scholarly journals How Much Have Variations in the Meridional Overturning Circulation Contributed to Sea Surface Temperature Trends since 1850? A Study with the EC-Earth Global Climate Model

2014 ◽  
Vol 27 (16) ◽  
pp. 6343-6357 ◽  
Author(s):  
Torben Schmith ◽  
Shuting Yang ◽  
Emily Gleeson ◽  
Tido Semmler
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Radiative Forcing ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract The surface of the world’s oceans has been warming since the beginning of industrialization. In addition to this, multidecadal sea surface temperature (SST) variations of internal origin exist. Evidence suggests that the North Atlantic Ocean exhibits the strongest multidecadal SST variations and that these variations are connected to the overturning circulation. This work investigates the extent to which these internal multidecadal variations have contributed to enhancing or diminishing the trend induced by the external radiative forcing, globally and in the North Atlantic. A model study is carried out wherein the analyses of a long control simulation with constant radiative forcing at preindustrial level and of an ensemble of simulations with historical forcing from 1850 until 2005 are combined. First, it is noted that global SST trends calculated from the different historical simulations are similar, while there is a large disagreement between the North Atlantic SST trends. Then the control simulation is analyzed, where a relationship between SST anomalies and anomalies in the Atlantic meridional overturning circulation (AMOC) for multidecadal and longer time scales is identified. This relationship enables the extraction of the AMOC-related SST variability from each individual member of the ensemble of historical simulations and then the calculation of the SST trends with the AMOC-related variability excluded. For the global SST trends this causes only a little difference while SST trends with AMOC-related variability excluded for the North Atlantic show closer agreement than with the AMOC-related variability included. From this it is concluded that AMOC variability has contributed significantly to North Atlantic SST trends since the mid nineteenth century.

scholarly journals Predictability and Decadal Variability of the North Atlantic Ocean State Evaluated from a Realistic Ocean Model

2017 ◽  
Vol 30 (2) ◽  
pp. 477-498 ◽  
Author(s):  
Florian Sévellec ◽  
Alexey V. Fedorov
Keyword(s):  
Atlantic Ocean ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Decadal Variability ◽  
North Atlantic Ocean ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
The North ◽  
The North Atlantic

This study investigates the excitation of decadal variability and predictability of the ocean climate state in the North Atlantic. Specifically, initial linear optimal perturbations (LOPs) in temperature and salinity that vary with depth, longitude, and latitude are computed, and the maximum impact on the ocean of these perturbations is evaluated in a realistic ocean general circulation model. The computations of the LOPs involve a maximization procedure based on Lagrange multipliers in a nonautonomous context. To assess the impact of these perturbations four different measures of the North Atlantic Ocean state are used: meridional volume and heat transports (MVT and MHT) and spatially averaged sea surface temperature (SST) and ocean heat content (OHC). It is shown that these metrics are dramatically different with regard to predictability. Whereas OHC and SST can be efficiently modified only by basin-scale anomalies, MVT and MHT are also strongly affected by smaller-scale perturbations. This suggests that instantaneous or even annual-mean values of MVT and MHT are less predictable than SST and OHC. Only when averaged over several decades do the former two metrics have predictability comparable to the latter two, which highlights the need for long-term observations of the Atlantic meridional overturning circulation in order to accumulate climatically relevant data. This study also suggests that initial errors in ocean temperature of a few millikelvins, encompassing both the upper and deep ocean, can lead to ~0.1-K errors in the predictions of North Atlantic sea surface temperature on interannual time scales. This transient error growth peaks for SST and OHC after about 6 and 10 years, respectively, implying a potential predictability barrier.

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.

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 Impact of Agulhas Leakage on the Atlantic Overturning Circulation in the CCSM4

2014 ◽  
Vol 27 (1) ◽  
pp. 101-110 ◽  
Author(s):  
Wilbert Weijer ◽  
Erik van Sebille
Keyword(s):  
North Atlantic ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Climate System Model ◽  
The North ◽  
Salinity Variability ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
The Impact

Abstract The impact of Agulhas leakage variability on the strength of the Atlantic meridional overturning circulation (AMOC) in the Community Climate System Model, version 4 (CCSM4) is investigated. In this model an advective connection exists that transports salinity anomalies from the Agulhas region into the North Atlantic on decadal (30–40 yr) time scales. However, there is no identifiable impact of Agulhas leakage on the strength of the AMOC, suggesting that the salinity variations are too weak to significantly modify the stratification in the North Atlantic. It is argued that this study is inconclusive with respect to an impact of Agulhas leakage on the AMOC. Salinity biases leave the South Atlantic and Indian Oceans too homogeneous, in particular erasing the observed salinity front in the Agulhas retroflection region. Consequently, salinity variability in the southeastern South Atlantic is found to be much weaker than observed.

Sign in / Sign up

Export Citation Format

Share Document