scholarly journals Changes of decadal SST Variations in the subpolar North Atlantic under strong CO2 forcing as an indicator for the ocean circulation’s contribution to Atlantic Multidecadal Variability

2020 ◽  
Author(s):  
Ralf Hand ◽  
Jürgen Bader ◽  
Daniela Matei ◽  
Rohit Ghosch ◽  
Johann Jungclaus
Keyword(s):  
Heat Transport ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Ocean Dynamics ◽  
Atlantic Multidecadal Variability ◽  
Ocean Heat Transport ◽  
Multidecadal Variability ◽  
The North ◽  
Sst Variability ◽  
Co2 Forcing

<p>The question, whether ocean dynamics are relevant for basin-scale North Atlantic decadal temperature variability is subject of ongoing discussions. Here, we analyze a set of simulations with a single climate model, consisting of a 2000-year pre-industrial control experiment, a 100-member historical ensemble, and a 100-member ensemble forced with an incremental CO2 increase by 1%/year. Compared to previous approaches, our setup offers the following advantages: First, the large ensemble size allows to robustly separate internally and externally forced variability and to robustly detect statistical links between different quantities. Second, the availability of different scenarios allows to investigate the role of the background state for drivers of the<br>variability. We find strong evidence that ocean dynamics, particularly ocean heat transport variations, form an important contribution to generate the Atlantic Multidecadal Variability (AMV) in the Max Planck Institute Earth System Model (MPI- ESM). Particularly the Northwest North Atlantic is substantially affected by ocean circulation for the historical and pre-industrial simulations. Anomalies of the Labrador Sea deep ocean density precede a change of the Atlantic Meridional Overturning Circulation (AMOC) and heat advection to the region south of Greenland.<br>Under strong CO2 forcing the AMV-SST regression pattern shows crucial changes: SST variability in the north western part of the North Atlantic is strongly reduced, so that the AMV pattern in this scenario is dominated by the low-latitude branch. We found a connection to changes in the deep water formation, that cause a strong reduction of the mean AMOC and its variability. Consequently, ocean heat transport convergence becomes less important for the SST variability south of Greenland.</p>

scholarly journals Changes of Decadal SST Variations in the Subpolar North Atlantic under Strong CO2 Forcing as an Indicator for the Ocean Circulation’s Contribution to Atlantic Multidecadal Variability

2020 ◽  
Vol 33 (8) ◽  
pp. 3213-3228 ◽  
Author(s):  
Ralf Hand ◽  
Jürgen Bader ◽  
Daniela Matei ◽  
Rohit Ghosh ◽  
Johann H. Jungclaus
Keyword(s):  
Heat Transport ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Northwestern Part ◽  
Atlantic Multidecadal Variability ◽  
Ocean Heat Transport ◽  
Multidecadal Variability ◽  
Sst Variability ◽  
Basin Scale

AbstractThe question of whether ocean dynamics are relevant for basin-scale North Atlantic decadal temperature variability is the subject of ongoing discussions. Here, we analyze a set of simulations with a single climate model consisting of a 2000-yr preindustrial control experiment, a 100-member historical ensemble, and a 100-member ensemble forced with an incremental CO2 increase by 1% yr−1. Compared to previous approaches, our setup offers the following advantages: First, the large ensemble size allows us to robustly separate internally and externally forced variability and to robustly detect statistical links between different quantities. Second, the availability of different scenarios allows us to investigate the role of the background state for drivers of the variability. We find strong evidence that ocean dynamics, particularly ocean heat transport variations, form an important contribution to generate the Atlantic multidecadal variability (AMV) in the Max Planck Institute Earth System Model (MPI-ESM). Particularly the northwest North Atlantic is substantially affected by ocean circulation for the historical and preindustrial simulations. Anomalies of the Labrador Sea deep ocean density precede a change of the Atlantic meridional overturning circulation (AMOC) and heat advection to the region south of Greenland. Under strong CO2 forcing, the AMV–SST regression pattern shows crucial changes: SST variability in the northwestern part of the North Atlantic is strongly reduced, so that the AMV pattern in this scenario is dominated by the low-latitude branch. We found a connection to changes in the deep-water formation that cause a strong reduction of the mean AMOC and its variability. Consequently, ocean heat transport convergence becomes less important for the SST variability south of Greenland.

scholarly journals Impact of Anomalous Ocean Heat Transport on the North Atlantic Oscillation

