[DLS] Malte Jansen (UChicago)

“Energetics of Atmosphere and Ocean Circulations—From Icy Moons to Climate Change”

Earth’s atmospheric circulation is a heat engine. In transporting heat from the radiatively heated warm surface to the colder tropospheric regions, where that heat is radiatively lost to space, it performs work that drives the winds. The winds in turn play a major role in driving the ocean circulation, which itself is not a heat engine (on today’s Earth).  The atmospheric heat engine is therefore fundamental to our understanding of atmosphere and ocean circulations. Unfortunately, the specifics of the heat engine are complex and involve hard-to-predict microscale processes, which somewhat limits the usefulness of this framework as a predictive theory. I will therefore here focus on a closely related but somewhat simplified approach, where the work performed by the heat engine is approximated in terms of the upward buoyancy flux. Kinetic energy is generated if light (warm) fluid is transported upward while dense (cold) fluid is transported downward; and, in a statistical steady state, that kinetic energy generation has to balance the dissipation. I will argue that this simple (known but under-appreciated) relationship provides important constraints for atmospheric and oceanic circulations. I will first apply it to infer kinetic energy dissipation (and related flow properties) for ice-covered oceans, including a potential “Snowball Earth” ocean as well as the oceans of icy moons, whose circulations are otherwise very poorly constrained. I will then discuss how the framework can be extended to Earth’s atmosphere to infer the global kinetic energy dissipation from two quantities: (1) the twice vertically integrated atmospheric radiative cooling profile and (2) a bulk Bowen ratio, measuring the ratio of sensible to latent vertical heat flux. I will also discuss how these two quantities are expected to change across a wide range of climates. For a “relatively” modest warming (as expected for the 21st century) we expect a strong cancellation between the effects of changes in the radiative cooling rate (which increases) and the bulk Bowen ratio (which decreases), leading to relatively little change in the global wind energy dissipation—broadly consistent with climate model simulations. However, this cancellation is not expected to hold over a wider range of climates. Instead we predict much weaker wind energy dissipation in much colder climates, and potentially stronger energy dissipation in much warmer climates.

EAPS Department Lecture Series
Weekly talks aimed to bring together the entire EAPS community, given by leading thinkers in the areas of geology, geophysics, geobiology, geochemistry, atmospheric science, oceanography, climatology, and planetary science. Runs concurrently with class 12.S501.

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