Nonlinear Climate Responses to Changes in Antarctic Intermediate Water

2013 ◽  
Vol 26 (22) ◽  
pp. 9175-9193 ◽  
Author(s):  
Jennifer A. Graham ◽  
David P. Stevens ◽  
Karen J. Heywood
Keyword(s):  
Climate Model ◽  
Potential Temperature ◽  
Heat Fluxes ◽  
Meridional Overturning Circulation ◽  
Intermediate Water ◽  
Antarctic Intermediate Water ◽  
Surface Ocean ◽  
The North ◽  
Meridional Overturning ◽  
Circumpolar Current

Abstract The global impact of changes in Antarctic Intermediate Water (AAIW) properties is demonstrated using idealized perturbation experiments in a coupled climate model. Properties of AAIW were altered between 10° and 20°S in the Atlantic, Pacific, and Indian Oceans separately. Potential temperature was changed by ±1°C, along with density-compensating changes in salinity. For each of the experiments, sea surface temperature responds to changes in AAIW when anomalies surface at higher latitudes (>30°). Anomalous sea-to-air heat fluxes leave density anomalies in the ocean, resulting in nonlinear responses to opposite-sign perturbations. In the Southern Ocean, these affect the meridional density gradient, leading to changes in Antarctic Circumpolar Current transport. The response to cooler, fresher AAIW is both greater in magnitude and significant over a larger area than that for warmer, saltier AAIW. The North Atlantic is particularly sensitive to cool, fresh perturbations, with density anomalies causing reductions in the meridional overturning circulation of up to 1 Sv (1 Sv ≡ 106 m3 s−1). Resultant changes in meridional ocean heat transport, along with surfacing anomalies, cause basinwide changes in the surface ocean and overlying atmosphere on multidecadal time scales.

Subpolar Gyre – AMOC – Atmosphere Interactions on Multidecadal Timescales in a Version of the Kiel Climate Model

2021 ◽  
Author(s):  
Jing Sun ◽  
Mojib Latif ◽  
Wonsun Park
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Deep Convection ◽  
Meridional Overturning Circulation ◽  
Subpolar Gyre ◽  
Surface Salinity ◽  
The North ◽  
Ocean Coupling ◽  
Meridional Overturning ◽  
The North Atlantic

<p>There is a controversy about the nature of multidecadal climate variability in the North Atlantic (NA) region, concerning the roles of ocean circulation and atmosphere-ocean coupling. Here we describe NA multidecadal variability from a version of the Kiel Climate Model, in which both subpolar gyre (SPG)-Atlantic Meridional Overturning Circulation (AMOC) and atmosphere-ocean coupling are essential. The oceanic barotropic streamfuntions, meridional overturning streamfunctions, and sea level pressure are jointly analyzed to derive the leading mode of Atlantic variability. This mode accounting for about 23.7 % of the total combined variance is oscillatory with an irregular periodicity of 25-50 years and an e-folding time of about a decade. SPG and AMOC mutually influence each other and together provide the delayed negative feedback necessary for maintaining the oscillation. An anomalously strong SPG, for example, drives higher surface salinity and density in the NA’s sinking region. In response, oceanic deep convection and AMOC intensify, which, with a time delay of about a decade, reduces SPG strength by enhancing upper-ocean heat content. The weaker gyre circulation leads to lower surface salinity and density in the sinking region, which eventually reduces deep convection and AMOC strength. There is a positive ocean-atmosphere feedback between the sea surface temperature and low-level atmospheric circulation over the Southern Greenland area, with related wind stress changes reinforcing SPG changes, thereby maintaining the (damped) multidecadal oscillation against dissipation. Stochastic surface heat-flux forcing associated with the North Atlantic Oscillation drives the eigenmode.</p>

scholarly journals Towards measuring the meridional overturning circulation from space

Ocean Science ◽  
2007 ◽  
Vol 3 (2) ◽  
pp. 223-228 ◽  
Author(s):  
D. Cromwell ◽  
A. G. P. Shaw ◽  
P. Challenor ◽  
R. E. Houseago-Stokes ◽  
R. Tokmakian
Keyword(s):  
Climate Model ◽  
Initial Step ◽  
Meridional Overturning Circulation ◽  
Ocean Bottom ◽  
Bottom Pressure ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
Ocean Climate Model ◽  
Future Work

Abstract. We present a step towards measuring the meridional overturning circulation (MOC), i.e. the full-depth water mass transport, in the North Atlantic using satellite data. Using the Parallel Ocean Climate Model, we simulate satellite observations of ocean bottom pressure and sea surface height (SSH) over the 20-year period from 1979–1998, and use a linear model to estimate the MOC. As much as 93.5% of the variability in the smoothed transport is thereby explained. This increases to 98% when SSH and bottom pressure are first smoothed. We present initial studies of predicting the time evolution of the MOC, with promising results. It should be stressed that this is an initial step only, and that to produce an actual working system for measuring the MOC from space would require considerable future work.

