scholarly journals Multiple Equilibria as a Possible Mechanism for Decadal Variability in the North Atlantic Ocean

2015 ◽  
Vol 28 (22) ◽  
pp. 8907-8922 ◽  
Author(s):  
Andreas Born ◽  
Juliette Mignot ◽  
Thomas F. Stocker
Keyword(s):  
Climate Variability ◽  
Conceptual Model ◽  
North Atlantic ◽  
Multiple Equilibria ◽  
Decadal Variability ◽  
Heat Content ◽  
Decadal Climate Variability ◽  
The North ◽  
The North Atlantic ◽  
Decadal Climate

Abstract Decadal climate variability in the North Atlantic has received increased attention in recent years, because modeling results suggest predictability of heat content and circulation indices several years ahead. However, determining the applicability of these results in the real world is challenging because of an incomplete understanding of the underlying mechanisms. Here, the authors show that recent attempts to reconstruct the decadal variations in one of the dominant circulation systems of the region, the subpolar gyre (SPG), are not always consistent. A coherent picture is partly recovered by a simple conceptual model solely forced by reanalyzed surface air temperatures. This confirms that surface heat flux indeed plays a leading role for this type of variability, as has been suggested in previous studies. The results further suggest that large variations in the SPG correspond to the crossing of a bifurcation point that is predicted from idealized experiments and an analytical solution of the model used herein. Performance of this conceptual model is tested against a statistical stochastic model. Hysteresis and the existence of two stable modes of the SPG circulation shape its response to forcing by atmospheric temperatures. The identification of the essential dynamics and the reduction to a minimal model of SPG variability provide a quantifiable basis and a framework for future studies on decadal climate variability and predictability.

scholarly journals The Impact of North Atlantic–Arctic Multidecadal Variability on Northern Hemisphere Surface Air Temperature

2010 ◽  
Vol 23 (21) ◽  
pp. 5668-5677 ◽  
Author(s):  
Vladimir A. Semenov ◽  
Mojib Latif ◽  
Dietmar Dommenget ◽  
Noel S. Keenlyside ◽  
Alexander Strehz ◽  
...  
Keyword(s):  
Climate Variability ◽  
Northern Hemisphere ◽  
North Atlantic ◽  
The Arctic ◽  
Decadal Climate Variability ◽  
Multidecadal Variability ◽  
The North ◽  
Hemisphere Surface ◽  
The North Atlantic ◽  
Decadal Climate

Abstract The twentieth-century Northern Hemisphere surface climate exhibits a long-term warming trend largely caused by anthropogenic forcing, with natural decadal climate variability superimposed on it. This study addresses the possible origin and strength of internal decadal climate variability in the Northern Hemisphere during the recent decades. The authors present results from a set of climate model simulations that suggest natural internal multidecadal climate variability in the North Atlantic–Arctic sector could have considerably contributed to the Northern Hemisphere surface warming since 1980. Although covering only a few percent of the earth’s surface, the Arctic may have provided the largest share in this. It is hypothesized that a stronger meridional overturning circulation in the Atlantic and the associated increase in northward heat transport enhanced the heat loss from the ocean to the atmosphere in the North Atlantic region and especially in the North Atlantic portion of the Arctic because of anomalously strong sea ice melt. The model results stress the potential importance of natural internal multidecadal variability originating in the North Atlantic–Arctic sector in generating interdecadal climate changes, not only on a regional scale, but also possibly on a hemispheric and even a global scale.

scholarly journals The impacts of oceanic deep temperature perturbations in the North Atlantic on decadal climate variability and predictability

2017 ◽  
Vol 51 (5-6) ◽  
pp. 2341-2357 ◽  
Author(s):  
Agathe Germe ◽  
Florian Sévellec ◽  
Juliette Mignot ◽  
Alexey Fedorov ◽  
Sébastien Nguyen ◽  
...  
Keyword(s):  
Climate Variability ◽  
North Atlantic ◽  
Decadal Climate Variability ◽  
The North ◽  
The North Atlantic ◽  
Deep Temperature ◽  
Decadal Climate

scholarly journals The mean state and variability of the North Atlantic circulation: a perspective from ocean reanalyses

