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Image: Aiden Jönsson/MISU/Stockholm University

Name

Aiden Jönsson, PhD candidate
Department of Meteorology, Stockholm University, Sweden

Title

Clouds and Earth’s hemispheric albedo symmetry: How do clouds affect hemispheric contrasts in heat and energy flows?

Abstract

Earth's Northern and Southern Hemispheres (NH and SH, respectively) have significantly different properties: the NH has a higher concentration of bright land surface area and aerosol emissions than the SH, making the Earth's clear-sky albedo hemispherically asymmetric. However, satellite observations have shown that higher cloud amount and reflectivity in the SH exactly compensate for this, making Earth's planetary albedo hemispherically symmetric. A physical explanation for this symmetry has not yet been found, but because it would give constraints for global cloud cover and its features, discovery of one may be a powerful tool in predicting the behavior of clouds in a changing climate.

The first chapter of this thesis investigates the hemispheric albedo symmetry in observations, and finds that its variability primarily stems from the tropics. General circulation models (GCMs) exhibit a large spread in albedo asymmetry biases; comparing these with observations reveals that the extratropics control mean-state modeled albedo asymmetry.

The second chapter compares the evolution of albedo asymmetries in GCMs when forced with increased CO2 concentrations. Models agree on an initial asymmetry response due to Arctic warming and albedo reductions, but diverge thereafter, with some models recovering their pre-industrial asymmetry. Those that recover their asymmetry do so via SH extratropical cloud loss and thus have stronger positive cloud feedbacks, illustrating that an albedo symmetry-maintaining mechanism could have implications for climate sensitivity.

Sources of modeled albedo asymmetry biases are investigated in a single atmospheric GCM using a perturbed parameter ensemble in the third chapter. The most significant parameters to simulated albedo asymmetry are those controlling warm rain formation, turbulent dissipation, and sea salt aerosol emissions. Parameters controlling warm rain formation and turbulent dissipation primarily affect extratropical low cloud cover, and those affecting ice particle formation disproportionately affects SH midlatitude albedo. Parameter settings that reproduce the observed albedo symmetry tend towards more strongly positive shortwave cloud feedbacks.

The link between hemispheric asymmetries in clouds and large-scale circulation is investigated with idealized atmospheric GCM experiments in the fourth chapter. Introducing hemispheric asymmetry in ocean heat fluxes that emulate heat divergence (convergence) in the SH (NH) drives an atmospheric response that qualitatively reproduces the observed cloud distribution. We conclude that the hemispheric albedo symmetry is not possible without implicating surface forcing from ocean circulation and heat transport.

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