scholarly journals Plateaus and jumps in the atmospheric radiocarbon record – Potential origin and value as global age markers for glacial-to-deglacial paleoceanography, a synthesis

2019 ◽  
Author(s):  
Michael Sarnthein ◽  
Kevin Küssner ◽  
Pieter M. Grootes ◽  
Blanca Ausin ◽  
Timothy Eglinton ◽  
...  
Keyword(s):  
North Pacific ◽  
Ice Cores ◽  
Meridional Overturning Circulation ◽  
Age Difference ◽  
Surface Ocean ◽  
The North ◽  
Deep Waters ◽  
Global Correlation ◽  
Carbon Release ◽  
Meridional Overturning

Abstract. Changes in the geometry of ocean Meridional Overturning Circulation (MOC) are crucial in controlling changes of climate and the carbon inventory of the atmosphere. However, the precise timing and global correlation of short-term glacial-to-deglacial changes of MOC in different ocean basins still present a major challenge. A possible solution is offered by the fine structure of jumps and plateaus in the record of radio-carbon (14C) concentration of the atmosphere and surface ocean that reflects changes in atmospheric 14C production as well as in the 14C exchange between air and sea and within the ocean. Boundaries of atmospheric 14C plateaus in the 14C record of Lake Suigetsu, now tied to Hulu U/Th model-ages instead of optical varve counts, provide a stratigraphic "rung ladder" of ~30 age tie points from 29 to 10 ka for correlation with and dating of planktic oceanic 14C records. The age difference between contemporary planktic and atmospheric 14C plateaus gives an estimate of the global distribution of 14C reservoir ages for surface waters of the Last Glacial Maximum (LGM) and deglacial Heinrich Stadial 1 (HS-1), as shown by 19 planktic 14C records. Clearly elevated and variable reservoir ages mark both high-latitude sites covered by sea ice and/or meltwater and upwelling regions. 14C ventilation ages of LGM deep waters reveal opposed geometries of Atlantic and Pacific MOC. Similar to today, Atlantic deep-water formation went along with an estuarine inflow of old abyssal waters from the Southern Ocean up to the northern North Pacific and an outflow of upper deep waters. Vice versa, 14C ventilation ages suggest a reversed MOC during early HS-1 and a ~1500 year long flushing of the deep North Pacific up to the South China Sea, when estuarine circulation geometry marked the North Atlantic, gradually starting near 19 ka. Elevated 14C ventilation ages of LGM deep waters reflect a major drawdown of carbon from the atmosphere. Inversely, the subsequent massive age drop and change in MOC induced two major events of carbon release to the atmosphere as recorded in Antarctic ice cores, shifts that highlight the significance of ocean MOC for atmospheric CO2 and its 14C inventory. These new features of MOC and the carbon cycle offer a challenge to model simulations that, in part because of insufficient spatial model resolution and reference data for testing the model results, still poorly reproduce them.

Future climate change scenarios shaped by inter-model differences in Atlantic Meridional Overturning Circulation response 

2021 ◽  
Author(s):  
Katinka Bellomo ◽  
Michela Angeloni ◽  
Susanna Corti ◽  
Jost von Hardenberg
Keyword(s):  
Climate Change ◽  
North Atlantic ◽  
Climate Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Future Climate ◽  
Future Climate Change ◽  
The North ◽  
Meridional Overturning ◽  
Climate Model Simulations

<div> <div> <div> <p>In climate model simulations of future climate change, the Atlantic Meridional Overturning Circulation (AMOC) is projected to decline. However, the impacts of this decline, relative to other changes, remain to be identified. Here we address this problem by analyzing 30 idealized abrupt-4xCO2 climate model simulations. We find that in models with larger AMOC decline, there is a minimum warming in the North Atlantic, a southward displacement of the Inter-tropical Convergence Zone (ITCZ) and a poleward shift of the mid-latitude jet. The changes in the models with smaller AMOC decline are drastically different: there is a relatively larger warming in the North Atlantic, the precipitation response exhibits a wet-get-wetter, dry-get-drier pattern, and there are smaller displacements of the mid-latitude jet. Our study indicates that the AMOC is a major source of inter-model uncertainty, and continued observational efforts are needed to constrain the AMOC response in future climate change.</p> </div> </div> </div>

scholarly journals The Variability of the Atlantic Meridional Overturning Circulation, the North Atlantic Oscillation, and the El Niño–Southern Oscillation in the Bergen Climate Model