2020 ◽  
Author(s):  
Laura Jackson ◽  
Clotilde Dubois ◽  
Gael Forget ◽  
Keith Haines ◽  
Matt Harrison ◽  
...  
Keyword(s):  
North Atlantic ◽  
Decadal Variability ◽  
Heat Content ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
The North ◽  
Ocean Reanalyses ◽  
The Mean ◽  
The North Atlantic ◽  
Mean State

<p>The observational network around the North Atlantic has improved significantly over the last few decades with the advent of Argo and satellite observations, and the more recent efforts to monitor the Atlantic Meridional Overturning Circulation (AMOC) using arrays such as RAPID and OSNAP. These have shown decadal timescale changes across the North Atlantic including in heat content, heat transport and the circulation. </p><p>However there are still significant gaps in the observational coverage, and significant uncertainties around some observational products. Ocean reanalyses integrate the observations with a dynamically consistent ocean model and are potentially tools that can be used to understand the observed changes. However the suitability of the reanalyses for the task must also be assessed.<br>We use an ensemble of global ocean reanalyses in comparison with observations in order to examine the mean state and interannual-decadal variability of the North Atlantic ocean since 1993. We assess how well the reanalyses are able to capture different processes and whether any understanding can be inferred. In particular we look at ocean heat content, transports, the AMOC and gyre strengths, water masses and convection. </p><p> </p>

scholarly journals Analyzing the Influence of the North Atlantic Ocean Variability on the Atlantic Meridional Mode on Decadal Time Scales

Atmosphere ◽  
2019 ◽  
Vol 11 (1) ◽  
pp. 3
Author(s):  
Sandro F. Veiga ◽  
Emanuel Giarolla ◽  
Paulo Nobre ◽  
Carlos A. Nobre
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
Decadal Variability ◽  
North Atlantic Ocean ◽  
Heat Content ◽  
Ocean Heat Content ◽  
Causality Analysis ◽  
Ocean Variability ◽  
The North ◽  
The North Atlantic

Important features of the Atlantic meridional mode (AMM) are not fully understood. We still do not know what determines its dominant decadal variability or the complex physical processes that sustain it. Using reanalysis datasets, we investigated the influence of the North Atlantic Ocean variability on the dominant decadal periodicity that characterizes the AMM. Statistical analyses demonstrated that the correlation between the sea surface temperature decadal variability in the Atlantic Ocean and the AMM time series characterizes the Atlantic multidecadal oscillation (AMO). This corroborates previous studies that demonstrated that the AMO precedes the AMM. A causal inference with a newly developed rigorous and quantitative causality analysis indicates that the AMO causes the AMM. To further understand the influence of the subsurface ocean on the AMM, the relationship between the ocean heat content (0–300 m) decadal variability and AMM was analyzed. The results show that although there is a significant zero-lag correlation between the ocean heat content in some regions of the North Atlantic (south of Greenland and in the eastern part of the North Atlantic) and the AMM, their cause-effect relationship on decadal time scales is unlikely. By correlating the AMO with the ocean heat content (0–300 m) decadal variability, the former precedes the latter; however, the causality analysis shows that the ocean heat content variability drives the AMO, corroborating several studies that point out the dominant role of the ocean heat transport convergence on AMO.

scholarly journals Trends, variability and predictive skill of the ocean heat content in North Atlantic: An analysis with the EC-Earth3 model

2021 ◽  
Author(s):  
Teresa Carmo-Costa ◽  
Roberto Bilbao ◽  
Pablo Ortega ◽  
Ana Teles-Machado ◽  
Emanuel Dutra
Keyword(s):  
North Atlantic ◽  
Decadal Variability ◽  
Heat Content ◽  
Ocean Heat Content ◽  
Regional Warming ◽  
The North ◽  
Predictive Skill ◽  
External Forcings ◽  
Modeling Uncertainties ◽  
The North Atlantic

