The Atmospheric General Circulation in Thermodynamic Coordinates and the response to Anthropogenic Forcing

The global atmospheric general circulation is viewed from a purely thermodynamic perspective by defining a stream function in latent heat (LH) versus dry static energy (DSE) coordinates. In such coordinates, the tropical circulations (Hadley and Walker cells) and fluxes by midlatitude synoptic eddies combine to form a single thermodynamic circulation. The circulation thus offers a unified description of global fluxes of heat and moisture, and is therefore named the hydrothermal circulation. Its amplitude and structure as well as how it may change during the 21st century with increased greenhouse-gas forcing is the subject of this study.

The hydrothermal circulation is found to have three branches. One is moist convection where LH is converted into DSE such that the stream lines approximately follow, and are bounded by, moist adiabats. This is followed by a branch of radiative damping where LH is virtually zero and DSE decreases. The last branch represents moistening and warming of near-surface air and is bounded by the Clausius-Clapeyron relationship.

In simulations of present-day (1980-2000) climate using the EC-Earth coupled climate model, the hydrothermal circulation has a maximum amplitude of -513 Sv. Using the same data, the meridional overturning circulation in moist isentropic coordinates, which represents meridional fluxes of heat and moisture, yields two hemisphere-wide cells with amplitudes 114 Sv and -187 Sv respectively. This suggests that only 114 + 187 = 301 Sv of the hydrothermal circulation can be explained by the meridional overturning. It is suggested that the remaining 212 Sv is explained by zonally oriented circulations of which the Walker circulation is shown to be the most prominent.

Comparing simulations of present day (1980-2000) to simulations of the late 21st century (2080-2100) it is found that the EC-Earth model predicts a widening and weakening of the hydrothermal circulation under the RCP 4.5 and RCP 8.5 scenarios, with more pronounced changes in the RCP 8.5 scenario. The widening is due to an increase in near-surface DSE and LH such that the moist convective branch of the future circulation is bounded by a higher moist adiabat than at present day. While the bounding moist adiabat is about 340 kJ/kg in the simulations of 1980-2000 it increases to about 350 kJ/kg in 2080-2100 with RCP 8.5 forcing. A widening is also observed for the meridional overturning in moist isentropic coordinates. Moreover, the amplitude of the hydrothermal circulation averaged over the years 2080-2100 is -466 Sv with RCP 8.5 forcing, thus a decrease from the -513 Sv in 1980-2000. However, the amplitudes of the meridional overturning cells in moist isentropic coordinates only change slightly, indicating that most of the weakening occurs in the zonally oriented flows. This weakening is consistent with predictions of weakened Walker circulation due to anthropogenic forcing.