The Arctic region shows a particularly high susceptibility to climate change, which historically manifests in an amplification of the near-surface warming in the Arctic relative to the global mean. This Arctic amplification (AA) has impacts on the climate system also beyond the northern polar regions, which highlights the importance to adequately represent it in numerical models. While state-of-the-art climate models widely agree on the presence of AA, they simulate a large spread in the magnitude of Arctic-amplified warming.
This thesis addresses the need to evaluate the performance of global climate models in projecting AA and its most important drivers. For the latter, the focus is on the three amplifying climate feedbacks (ACFs) that largely drive the meridional warming structure leading to AA. The ACFs include the sea-ice-albedo feedback (SIAF), the Planck feedback, and the lapse-rate feedback (LRF). These feedbacks arise from the relevant changes in Arctic sea ice, near-surface temperatures, and the deviation from the near-surface temperature change through the atmosphere, respectively.
In the thesis, two observational constraints are presented to narrow the range of climate models of the sixth Coupled Model Intercomparison Project (CMIP6) regarding their projection of AA and the ACFs in both past and future climate. While for the past, the models representation of near-surface processes can often be directly evaluated against observations, it is particularly the LRF that is difficult to constrain as it incorporates the entire atmospheric warming structure. As a consequence, the historical constraint focuses on the LRF, while the future constraint gives a prediction range for the evolution of AA and all three ACFs through the 21st century.
The main results are highlighted in the view of the changing atmospheric energy budget (AEB) of the Arctic under anthropogenic climate forcing. The AEB provides a framework to address Arctic climate change at large scales, and further helps to decide on the relevant aspects that provide appropriate metrics for constraining both AA and the ACFs. In other words, the perspective of a changing Arctic AEB highlights important alterations of the energetics under climate change, that further link to changes in climate aspects that partly explain the inter-model spread in simulated AA and the ACFs.
The main results of the cumulative thesis are formulated on the basis of three published research papers, papers I, II, and III.
Paper I addresses the Arctic AEB which is typically characterised by an equilibrium between net radiative cooling and advective heating, and mostly an absence of convection. This radiative-advective equilibrium (RAE) approximates well the energy budget and thermal structure of the Arctic atmosphere. The main outcome of paper I is that with continuous warming as simulated by CMIP6 models in an idealised setup, a deviation from the RAE increasingly develops, resulting from sea ice retreat and increased ocean-to-atmosphere heat fluxes. These changes are further concomitant with a depletion of the typical surface-based temperature inversion and a decrease in advective heating, which is byword for the convergence of atmospheric energy transport in the Arctic.
Since the RAE currently explains much of the basic thermal structure of the Arctic atmosphere, those changes have the potential to further mediate the LRF.
Paper II builds on paper I and evaluates the performance of climate models in representing the key aspects of the Arctic LRF in CMIP6 historical simulations that have the best estimates of the transient climate forcings during the observational period.
In particular it is found that CMIP6 models that realistically simulate both the lower thermal structure of the atmosphere and the poleward atmospheric energy transport are more trustworthy in informing about the LRF and how much it contributed to Arctic warming during the past few decades. The evaluation is based on observations of surface-based temperature inversions during the year-long Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, and atmospheric energy transport convergence computations from reanalyses.
Paper III expands the constraint approach of paper II and carries out an emergent constraint (EC) on future AA and the ACFs that further elaborates on the physical relationships between the constraining metrics and future climate projections. Previous work has highlighted that parts of the inter-model spread in simulated AA is explained through the spread in contemporaneous sea ice loss across climate models.
The thesis confirms this link by showing that CMIP6 models with a stronger climatological sea ice loss project a stronger AA in the future under the assumption of a high emission scenario.
By further linking the degree of future ice loss to the current-climate sea ice amount in CMIP6 models, paper III facilitates an EC on the future evolution of AA and the ACFs. In particular, models with a lower contemporary sea ice amount project a larger magnitude of AA by setting the stage for stronger climatological ice loss and near-surface warming, linking to the relevant ACFs.
From the corresponding prediction it is evident that AA is expected to continue at a warming rate that is more than twice or three times larger than global-mean warming. Furthermore, the three ACFs continue to contribute to Arctic warming, with the SIAF leading the warming contribution response.
Lastly, the consideration of statistically strong and physically plausible relationships across climate models makes the EC a valuable technique to constrain climate model simulations in conjunction with observations.
This thesis highlights the potential of combining the advantages of both presented constraints: Using multiple process-relevant aspects instead of one singular metric (paper II), but considering the mechanistic couplings between these metrics and the climate projection of interest (paper III) will improve our model-evaluation techniques and further help guiding the design of future climate simulations.:Summary of the dissertation
List of papers
Author’s contribution
Supervision statement
1 Introduction
2 Research focus
3 The Arctic atmospheric energy budget
3.1 The atmospheric column model
3.2 The annual atmospheric energy budget
4 Arctic amplification and climate feedbacks
4.1 Amplifying climate feedbacks
4.2 A comment on process coupling
5 Methods and data
5.1 Energy budget equations
5.2 Quantifying Arctic amplification and climate feedbacks
5.3 Climate model data
5.3.1 CMIP6 idealised simulations
5.3.2 CMIP6 historical simulations
5.3.3 CMIP6 ssp585 simulations
5.4 Observational constraints
5.4.1 Constraint on historical Arctic lapse-rate feedback
5.4.2 Constraint on future Arctic amplification and relevant climate feedbacks
6 Results
6.1 Paper I - Deviations from the Arctic radiative-advective equilibrium under anthropogenic climate change
6.2 Paper II - Constraining the Arctic lapse-rate feedback during past decades by contemporary observations
6.3 Paper III - Constraining future Arctic amplification and the relevant climate feedbacks based on the recent sea ice climatology
7 Summary and outlook
References
Lists
Acknowledgements
Appendix A: Paper I
Appendix B: Paper II
Appendix C: Paper III
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:91545 |
| Date | 21 May 2024 |
| Creators | Linke, Olivia |
| Contributors | Universität Leipzig |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/acceptedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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