Contrasting internally and externally generated Atlantic Multidecadal Variability and the role for AMOC in CMIP6

Jon Robson¹, Rowan Sutton¹, Matthew B Menary²

¹Department of Meteorology, National Centre for Atmospheric Science, University of Reading, Reading, UK.

Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences

|October 22, 2023

Summary

This article examines whether long-term temperature fluctuations in the North Atlantic are driven by internal ocean currents or external factors like human-induced climate change. By analyzing climate model simulations, the authors show that internal variability is tied to ocean circulation, while external forcing primarily affects surface heat exchange. The findings suggest that ocean dynamics remain a primary driver of observed climate patterns, though models still require improvements to fully capture these complex interactions.

Area of Science:

Climate dynamics within atmospheric science
Oceanography and Atlantic Multidecadal Variability research

Background:

Uncertainty persists regarding the primary drivers of long-term North Atlantic temperature fluctuations. Prior research has shown that these shifts might stem from internal ocean processes or external climate forcing. That ambiguity drove the need for a comprehensive assessment of current climate models. No prior work had resolved how these distinct mechanisms manifest in the latest generation of simulations. This gap motivated a detailed look at the Coupled Model Intercomparison Project phase 6 data. Scientists have long debated whether ocean circulation or surface heat fluxes dominate these patterns. Previous studies often struggled to separate these overlapping signals effectively. This paper addresses these conflicting perspectives by evaluating the multivariate signatures of climate variability.

Purpose Of The Study:

The aim of this article is to evaluate the drivers of long-term North Atlantic temperature fluctuations. Researchers seek to determine whether these patterns arise from internal ocean circulation or external climate forcing. This study addresses the ongoing debate regarding the role of the Atlantic Meridional Overturning Circulation. The authors investigate how different forcing mechanisms manifest in modern climate simulations. They intend to clarify why previous hypotheses have yielded conflicting results in the scientific literature. By assessing the sixth coupled model intercomparison project, the team provides a updated perspective on current model capabilities. This work aims to isolate the specific processes that govern internal versus external climate signals. The study ultimately seeks to reconcile model outputs with observed historical data.

Main Methods:

The review approach involves a systematic evaluation of historical simulations from the sixth coupled model intercomparison project. Investigators isolate distinct processes by examining the multivariate expression of climate signals. This strategy enables the separation of internally generated variability from externally forced components. The team compares these model outputs against established historical observations to test for consistency. They utilize specific indices to determine how sensitive the importance of external forcing remains. By focusing on surface-flux mechanisms, the authors identify how external factors influence the climate system. This methodology provides a framework for contrasting different drivers of long-term temperature fluctuations. The analysis synthesizes existing data to clarify the role of ocean dynamics in these complex systems.

Main Results:

The strongest finding indicates that internal variability in climate models is consistent with a significant role for ocean circulation. The researchers report that externally forced variability functions largely as a surface-flux mechanism with little ocean involvement. They observe that the internal multivariate fingerprint closely resembles historical climate records. In contrast, the externally forced fingerprint appears inconsistent with observed data. The importance of external forcing shows high sensitivity to the specific index chosen for the analysis. This sensitivity arises from the presence of globally coherent signals within the model simulations. The authors demonstrate that these distinct processes can be effectively isolated through multivariate exploration. Their findings suggest that ocean dynamics remain a primary driver of observed climate patterns in the North Atlantic.

Conclusions:

The authors propose that internal climate variability is closely linked to ocean circulation patterns. They suggest that external forcing operates mainly through surface heat exchange rather than deep ocean dynamics. The researchers note that the internal multivariate signature aligns well with historical observations. Conversely, the externally forced signature appears to conflict with observed climate data. These results imply that ocean dynamics remain a primary influence on observed temperature shifts. The team highlights that current models still exhibit notable deficiencies in representing these complex interactions. They caution that a stronger influence from externally forced dynamical changes cannot be entirely excluded. This synthesis underscores the ongoing challenge of distinguishing between natural and human-induced climate signals.

Keywords:

AMOC AMV Atlantic Ocean multidecadal variability ocean circulation climate forcing North Atlantic temperature CMIP6 simulations

Frequently Asked Questions

The researchers propose that internal variability relies on ocean circulation, specifically the Atlantic Meridional Overturning Circulation. In contrast, externally forced variability functions primarily as a surface-flux mechanism with minimal involvement from deep ocean processes.

The authors utilize the Coupled Model Intercomparison Project phase 6, or CMIP6, historical simulations to evaluate climate model performance. This dataset allows for the isolation of multivariate expressions of climate variability that were previously difficult to distinguish.

The researchers indicate that ocean dynamics are necessary to replicate observed internal variability patterns. Without accounting for these specific circulation processes, models fail to match the multivariate fingerprint seen in historical climate records.

The authors use multivariate fingerprints to differentiate between internal and external signals. This approach allows them to map how different forcing agents manifest in temperature and heat flux data across the North Atlantic region.

The team measures the consistency between model-generated fingerprints and observed climate data. They observe that internal model signatures match historical records, whereas externally forced signatures show significant inconsistencies with those same observations.

The researchers propose that while ocean dynamics remain central, current models are still deficient in several areas. They suggest that future work must address these limitations to rule out potential contributions from externally forced dynamical changes.

Related Concept Videos