<p>As the global climate is warming, with important regional differences, there is a growing need to (i) better understand whether and how internal variability controls the regional warming trends, and (ii) to identify the regions in which both the trends and the superimposed interannual variability are predictable. In this study we investigate trends, variability and predictive skill of the upper ocean heat content in the North Atlantic basin. This region is a source of decadal variability, in which internal ocean processes can locally modulate the global warming trends and add additional prediction skill. The analysis is focused on the period 1970-2014, and combines the study of an ensemble of ocean reanalyses, with two sets of CMIP6 experiments performed with the Earth system model EC-Earth3: (i) a 10-member historical ensemble; and (ii) an initialized 10-member retrospective decadal prediction system. External forcings are found to be important for the development of the regional trends, but on their own are unable to reproduce the exact geographical pattern. Our results also show that not all regions in the North Atlantic are equally predictable, which is explained by different contributions of the forcings and internal climate variability. While high levels of predictive skill in regions like the Eastern Subpolar North Atlantic, or the Irminger and Iceland Seas are clearly enabled by initialization, with a negligible influence of the external forcings, skill in others areas like the Subtropical North Atlantic, or the Gulf Stream extension mostly comes from the externally forced trends. The Labrador Sea is a particular case where predictive skill has both an external and internal origin. Large observational and modeling uncertainties affect the Central Subpolar North Atlantic, the only region exhibiting a cooling during the study period, uncertainties that might explain its very poor predictive skill. <em>We</em><em><span> would like to acknowledge the financial support from FCT through projects </span></em><em><span>FCT-UIDB/50019/2020</span></em><em><span> and PD/BD/142785/2018.</span></em></p>

North Atlantic decadal variability in a coupled global model and relevance to observations

2020 ◽  
Author(s):  
Yochanan Kushnir ◽  
Dog Run (Donna) Lee ◽  
Mingfang Ting
Keyword(s):  
Climate Change ◽  
North Atlantic ◽  
Time Scale ◽  
Decadal Variability ◽  
North Atlantic Ocean ◽  
Heat Content ◽  
Decadal Climate Variability ◽  
North Atlantic Current ◽  
Decadal Time Scale ◽  
The North

<p>This study focuses on the decadal time scale variability of the North Atlantic Ocean-Atmosphere system. This time scale is relevant to preparedness and adaptation as society becomes increasingly threatened by the adverse impact of anthropogenic climate change. North Atlantic decadal climate variability has been related to interaction between the subpolar and subtropical gyre and manifested in persistent multi-year SST and heat content anomalies and shifts in the latitude of the Gulf Stream/North Atlantic Current (GS/NAC). We apply a space-time analysis to annual, North Atlantic, upper ocean heat content (OHC) anomalies from the National Center for Atmospheric Research (NCAR), Community Earth System Model (CESM) long pre-industrial control run. The analysis reveals decadal anomalies associated with two patterns: a dipole centered on the GS/NAC, in the western side of the Basin that oscillates quasi-regularly, reversing its sign every of 6 to 7 years. The second pattern is centered in the eastern side of the basin and lags the first by about 5 years, implying that heat is transported between the subtropical and subpolar gyres. Analysis of surface windstress anomalies connected with these OHC fluctuations implies that the latter are forced by stochastic atmospheric variability. Further analysis compares the model patterns with observations to determine their relevance and predictability and assesses their response to climate change.</p>

The North Atlantic Ocean as a Modulator of Vegetation Greening/Browning in the Northern High Latitudes?

