On Multidecadal Variability of the Atlantic Meridional Overturning Circulation in the Community Climate System Model Version 3

2008 ◽  
Vol 21 (21) ◽  
pp. 5524-5544 ◽  
Author(s):  
Gokhan Danabasoglu
Keyword(s):  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
System Model ◽  
Subpolar Gyre ◽  
Climate System Model ◽  
Multidecadal Variability ◽  
Meridional Overturning ◽  
The Mean

Abstract Multidecadal variability of the Atlantic meridional overturning circulation (MOC) is investigated diagnostically in the NCAR Community Climate System Model version 3 (CCSM3) present-day simulations, using the highest (T85 × 1) resolution version. This variability has a 21-yr period and is present in many other ocean fields in the North Atlantic. In MOC, the oscillation amplitude is about 4.5 Sv (1 Sv ≡ 106 m3 s−1), corresponding to 20% of the mean maximum MOC transport. The northward heat transport (NHT) variability has an amplitude of about 0.12 PW, representing 10% of the mean maximum NHT. In sea surface temperature (SST) and sea surface salinity (SSS), the peak-to-peak changes can be as large as 6°–7°C and 3 psu, respectively. The Labrador Sea region is identified as the deep-water formation (DWF) site associated with the MOC oscillations. In contrast with some previous studies, temperature and salinity contributions to the total density in this DWF region are almost equal and in phase. The heat and freshwater budget analyses performed for the DWF site indicate a complex relationship between the DWF, MOC, North Atlantic Oscillation (NAO), and subpolar gyre circulation anomalies. Their complicated interactions appear to be responsible for the maintenance of this multidecadal oscillation. In these interactions, the atmospheric variability associated with the model’s NAO plays a prominent role. In particular, the NAO modulates the subpolar gyre strength and contributes to the formation of the temperature and salinity anomalies that lead to positive/negative density anomalies at the DWF site. In addition, the wind stress curl anomalies occurring during the transition phase between the positive and negative NAO states produce fluctuations of the subtropical–subpolar gyre boundary, thus creating midlatitude SST and SSS anomalies. Comparisons with observations show that neither the pattern nor the magnitude of this dominant SST variability is realistic.

Impact of Arctic gateways closure on the Atlantic Meridional Overturning Circulation in the Pliocene

2021 ◽  
Author(s):  
Julia Weiffenbach ◽  
Michiel Baatsen ◽  
Anna von der Heydt
Keyword(s):  
Boundary Conditions ◽  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
The Arctic ◽  
Bering Strait ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning

<p>The mid-Pliocene climate is the most recent geological period with a greenhouse gas concentration of approximately 400 ppmv, similar to the present day. Proxy reconstructions indicate enhanced warming in the high North Atlantic in the mid-Pliocene, which has been suggested to be a response to a stronger Atlantic Meridional Overturning Circulation (AMOC). PlioMIP2 ensemble results show a stronger AMOC and simulated North Atlantic sea surface temperatures (SSTs) match reconstructions better than PlioMIP1. A major difference between PlioMIP1 and PlioMIP2 is the closure of the Bering Strait and Canadian Archipelago in the Pliocene. Previous studies have shown that closure of these Arctic gateways leads to an enhanced AMOC due to altered freshwater fluxes in the Arctic.</p><p>Analysis of our Community Earth System Model (CESM1) simulations shows that the simulated increase in North Atlantic SSTs and strengthened AMOC in the Pliocene is a result of Pliocene boundary conditions rather than CO<sub>2</sub> concentration increase. Here we compare results from two runs with pre-industrial boundary conditions and 280 and 560 ppmv CO<sub>2</sub> concentrations and three runs with PlioMIP2 boundary conditions and 280, 400 and 560 ppmv CO<sub>2</sub> concentrations. Results show a 10-15% stronger AMOC in the Pliocene simulations as well as enhanced warming and saltening of the North Atlantic sea surface. While there is a stronger AMOC, the Atlantic northward ocean heat transport (OHT) in the Pliocene simulations only increases 0-3% with respect to the pre-industrial. Analysis indicates there is an altered relationship between the AMOC and OHT in the Pliocene, pointing to fundamentally different behavior of the AMOC in the Pliocene simulations. This is supported by a specific spatial pattern of deep water formation (DWF) areas in the Pliocene simulations that is significantly different from that of the pre-industrial. In the Pliocene simulations, DWF areas adjacent to south Greenland disappear and new DWF areas appear further southwards in the Labrador Sea off the coast of Newfounland. These results indicate that insight into the effect of the palaeogeographic boundary conditions is crucial to understanding the Pliocene climate and its potential as a geological equivalent to a future greenhouse climate.</p>

scholarly journals Changes in extreme regional sea surface height due to an abrupt weakening of the Atlantic meridional overturning circulation

