scholarly journals Can sparse proxy data constrain the strength of the Atlantic meridional overturning circulation?

2014 ◽  
Vol 7 (1) ◽  
pp. 419-432 ◽  
Author(s):  
T. Kurahashi-Nakamura ◽  
M. Losch ◽  
A. Paul
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Atlantic Meridional Overturning Circulation ◽  
Temperature Data ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Proxy Data ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Surface Temperature Data

Abstract. In a feasibility study, the potential of proxy data for the temperature and salinity during the Last Glacial Maximum (LGM, about 19 000 to 23 000 years before present) in constraining the strength of the Atlantic meridional overturning circulation (AMOC) with a general ocean circulation model was explored. The proxy data were simulated by drawing data from four different model simulations at the ocean sediment core locations of the Multiproxy Approach for the Reconstruction of the Glacial Ocean surface (MARGO) project, and perturbing these data with realistic noise estimates. The results suggest that our method has the potential to provide estimates of the past strength of the AMOC even from sparse data, but in general, paleo-sea-surface temperature data without additional prior knowledge about the ocean state during the LGM is not adequate to constrain the model. On the one hand, additional data in the deep-ocean and salinity data are shown to be highly important in estimating the LGM circulation. On the other hand, increasing the amount of surface data alone does not appear to be enough for better estimates. Finally, better initial guesses to start the state estimation procedure would greatly improve the performance of the method. Indeed, with a sufficiently good first guess, just the sea-surface temperature data from the MARGO project promise to be sufficient for reliable estimates of the strength of the AMOC.

scholarly journals Can sparse proxy data constrain the strength of the Atlantic meridional overturning circulation?

2013 ◽  
Vol 6 (3) ◽  
pp. 4417-4445
Author(s):  
T. Kurahashi-Nakamura ◽  
M. Losch ◽  
A. Paul
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Atlantic Meridional Overturning Circulation ◽  
Temperature Data ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Proxy Data ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Surface Temperature Data

Abstract. In a feasibility study, the potential of proxy data for the temperature and salinity during the Last Glacial Maximum (LGM, about 19 000 to 23 000 yr before present) in constraining the strength of the Atlantic meridional overturning circulation (AMOC) in a general ocean circulation model was explored. The proxy data were simulated by drawing data from four different model simulations at the ocean sediment core locations of the Multiproxy Approach for the Reconstruction of the Glacial Ocean surface (MARGO) project and perturbing these data with realistic noise estimates. The results suggest that our method has the potential of providing estimates of the past strength of the AMOC even from sparse data, but in general paleo-sea surface temperature data without additional prior knowledge about the ocean state during the LGM is not adequate to constrain the model. On the one hand, additional salinity data in the deep ocean and at the sea surface are shown to be highly important in estimating the LGM circulation and as expected, reducing proxy-data errors improves the solutions. Whereas increasing the amount of surface data alone does not appear to be enough for better estimates. Finally, better initial guesses to start the state estimation procedure from greatly improve the performance of the method. Indeed, with a sufficiently good first guess, just the sea-surface temperature data from the MARGO project appear sufficient for reliable estimates of the strength of the AMOC.

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 Effect of Atlantic Meridional Overturning Circulation Changes on Tropical Atlantic Sea Surface Temperature Variability: A 2½-Layer Reduced-Gravity Ocean Model Study

2010 ◽  
Vol 23 (2) ◽  
pp. 312-332 ◽  
Author(s):  
Caihong Wen ◽  
Ping Chang ◽  
Ramalingam Saravanan
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Atlantic Meridional Overturning Circulation ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Tropical Atlantic ◽  
Subsurface Temperature ◽  
The North ◽  
Meridional Overturning

Abstract Previous coupled climate model simulations reveal that a dipole-like SST pattern with cooler (warmer) temperature over the north (south) tropical Atlantic emerges in response to a slowdown of the Atlantic meridional overturning circulation (AMOC). Using a 2½-layer reduced-gravity ocean model, a systematic investigation into oceanic processes controlling the tropical Atlantic sea surface temperature (SST) response to AMOC changes by varying the strength of northward mass transport at the open boundaries was conducted. It is found that the North Brazil Current (NBC) reverses its direction in response to a shutdown of the AMOC. Such a circulation change causes a decrease in upper equatorial ocean stratification and warming in the Gulf of Guinea and off the coast of Africa. These findings point to the importance of oceanic dynamics in the equatorial SST response to AMOC changes. Sensitivity experiments further show that the SST response relates nonlinearly to AMOC changes. The strength of the SST response increases dramatically when the AMOC strength falls below a threshold value. This nonlinear threshold behavior depends on the position of a subsurface temperature gradient forming along the boundary between the northern subtropical gyre and the tropical gyre that interacts with the western boundary current. The analysis suggests that, in order for the oceanic dynamics to have a dominant influence on tropical Atlantic SST in response to AMOC changes, two conditions must be satisfied: 1) the AMOC must weaken substantially so that the NBC flows equatorward, permitting water mass exchange between the northern subtropical and tropical gyres, and 2) the subsurface temperature front must be located in an optimal location where subsurface temperature anomalies induced by AMOC change are able to enter the equatorial zone.

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.

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