2021 ◽  
Author(s):  
Leonard F. Borchert ◽  
Alexander J. Winkler
Keyword(s):  
Climate Variability ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Internal Variability ◽  
Dominant Mode ◽  
High Latitudes ◽  
Model Simulations ◽  
Internal Climate Variability ◽  
The North ◽  
The North Atlantic

<p>Vegetation in the northern high latitudes shows a characteristic pattern of persistent changes as documented by multi-decadal satellite observations. The prevailing explanation that these mainly increasing trends (greening) are a consequence of external CO<sub>2</sub> forcing, i.e., due to the ubiquitous effect of CO2-induced fertilization and/or warming of temperature-limited ecosystems, however does not explain why some areas also show decreasing trends of vegetation cover (browning). We propose here to consider the dominant mode of multi-decadal internal climate variability in the north Atlantic region, the Atlantic Multidecadal Variability (AMV), as the missing link in the explanation of greening and browning trend patterns in the northern high latitudes. Such a link would also imply potential for decadal predictions of ecosystem changes in the northern high latitudes.</p><p>An analysis of observational and reanalysis data sets for the period 1979-2019 shows that locations characterized by greening trends largely coincide with warming summer temperature and increasing precipitation. Wherever either cooling or decreasing precipitation occurs, browning trends are observed over this period. These precipitation and temperature patterns are significantly correlated with a North Atlantic sea surface temperature index that represents the AMV signal, indicating its role in modulating greening/browning trend patterns in the northern high latitudes.</p><p>Using two large ensembles of coupled Earth system model simulations (100 members of MPI-ESM-LR Grand Ensemble and 32 members of the IPSL-CM6A-LR Large Ensemble), we separate the greening/browning pattern caused by external CO<sub>2</sub> forcing from that caused by internal climate variability associated with the AMV. These sets of model simulations enable a clean separation of the externally forced signal from internal variability. While the greening and browning patterns in the simulations do not agree with observations in terms of magnitude and location, we find consistent internally generated greening/browning patterns in both models caused by changes in temperature and precipitation linked to the AMV signal. These greening/browning trend patterns are of the same magnitude as those caused by the external forcing alone. Our work therefore shows that internally-generated changes of vegetation in the northern lands, driven by AMV, are potentially as large as those caused by external CO<sub>2</sub> forcing. We thus argue that the observed pattern of greening/browning in the northern high latitudes could originate from the combined effect of rising CO<sub>2</sub> as well as the AMV.</p>

scholarly journals Bunker Cave stalagmites: an archive for central European Holocene climate variability

2012 ◽  
Vol 8 (3) ◽  
pp. 1687-1720 ◽  
Author(s):  
J. Fohlmeister ◽  
A. Schröder-Ritzrau ◽  
D. Scholz ◽  
C. Spötl ◽  
D. F. C. Riechelmann ◽  
...  
Keyword(s):  
Climate Variability ◽  
North Atlantic ◽  
Little Ice Age ◽  
Ice Age ◽  
Holocene Climate ◽  
Central European ◽  
The North ◽  
The North Atlantic ◽  
Meteoric Precipitation ◽  
Precipitation And Temperature

Abstract. Holocene climate was characterised by variability on multi-centennial to multi-decadal time scales. In central Europe, these fluctuations were most pronounced during winter. Here we present a new record of past winter climate variability for the last 10.8 ka based on four speleothems from Bunker Cave, Western Germany. Due to its central European location, the cave site is particularly well suited to record changes in precipitation and temperature in response to changes in the North Atlantic realm. We present high resolution records of δ18O, δ13C values and Mg/Ca ratios. We attribute changes in the Mg/Ca ratio to variations in the meteoric precipitation. The stable C isotope composition of the speleothems most likely reflects changes in vegetation and precipitation and variations in the δ18O signal are interpreted as variations in meteoric precipitation and temperature. We found cold and dry periods between 9 and 7 ka, 6.5 and 5.5 ka, 4 and 3 ka as well as between 0.7 to 0.2 ka. The proxy signals in our stalagmites compare well with other isotope records and, thus, seem representative for central European Holocene climate variability. The prominent 8.2 ka event and the Little Ice Age cold events are both recorded in the Bunker cave record. However, these events show a contrasting relationship between climate and δ18O, which is explained by different causes underlying the two climate anomalies. Whereas the Little Ice Age is attributed to a pronounced negative phase of the North Atlantic Oscillation, the 8.2 ka event was triggered by cooler conditions in the North Atlantic due to a slowdown of the Thermohaline Circulation.

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