Ocean Science ◽  
2014 ◽  
Vol 10 (6) ◽  
pp. 881-891 ◽  
Author(s):  
S.-E. Brunnabend ◽  
H. A. Dijkstra ◽  
M. A. Kliphuis ◽  
B. van Werkhoven ◽  
H. E. Bal ◽  
...  
Keyword(s):  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Sea Surface Height ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Sea Level Changes ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning

Abstract. As an extreme scenario of dynamical sea level changes, regional sea surface height (SSH) changes that occur in the North Atlantic due to an abrupt weakening of the Atlantic meridional overturning circulation (AMOC) are simulated. Two versions of the same ocean-only model are used to study the effect of ocean model resolution on these SSH changes: a high-resolution (HR) strongly eddying version and a low-resolution (LR) version in which the effect of eddies is parameterised. The weakening of the AMOC is induced in both model versions by applying strong freshwater perturbations around Greenland. A rapid decrease of the AMOC in the HR version induces much shorter return times of several specific regional and coastal extremes in North Atlantic SSH than in the LR version. This effect is caused by a change in main eddy pathways associated with a change in separation latitude of the Gulf Stream.

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).

scholarly journals A Multimodel Study of Sea Surface Temperature and Subsurface Density Fingerprints of the Atlantic Meridional Overturning Circulation

2013 ◽  
Vol 26 (22) ◽  
pp. 9155-9174 ◽  
Author(s):  
Christopher D. Roberts ◽  
Freya K. Garry ◽  
Laura C. Jackson
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Climate System ◽  
Sea Surface ◽  
Labrador Sea ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract The Atlantic meridional overturning circulation (AMOC) is an important component of the North Atlantic climate system. Here, simulations from 10 coupled climate models are used to calculate patterns of sea surface temperature (SST) and subsurface density change associated with decadal AMOC variability. The models are evaluated using observational constraints and it is shown that all 10 models suffer from North Atlantic Deep Water transports that are too shallow, although the biases are least severe in the Community Climate System Model, version 4 (CCSM4). In the models that best compare with observations, positive AMOC anomalies are associated with reduced Labrador Sea stratification and increased midocean (800–1800 m) densities in the subpolar gyre. Maximum correlations occur when AMOC anomalies lag Labrador Sea stratification and subsurface density anomalies by 2–6 yr and 0–3 yr, respectively. In all 10 models, North Atlantic warming follows positive AMOC anomalies, but the patterns and magnitudes of SST change are variable. A simple detection and attribution analysis is then used to evaluate the utility of Atlantic midocean density and Labrador Sea stratification indices for detecting changes to the AMOC in the presence of increasing CO2 concentrations. It is shown that trends in midocean density are identifiable (although not attributable) significantly earlier than trends in the AMOC. For this reason, subsurface density observations could be a useful complement to transport observations made at specific latitudes and may help with the more rapid diagnosis of basin-scale changes in the AMOC. Using existing observations, it is not yet possible to detect a robust trend in the AMOC using either midocean densities or transport observations from 26.5°N.

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.

The Adequacy of Observing Systems in Monitoring the Atlantic Meridional Overturning Circulation and North Atlantic Climate

2010 ◽  
Vol 23 (19) ◽  
pp. 5311-5324 ◽  
Author(s):  
S. Zhang ◽  
A. Rosati ◽  
T. Delworth
Keyword(s):  
North Atlantic ◽  
Coupled System ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
The North ◽  
North Atlantic Climate ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
Observing Systems

Abstract The Atlantic meridional overturning circulation (AMOC) has an important influence on climate, and yet adequate observations of this circulation are lacking. Here, the authors assess the adequacy of past and current widely deployed routine observing systems for monitoring the AMOC and associated North Atlantic climate. To do so, this study draws on two independent simulations of the twentieth century using an Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled climate model. One simulation is treated as “truth” and is sampled according to the observing system being evaluated. The authors then assimilate these synthetic “observations” into the second simulation within a fully coupled system that instantaneously exchanges information among all coupled components and produces a nearly balanced and coherent estimate for global climate states including the North Atlantic climate system. The degree to which the assimilation recovers the truth is an assessment of the adequacy of the observing system being evaluated. As the coupled system responds to the constraint of the atmosphere or ocean, the assessment of the recovery for climate quantities such as Labrador Sea Water (LSW) and the North Atlantic Oscillation increases the understanding of the factors that determine AMOC variability. For example, the low-frequency sea surface forcings provided by the atmospheric and sea surface temperature observations are found to excite a LSW variation that governs the long-time-scale variability of the AMOC. When the most complete modern observing system, consisting of atmospheric winds and temperature, is used along with Argo ocean temperature and salinity down to 2000 m, a skill estimate of AMOC reconstruction is 90% (out of 100% maximum). Similarly encouraging results hold for other quantities, such as the LSW. The past XBT observing system, in which deep-ocean temperature and salinity were not available, has a lesser ability to recover the truth AMOC (the skill is reduced to 52%). While these results raise concerns about the ability to properly characterize past variations of the AMOC, they also hold promise for future monitoring of the AMOC and for initializing prediction models.