2005 ◽  
Vol 18 (23) ◽  
pp. 4955-4969 ◽  
Author(s):  
Fabio D’Andrea ◽  
Arnaud Czaja ◽  
John Marshall
Keyword(s):  
North Atlantic Oscillation ◽  
Heat Transport ◽  
Time Scales ◽  
North Atlantic ◽  
Ocean Dynamics ◽  
Ocean Heat Transport ◽  
The North ◽  
The North Atlantic ◽  
Wind Driven Circulation ◽  
The North Atlantic Oscillation

Abstract Coupled atmosphere–ocean dynamics in the North Atlantic is studied by means of a simple model, featuring a baroclinic three-dimensional atmosphere coupled to a slab ocean. Anomalous oceanic heat transport due to wind-driven circulation is parameterized in terms of a delayed response to the change in wind stress curl due to the North Atlantic Oscillation (NAO). Climate variability for different strengths of ocean heat transport efficiency is analyzed. Two types of behavior are found depending on time scale. At interdecadal and longer time scales, a negative feedback is found that leads to a reduction in the spectral power of the NAO. By greatly increasing the efficiency of ocean heat transport, the NAO in the model can be made to completely vanish from the principal modes of variability at low frequency. This suggests that the observed NAO variability at these time scales must be due to mechanisms other than the interaction with wind-driven circulation. At decadal time scales, a coupled oscillation is found in which SST and geopotential height fields covary.

scholarly journals Atlantic Multidecadal Variability and the U.K. ACSIS Program

2018 ◽  
Vol 99 (2) ◽  
pp. 415-425 ◽  
Author(s):  
R. T. Sutton ◽  
G. D. McCarthy ◽  
J. Robson ◽  
B. Sinha ◽  
A. T. Archibald ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Heat Content ◽  
Climate Impacts ◽  
Environmental Research ◽  
Atlantic Multidecadal Variability ◽  
Multidecadal Variability ◽  
The North ◽  
Holistic Understanding ◽  
The North Atlantic

Abstract Atlantic multidecadal variability (AMV) is the term used to describe the pattern of variability in North Atlantic sea surface temperatures (SSTs) that is characterized by decades of basinwide warm or cool anomalies, relative to the global mean. AMV has been associated with numerous climate impacts in many regions of the world including decadal variations in temperature and rainfall patterns, hurricane activity, and sea level changes. Given its importance, understanding the physical processes that drive AMV and the extent to which its evolution is predictable is a key challenge in climate science. A leading hypothesis is that natural variations in ocean circulation control changes in ocean heat content and consequently AMV phases. However, this view has been challenged recently by claims that changing natural and anthropogenic radiative forcings are critical drivers of AMV. Others have argued that changes in ocean circulation are not required. Here, we review the leading hypotheses and mechanisms for AMV and discuss the key debates. In particular, we highlight the need for a holistic understanding of AMV. This perspective is a key motivation for a major new U.K. research program: the North Atlantic Climate System Integrated Study (ACSIS), which brings together seven of the United Kingdom’s leading environmental research institutes to enable a broad spectrum approach to the challenges of AMV. ACSIS will deliver the first fully integrated assessment of recent decadal changes in the North Atlantic, will investigate the attribution of these changes to their proximal and ultimate causes, and will assess the potential to predict future changes.

scholarly journals Contributions of Atmospheric Stochastic Forcing and Intrinsic Ocean Modes to North Atlantic Ocean Interdecadal Variability

2020 ◽  
Vol 33 (6) ◽  
pp. 2351-2370 ◽  
Author(s):  
Olivier Arzel ◽  
Thierry Huck
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
General Circulation ◽  
Thermal Structure ◽  
Ocean Dynamics ◽  
Dimensionless Number ◽  
Stochastic Forcing ◽  
The North ◽  
Wide Range ◽  
The North Atlantic

AbstractAtmospheric stochastic forcing associated with the North Atlantic Oscillation (NAO) and intrinsic ocean modes associated with the large-scale baroclinic instability of the North Atlantic Current (NAC) are recognized as two strong paradigms for the existence of the Atlantic multidecadal oscillation (AMO). The degree to which each of these factors contribute to the low-frequency variability of the North Atlantic is the central question in this paper. This issue is addressed here using an ocean general circulation model run under a wide range of background conditions extending from a supercritical regime where the oceanic variability spontaneously develops in the absence of any atmospheric noise forcing to a damped regime where the variability requires some noise to appear. The answer to the question is captured by a single dimensionless number Γ measuring the ratio between the oceanic and atmospheric contributions, as inferred from the buoyancy variance budget of the western subpolar region. Using this diagnostic, about two-thirds of the sea surface temperature (SST) variance in the damped regime is shown to originate from atmospheric stochastic forcing whereas heat content is dominated by internal ocean dynamics. Stochastic wind stress forcing is shown to substantially increase the role played by damped ocean modes in the variability. The thermal structure of the variability is shown to differ fundamentally between the supercritical and damped regimes, with abrupt modifications around the transition between the two regimes. Ocean circulation changes are further shown to be unimportant for setting the pattern of SST variability in the damped regime but are fundamental for a preferred time scale to emerge.

