The climate system as a whole can be viewed as a highly complex thermal/heat engine, in which numerous processes continuously interact to transform heat into work and vice-versa. As any physical system, the climate system obeys the basic laws of thermodynamics, and we may therefore expect the tools of non-equilibrium thermodynamics to be particularly useful in describing and synthesising its properties. The main aim of this short course will be twofold. Part 1 will provide an advanced introduction to the fundamentals of equilibrium and non-equilibrium thermodynamics, irreversible processes and energetics of multicomponent stratified fluids. Part 2 will illustrate the usefulness of this viewpoint to summarize the main features of the climate system in terms of thermodynamic cycles, as well as a diagnostic tool to constrain the behaviour of climate models. Although the aim is for this to be a self-contained module, some basic knowledge of the subject would be beneficial to the participants. Registration is not needed, but indication of interest would be helpful for planning purposes.

Part 1 (2 hours) will have the following learning objectives:
• Equilibrium thermodynamics, master thermodynamic potentials, partial thermodynamic properties
• Interdependence of energy conservation and irreversible entropy production
• Mutually consistent definitions of heat and work in the atmosphere and oceans
• Convexity of the internal energy and the concept of exergy and available potential energy (APE). Local versus global theories of APE. Problems related to the definition and construction of reference states and of the ‘environment’.
• Standard and non-standard theories of irreversible processes. Are all irreversible processes necessarily dissipative? Irreversibility parameter.
• Non-equilibrium theory of sensible and latent heat fluxes at the air-sea interface, reversible and irreversible phase changes.
• Theories for the thermodynamic efficiency of the atmospheric and oceanic heat engines: APE versus entropy-based Carnot approaches. Does humidity really make the atmospheric heat engine less efficient? Maximum work versus maximum power.
• Exact partitions of potential energy into sign-definite components. Applications to exact mean/eddy partitions. Concepts of local baroclinic life cycle.

Part 2 (1 hour) will illustrate practical applications rooted in recent research and will cover topics such as:
• Means of energy exchange throughout the atmosphere and in the oceans
• Representation of irreversible processes in climate models.
• Importance of extratropical eddies in shaping the meridional energy transport, and how this links to the general circulation of the atmosphere
• Link to observations, consistency of current climate models with theory. Using theory to improve climate models in the future.