Diabatic jet and storm track feedbacks

Thesis

Condensation and freezing of water releases latent heat that influences the atmospheric circulation across a wide range of scales and through multiple mechanisms. On all scales, the precise location of latent heating is crucial in determining its effects. As the atmosphere warms, latent heating is expected to intensify, which makes it increasingly important to understand its role in the climate system. Yet we lack a theory that reconciles the many ways in which latent heating shapes the atmospheric circulation.

In this thesis, I take a step towards meeting this challenge by mapping the relationship between latent heating and atmospheric circulation across spatio-temporal scales. On synoptic scales, I identify an asymmetric dependence on meridional wind, with heating concentrated on the warm flank of the jet, energising the flow. In spatio-temporal averages, this asymmetry is smeared out, and the mean heating instead appears on the cold flank and therefore appears as an energy sink.

Diabatic heating enters the thermodynamic energy equation as a linear source term. It may therefore seem sufficient to diagnose the mean latent heating alone — as many studies have done — to assess the total effect of latent heating on the mean atmospheric circulation. However, because heating is itself coupled to the dynamics, it introduces a nonlinearity: latent heating depends on vertical velocity, which in turn depends on horizontal temperature advection, which is not accounted for when averaging.

Here, I use novel atmospheric general circulation model experiments to isolate the effects of coupled heating–dynamics interactions from those of climatological latent heating. I show that the nonlinear coupling between latent heating and the dynamics enhances climatic zonal asymmetries, increases the efficiency of eddy heat fluxes, intensifies the strongest storms by ∼ 20%, and sustains the peak intensity of the strongest storms despite baroclinicity weakening under global warming.

Authors: Auestad, HS

• DOI: 10.5287/ora-nob4bypxr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: atmopsheric dynamics; latent heat; diabatic; storm tracks; midlatitude
• Subjects: Physics; Climate; Fluid dynamics