scholarly journals Subtle influence of the Atlantic Meridional Overturning Circulation (AMOC) on seasonal sea surface temperature (SST) hindcast skill in the North Atlantic

2021 ◽  
Vol 2 (3) ◽  
pp. 739-757
Author(s):  
Julianna Carvalho-Oliveira ◽  
Leonard Friedrich Borchert ◽  
Aurélie Duchez ◽  
Mikhail Dobrynin ◽  
Johanna Baehr
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Overturning Circulation ◽  
Seesaw Mechanism ◽  
Meridional Overturning ◽  
Start Dates

Abstract. We investigate the impact of the strength of the Atlantic Meridional Overturning Circulation (AMOC) at 26∘ N on the prediction of North Atlantic sea surface temperature anomalies (SSTAs) a season ahead. We test the dependence of sea surface temperate (SST) predictive skill in initialised hindcasts on the phase of the AMOC at 26∘N, invoking a seesaw mechanism driven by AMOC fluctuations, with positive SSTAs north of 26∘ N and negative SSTAs south of 26∘ N after a strong AMOC and vice versa. We use initialised simulations with the MPI-ESM-MR (where MR is mixed resolution) seasonal prediction system. First, we use an assimilation experiment between 1979–2014 to confirm that the AMOC leads a SSTA dipole pattern in the tropical and subtropical North Atlantic, with the strongest AMOC fingerprints after 2–4 months. Going beyond previous studies, we find that the AMOC fingerprint has a seasonal dependence and is sensitive to the length of the observational window used, i.e. stronger over the last decade than for the entire time series back to 1979. We then use a set of ensemble hindcast simulations with 30 members, starting each February, May, August and November between 1982 and 2014. We compare the changes in skill between composites based on the AMOC phase a month prior to each start date to simulations without considering the AMOC phase and find subtle influence of the AMOC mechanism on seasonal SST prediction skill. We find higher subtropical SST hindcast skill at a 2–4-month lead time for June–July–August (JJA) SSTA composites based on the AMOC phase at May start dates than for the full time period. In other regions and seasons, we find a negligible impact of the AMOC seesaw mechanism on seasonal SST predictions due to atmospheric influence, calling for caution when considering such a mechanism. Our method shows that, for May start dates following strong AMOC phases, summer SST hindcast skill over the subtropics increases significantly compared to that of weak AMOC phases. This suggests that in the assessment of SST skill for a season ahead an eye should be kept on the initial AMOC state.

scholarly journals Simultaneous Evolution of Gyre and Atlantic Meridional Overturning Circulation Anomalies as an Eigenmode of the North Atlantic System

2017 ◽  
Vol 30 (17) ◽  
pp. 6737-6755
Author(s):  
Bowen Zhao ◽  
Thomas Reichler ◽  
Courtenay Strong ◽  
Cecile Penland
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Half Cycle ◽  
Subpolar Gyre ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

The authors identify an interdecadal oscillatory mode of the North Atlantic atmosphere–ocean system in a general circulation model (GFDL CM2.1) via a linear inverse model (LIM). The oscillation mechanism is mostly embedded in the subpolar gyre: anomalous advection generates density anomalies in the eastern subpolar gyre, which propagate along the mean gyre circulation and reach the subpolar gyre center around 10 years later, when associated anomalous advection of the opposite sign starts the other half cycle. As density anomalies reach the Labrador Sea deep convection region, Atlantic meridional overturning circulation (AMOC) anomalies are also induced. Both the gyre and AMOC anomalies then propagate equatorward slowly, following the advection of density anomalies. The oscillation is further demonstrated to be more likely an ocean-only mode while excited by the atmospheric forcing; in particular, it can be approximated as a linearly driven damped oscillator that is partly excited by the North Atlantic Oscillation (NAO). The slowly evolving interdecadal oscillation significantly improves and prolongs the LIM’s prediction skill of sea surface temperature (SST) evolution.

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