scholarly journals On the Interpretation of the North Atlantic Averaged Sea Surface Temperature

2020 ◽  
Vol 33 (14) ◽  
pp. 6025-6045
Author(s):  
Jing Sun ◽  
Mojib Latif ◽  
Wonsun Park ◽  
Taewook Park
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Climate Models ◽  
Ocean Dynamics ◽  
Sea Surface ◽  
The North ◽  
Sst Variability ◽  
The North Atlantic ◽  
Different Time Scales

AbstractThe North Atlantic (NA) basin-averaged sea surface temperature (NASST) is often used as an index to study climate variability in the NA sector. However, there is still some debate on what drives it. Based on observations and climate models, an analysis of the different influences on the NASST index and its low-pass filtered version, the Atlantic multidecadal oscillation (AMO) index, is provided. In particular, the relationships of the two indices with some of its mechanistic drivers including the Atlantic meridional overturning circulation (AMOC) are investigated. In observations, the NASST index accounts for significant SST variability over the tropical and subpolar NA. The NASST index is shown to lump together SST variability originating from different mechanisms operating on different time scales. The AMO index emphasizes the subpolar SST variability. In the climate models, the SST-anomaly pattern associated with the NASST index is similar. The AMO index, however, only represents pronounced SST variability over the extratropical NA, and this variability is significantly linked to the AMOC. There is a sensitivity of this linkage to the cold NA SST bias observed in many climate models. Models suffering from a large cold bias exhibit a relatively weak linkage between the AMOC and AMO and vice versa. Finally, the basin-averaged SST in its unfiltered form, which has been used to question a strong influence of ocean dynamics on NA SST variability, mixes together multiple types of variability occurring on different time scales and therefore underemphasizes the role of ocean dynamics in the multidecadal variability of NA SSTs.

scholarly journals Variable External Forcing Obscures the Weak Relationship between the NAO and North Atlantic Multidecadal SST Variability

2019 ◽  
Vol 32 (13) ◽  
pp. 3847-3864 ◽  
Author(s):  
Jeremy M. Klavans ◽  
Amy C. Clement ◽  
Mark A. Cane
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
External Forcing ◽  
Atlantic Multidecadal Variability ◽  
The North ◽  
Sst Variability ◽  
Atmospheric Noise ◽  
The North Atlantic Oscillation ◽  
The Relationship ◽  
Spurious Signals

Abstract North Atlantic sea surface temperatures (SST) exhibit a lagged response to the North Atlantic Oscillation (NAO) in both models and observations, which has previously been attributed to changes in ocean heat transport. Here we examine the lagged relationship between the NAO and Atlantic multidecadal variability (AMV) in the context of the two other major components of the AMV: atmospheric noise and external forcing. In preindustrial control runs, we generally find that after accounting for spurious signals introduced by filtering, the SST response to the NAO is only statistically significant in the subpolar gyre. Further, the lagged SST response to the NAO is small in magnitude and offers a limited contribution to the AMV pattern, statistics, or predictability. When climate models include variable external forcing, the relationship between the NAO and AMV is obscured and becomes inconsistent. In these historically forced runs, knowledge of the prior NAO offers reduced predictability. The differences between the preindustrial and the historically forced ensembles suggest that we do not yet have enough observational data to surmise the true NAO–AMV relationship and add evidence that external forcing plays a substantial role in producing the AMV.

scholarly journals The Multidecadal Atlantic SST—Sahel Rainfall Teleconnection in CMIP5 Simulations

2014 ◽  
Vol 27 (2) ◽  
pp. 784-806 ◽  
Author(s):  
Elinor R. Martin ◽  
Chris Thorncroft ◽  
Ben B. B. Booth
Keyword(s):  
North Atlantic ◽  
Rainfall Variability ◽  
Coupled Model ◽  
Multidecadal Variability ◽  
The North ◽  
Sst Variability ◽  
Tropical North Atlantic ◽  
Sahel Rainfall ◽  
Intercomparison Project ◽  
The Indian Ocean