2005 ◽  
Vol 18 (13) ◽  
pp. 2361-2375 ◽  
Author(s):  
Juliette Mignot ◽  
Claude Frankignoul
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Deep Convection ◽  
Meridional Overturning Circulation ◽  
Tropical Atlantic ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
The North Atlantic Oscillation ◽  
The Tropical Pacific

Abstract The link between the interannual to interdecadal variability of the Atlantic meridional overturning circulation (AMOC) and the atmospheric forcing is investigated using 200 yr of a control simulation of the Bergen Climate Model, where the mean circulation cell is rather realistic, as is also the location of deep convection in the northern North Atlantic. The AMOC variability has a slightly red frequency spectrum and is primarily forced by the atmosphere. The maximum value of the AMOC is mostly sensitive to the deep convection in the Irminger Sea, which it lags by about 5 yr. The latter is mostly forced by a succession of atmospheric patterns that induce anomalous northerly winds over the area. The impact of the North Atlantic Oscillation on deep convection in the Labrador and Greenland Seas is represented realistically, but its influence on the AMOC is limited to the interannual time scale and is primarily associated with wind forcing. The tropical Pacific shows a strong variability in the model, with too strong an influence on the North Atlantic. However, its influence on the tropical Atlantic is realistic. Based on lagged correlations and the release of fictitious Lagrangian drifters, the tropical Pacific seems to influence the AMOC with a time lag of about 40 yr. The mechanism is as follows: El Niño events induce positive sea surface salinity anomalies in the tropical Atlantic that are advected northward, circulate in the subtropical gyre, and then subduct. In the ocean interior, part of the salinity anomaly is advected along the North Atlantic current, eventually reaching the Irminger and Labrador Seas after about 35 yr where they destabilize the water column and favor deep convection.

scholarly journals Early-onset of Atlantic Meridional Overturning Circulation weakening in response to atmospheric CO2 concentration

2021 ◽  
Vol 4 (1) ◽  
Author(s):  
Mihai Dima ◽  
Denis R. Nichita ◽  
Gerrit Lohmann ◽  
Monica Ionita ◽  
Mirela Voiculescu
Keyword(s):  
Numerical Simulations ◽  
North Atlantic ◽  
Co2 Concentration ◽  
Atlantic Meridional Overturning Circulation ◽  
Heat Fluxes ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning

AbstractThe Atlantic Meridional Overturning Circulation (AMOC), a tipping component of the climate system, is projected to slowdown during the 21st century in response to increased atmospheric CO2 concentration. The rate and start of the weakening are associated with relatively large uncertainties. Observed sea surface temperature-based reconstructions indicate that AMOC has been weakening since the mid-20th century, but its forcing factors are not fully understood. Here we provide dynamical observational evidence that the increasing atmospheric CO2 concentration affects the North Atlantic heat fluxes and precipitation rate, and weakens AMOC, consistent with numerical simulations. The inferred weakening, starting in the late 19th century, earlier than previously suggested, is estimated at 3.7 ± 1.0 Sv over the 1854–2016 period, which is larger than it is shown in numerical simulations (1.4 ± 1.4 Sv).

Was the onset of interhemispheric AMOC slightly prior to Antarctic glaciation at the Eocene-Oligocene transition?

2020 ◽  
Author(s):  
Meir Abelson ◽  
Jonathan Erez
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Atlantic Sector ◽  
The North ◽  
Global Cooling ◽  
Circulation Cell ◽  
Meridional Overturning ◽  
Circumpolar Current

<p>A compilation of benthic δ<sup>18</sup>O from the whole Atlantic and the Southern Ocean (Atlantic sector), shows two major jumps in the interbasinal gradient of d<sup>18</sup>O (Δδ<sup>18</sup>O) during the Eocene and the Oligocene: One at ~40 Ma and the second concomitant with the isotopic event of the Eocene-Oligocene transition (EOT), ~33.7 Ma ago. From previously published circulation models, we show that the first Δδ<sup>18</sup>O jump reflects the thermal isolation of Antarctica associated with the proto-Antarctic circumpolar current (ACC). The second marks the onset of interhemispheric northern-sourced circulation cell, similar to the modern Atlantic meridional overturning circulation (AMOC). The onset of AMOC-like circulation probably slightly preceded (100-300 ky) the EOT, as we show by the high resolution profiles of δ<sup>18</sup>O and δ<sup>13</sup>C previously published from DSDP/ODP sites in the Southern Ocean and South Atlantic. We suggest that while the shallow proto-ACC supplied the energy for deep ocean convection in the Southern Hemisphere, the onset of the interhemispheric northern circulation cell was due to the significant EOT intensification of deepwater formation in the North Atlantic driven by the Nordic anti-estuarine circulation. This onset of the interhemispheric northern-sourced circulation cell could have prompted the EOT global cooling.</p>

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