4) ◽  
Author(s):  
Joël J.‐M. Hirschi ◽  
Bernard Barnier ◽  
Claus Böning ◽  
Arne Biastoch ◽  
Adam T. Blaker ◽  
...  
Keyword(s):  
High Resolution ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
High Resolution Models

The effects of KT ≠ KS in a Stommel-like model of the upper Atlantic Meridional Overturning Circulation under steady surface flux forcing

2019 ◽  
Vol 77 (1) ◽  
pp. 243-266
Author(s):  
A.E. Gargett
Keyword(s):  
Steady State ◽  
Water Mass ◽  
Surface Flux ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Small Scale ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
North Polar ◽  
Stratified Regime

This study examines a simple 6-box model of a single pole-to-pole ocean basin. Each of a northern "polar gyre," a southern "polar gyre," and an "equatorial gyre," consisting of north and south subtropical gyres plus the equatorial region, is represented by two boxes: a surface box receiving constant fluxes of both temperature (heat) and salt (freshwater) and a deep box. The model includes four dominant processes: surface flux forcing, horizontal meridional advection driven by Southern Ocean winds, horizontal eddy diffusion at gyre boundaries, and convection, as well as the process of vertical diffusion by small-scale processes. Provided that heat loss from the northern polar gyre is sufficiently larger than that from the southern polar gyre, a steady-state Atlantic Meridional Overturning Circulation (AMOC)-like system, i. e., one with sinking in the north polar gyre and upwelling in a weakly stratified southern polar gyre, is obtained at present values of RF ≡ βFS / αFT, the ratio of surface forcing by fluxes of temperature (T ) and salinity (S ) in the equatorial gyre. Despite the fact that vertical diffusive fluxes are much smaller than those associated with all the other processes, it is shown that implementation in this model of a simple water mass–based representation of different vertical diffusivities for T and S, the two water properties that, with pressure, determine the density of seawater, can lead to profound change in the steady-state modes of the system. With equal diffusivities, the AMOC-like mode with north polar convection shifts abruptly to a mode with equatorial convection at sufficiently large values of RF. With unequal diffusivities, this mode boundary is replaced by an intermediate region of RF values in which all three gyres are stratified. The existence and extent of this stratified regime is shown to result predominantly from the differences between vertical turbulent diffusivities of T and S in the "salt fingering" equatorial gyre. Existence of a stratified regime at values of RF somewhat larger that present implies a tendency towards stable stratification throughout the oceans if, under climate change, the equatorial diffusivity difference were to increase as a result of water mass changes in the subtropical gyres and/or an increase in RF as a result of increased atmospheric freshwater fluxes and/or decreased heat fluxes. This tendency towards an everywhere-stratified ocean is independent of that expected from increased freshwater addition to surface polar oceans due to ice melt.

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