Abstract This study uses models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to evaluate and investigate Sahel rainfall multidecadal variability and teleconnections with global sea surface temperatures (SSTs). Multidecadal variability is lower than observed in all historical simulations evaluated. Focus is on teleconnections with North Atlantic SST [Atlantic multidecadal variability (AMV)] as it is more successfully simulated than the Indian Ocean teleconnection. To investigate why some models successfully simulated this teleconnection and others did not, despite having similarly large AMV, two groups of models were selected. Models with large AMV were highlighted as good (or poor) by their ability to simulate relatively high (low) Sahel multidecadal variability and have significant (not significant) correlation between multidecadal Sahel rainfall and an AMV index. Poor models fail to capture the teleconnection between the AMV and Sahel rainfall because the spatial distribution of SST multidecadal variability across the North Atlantic is incorrect. A lack of SST signal in the tropical North Atlantic and Mediterranean reduces the interhemispheric SST gradient and, through circulation changes, the rainfall variability in the Sahel. This pattern was also evident in the control simulations, where SST and Sahel rainfall variability were significantly weaker than historical simulations. Errors in SST variability were suggested to result from a combination of weak wind–evaporation–SST feedbacks, poorly simulated cloud amounts and feedbacks in the stratocumulus regions of the eastern Atlantic, dust–SST–rainfall feedbacks, and sulfate aerosol interactions with clouds. By understanding the deficits and successes of CMIP5 historical simulations, future projections and decadal hindcasts can be examined with additional confidence.

scholarly journals The North Atlantic Warming Hole as Part of a Century-Long Fluctuating Phenomenon

2021 ◽  
Author(s):  
Arnold Taylor
Keyword(s):  
Global Warming ◽  
Heat Transport ◽  
Temperature Change ◽  
North Atlantic ◽  
Meridional Circulation ◽  
Ocean Heat Transport ◽  
The North ◽  
Published Evidence ◽  
North Atlantic Climate ◽  
The North Atlantic

Despite global warming, a region of the North Atlantic has been observed to cool, a phenomenon known as theáNorth Atlantic Warming Holeá(NAWH). The causes of the NAWH remain under debate but its emergence has been linked to a slowdown of the meridional circulation leading to a reduced ocean heat transport into the warming hole region. This note uses previously published evidence to suggest that the pattern of temperature change is not unique but may have been a recurring feature during the last century and a half, fluctuating between a positive and negative phase. It appears global warming has amplified one of these phases in the North Atlantic climate.

North Atlantic Ocean Circulation Response to Stochastic Mesoscale Weather Systems

2021 ◽  
Author(s):  
Shenjie Zhou ◽  
Xiaoming Zhai ◽  
Ian Renfrew
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Models ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Ensemble Prediction ◽  
Subpolar Gyre ◽  
Atmospheric Forcing ◽  
The North ◽  
The North Atlantic

<p>The ocean is forced by the atmosphere on a range of spatial and temporal scales. In ocean and climate models the resolution of the atmospheric forcing sets a limit on the scales that are represented. For typical climate models this means mesoscale (< 400 km) atmospheric forcing is absent. Previous studies have demonstrated that mesoscale forcing significantly affects key ocean circulation systems such as the North Atlantic Subpolar gyre and the Atlantic Meridional Overturning Circulation (AMOC). However, the approach of these studies has either been ad hoc or limited in resolution. Here we present ocean model simulations with and without realistic mesoscale atmospheric forcing that represents scales down to 10 km. We use a novel stochastic parameterization – based on a cellular automaton algorithm that is common in weather forecasting ensemble prediction systems<sup> </sup>– to represent spatially coherent weather systems over a range of scales, including down to the smallest resolvable by the ocean grid. The parameterization is calibrated spatially and temporally using marine wind observations. The addition of mesoscale atmospheric forcing leads to coherent patterns of change in the sea surface temperature and mixed-layer depth. It also leads to non-negligible changes in the volume transport in the North Atlantic subtropical gyre (STG) and subpolar gyre (SPG) and in the AMOC. A non-systematic basin-scale circulation response to the mesoscale wind perturbation emerges – an in-phase oscillation in northward heat transport across the gyre boundary, partly driven by the constantly enhanced STG, correspoding to an oscillatory behaviour in SPG and AMOC indices with a typical time scale of 5-year, revealing the importance of ocean dynamics in generating non-local ocean response to the stochastic mesoscale atmospheric forcing. Atmospheric convection-permitting regional climate simulations predict changes in the intensity and frequency of mesoscale weather systems this century, so representing these systems in coupled climate models could bring higher fidelity in future climate projections.</p>

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