Estimating Oceanic Heat Content Change Using Isotherms

Abstract This paper presents a new analysis of ocean heat content changes over the last 50 yr using isotherms by calculating the mean temperature above the 14°C isotherm and the depth of the 14°C isotherm as separate variables. A new quantity called the “relative heat content” (“RHC”) is introduced, which represents the minimum local heat content change over time, relative to a fixed isotherm. It is shown how mean temperature and isotherm depth changes make separable and additive contributions to changes in RHC. Maps of RHC change between 1970 and 2000 show similar spatial patterns to a traditional fixed-depth ocean heat content change to 220 m. However, the separate contributions to RHC suggest a more spatially uniform contribution from warming above the isotherm, while isotherm depth changes show wind-driven signals, of which some are identifiable as being related to the North Atlantic Oscillation. The time series show that the warming contribution to RHC dominates the global trend, while the depth contribution only dominates on the basin scale in the North Atlantic. The RHC shows minima associated with the major volcanic eruptions (particularly in the Indian Ocean), and these are entirely contributed by mean temperature changes rather than isotherm depth changes. The depth change contributions to RHC are strongly affected by the recently reported XBT fall-rate bias, whereas the mean temperature contributions are not. Therefore, only the isotherm depth change contributions to RHC will need to be reassessed as fall-rate-corrected data become available.

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On the North Atlantic Ocean Heat Content Change between 1955–70 and 1980–95

Journal of Climate ◽

10.1175/jcli-d-11-00187.1 ◽

2012 ◽

Vol 25 (10) ◽

pp. 3619-3628 ◽

Cited By ~ 14

Author(s):

Xiaoming Zhai ◽

Luke Sheldon

Keyword(s):

Atlantic Ocean ◽

North Atlantic ◽

Gulf Stream ◽

Large Scale ◽

North Atlantic Ocean ◽

Heat Content ◽

Ocean Heat Content ◽

The North ◽

Content Change ◽

The North Atlantic

Abstract The upper-ocean heat content of the North Atlantic has undergone significant changes over the last 50 years but the underlying physical mechanisms are not yet well understood. In the present study, the authors examine the North Atlantic ocean heat content change in the upper 700 m between the 1955–70 and 1980–95 periods. Consistent with previous studies, the large-scale pattern consists of warming of the tropics and subtropics and cooling of the subpolar ocean. However, this study finds that the most significant heat content change in the North Atlantic during these two time periods is the warming of the Gulf Stream region. Numerical experiments strongly suggest that this warming in the Gulf Stream region is largely driven by changes of the large-scale wind forcing. Furthermore, the increased ocean heat content in the Gulf Stream region appears to feedback on to the atmosphere, resulting in warmer surface air temperature and enhanced precipitation there.

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The Rapid Warming of the North Atlantic Ocean in the Mid-1990s in an Eddy-Permitting Ocean Reanalysis (1982–2013)

Journal of Climate ◽

10.1175/jcli-d-15-0438.1 ◽

2016 ◽

Vol 29 (15) ◽

pp. 5417-5430 ◽

Cited By ~ 5

Author(s):

Chunxue Yang ◽

Simona Masina ◽

Alessio Bellucci ◽

Andrea Storto

Keyword(s):

Atlantic Ocean ◽

Heat Transport ◽

North Atlantic ◽

North Atlantic Ocean ◽

Ocean Reanalysis ◽

The North ◽

Mean Temperature ◽

The Mean ◽

The North Atlantic ◽

Mean State

Abstract The rapid warming in the mid-1990s in the North Atlantic Ocean is investigated by means of an eddy-permitting ocean reanalysis. Both the mean state and variability, including the mid-1990s warming event, are well captured by the reanalysis. An ocean heat budget applied to the subpolar gyre (SPG) region (50°–66°N, 60°–10°W) shows that the 1995–99 rapid warming is primarily dictated by changes in the heat transport convergence term while the surface heat fluxes appear to play a minor role. The mean negative temperature increment suggests a warm bias in the model and data assimilation corrects the mean state of the model, but it is not crucial to reconstruct the time variability of the upper-ocean temperature. The decomposition of the heat transport across the southern edge of the SPG into time-mean and time-varying components shows that the SPG warming is mainly associated with both the anomalous advection of mean temperature and the mean advection of temperature anomalies across the 50°N zonal section. The relative contributions of the Atlantic meridional overturning circulation (AMOC) and gyre circulation to the heat transport are also analyzed. It is shown that both the overturning and gyre components are relevant to the mid-1990s warming. In particular, the fast adjustment of the barotropic circulation response to the NAO drives the anomalous transport of mean temperature at the subtropical/subpolar boundary, while the slowly evolving AMOC feeds the large-scale advection of thermal anomalies across 50°N. The persistently positive phase of the NAO during the years prior to the rapid warming likely favored the cross-gyre heat transfer and the following SPG warming.

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

10.5194/egusphere-egu2020-5575 ◽

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

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.&#160;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. 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.&#160;&#160;

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

Atmosphere ◽

10.3390/atmos11010003 ◽

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.

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Atlantic meridional ocean heat transport at 26° N: impact on subtropical ocean heat content variability

Ocean Science ◽

10.5194/os-9-1057-2013 ◽

2013 ◽

Vol 9 (6) ◽

pp. 1057-1069 ◽

Cited By ~ 10

Author(s):

M. Sonnewald ◽

J. J.-M. Hirschi ◽

R. Marsh ◽

E. L. McDonagh ◽

B. A. King

Keyword(s):

Heat Transport ◽

North Atlantic ◽

Heat Content ◽

Ocean Heat Content ◽

Box Model ◽

Ocean Heat Transport ◽

Overturning Circulation ◽

The North ◽

Meridional Overturning ◽

The North Atlantic

Abstract. Local climate is significantly affected by changes in the oceanic heat content on a range of timescales. This variability is driven by heat fluxes from both the atmosphere and the ocean. In the Atlantic the meridional overturning circulation is the main contributor to the oceanic meridional heat transport for latitudes south of about 50° N. The RAPID project has been successfully monitoring the Atlantic meridional overturning at 26° N since 2004. This study demonstrates how these data can be used to estimate the variability of the basin-wide ocean heat content in the upper 800 m between 26° and 36° N. Traditionally the atmosphere is seen to dominate the ocean heat content variability. However, previous studies have looked at smaller areas in the Gulf Stream region, finding that the ocean dominates deseasoned fluctuations of ocean heat content, while studies of the whole North Atlantic region suggest that the atmosphere may be dominant. In our study we use a box model to investigate fluctuations of the ocean heat content in the subtropical North Atlantic between 26° and 36° N. The box model approach is validated using 19 yr of high-resolution general circulation model (GCM) data. We find that in both the GCM- and RAPID-based data the ocean heat transport dominates the deseasoned heat content variability, while the atmosphere's impact on the ocean heat content evolution stabilizes after 6 months. We demonstrate that the utility of the RAPID data goes beyond monitoring the overturning circulation at 26° N, and that it can be used to better understand the causes of ocean heat content variability in the North Atlantic. We illustrate this for a recent decrease in ocean heat content which was observed in the North Atlantic in 2009 and 2010. Our results suggest that most of this ocean heat content reduction can be explained by a reduction of the meridional ocean heat transport during this period.

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

10.5194/egusphere-egu21-3077 ◽

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

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. We would like to acknowledge the financial support from FCT through projects&#160;FCT-UIDB/50019/2020&#160;and&#160;PD/BD/142785/2018.

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