The heat transport of the hydrological cycle: insights from heat tags

Abstract:

Traditionally, the poleward transport of column moist static energy (MSE) is partitioned into contributions from dry static energy (DSE) and moisture (in energy units). This leads to an apparent, and confusing, competition between dry and moist processes: in the tropical Hadley circulation, a large divergence of DSE out of the tropics is largely compensated by convergence of moisture into the tropics, such that their sum leads to a residual of MSE transport out of the tropics. But, of course, the moisture that has converged into the tropics drives latent heating, which provides a source of the same DSE that is exported out of the tropics. This exported, supposedly ‘dry’ heat, is then taken up by eddy driven circulations without a precise sense of its origin through latent heat release. This confusion is also apparent in the competition between increased moisture transport and decreased dry static energy transport under greenhouse warming.

To clarify the contribution of moist processes to poleward transport, we use “heat tags”, which are passive tracers, numerically implemented in a GCM, that record the separate heat content contributions from different diabatic processes. Using heat tags in an idealized moist aquaplanet model, we construct a new partitioning of poleward heat transport into moist contributions to MSE, from water vapor, subgrid-scale convective heating, and resolved large-scale condensational heating; and dry contributions to MSE, from vertical diffusion (including surface sensible heating) and radiative heating. The moist terms, which represent the energy put into the atmosphere through evaporation, capture the poleward heat transport of the hydrological cycle.

This revised partitioning removes the apparent competition between the DSE and moisture transport, revealing a single signed poleward transport of moist process tags that reflects the overall flow of MSE out of the tropics. From the midlatitudes up to the polar regions, MSE transport is dominated by poleward eddy transport of the sum of the moist tags. In response to greenhouse warming, while DSE and moisture transport increase in magnitude with opposite signs, the moist tag transport response largely consists of a simple increase of moist tag energy transport from the tropics to the midlatitudes.