The global atmospheric circulation in moist isentropic coordinates

This paper compares the zonal mean global circulation using two different isentropic coordinates: liquid water entropy, which is closely related to the potential temperature; and moist entropy which is closely related to the equivalent potential temperature. By combining these two analyses, we show that in the midlatitudes, there is a large poleward mass transport of warm, moist air near the Earth's surface, which accounts for a significant portion of the total global atmospheric circulation. The air parcels in this flow have a high entropy and are expected to rise into the upper troposphere as they move further poleward.

NCEP Reanalysis data is used to obtain the zonal mean circulation using both the liquid and moist entropy as vertical coordinates. The zonal mean circulation in liquid water entropy coordinates is similar to that obtained by Held and Schneider (1999) using potential temperature, while the circulation in moist entropy coordinates is similar to the results presented by Czaja and Marshal (2006) for the circulation in equivalent potential temperature. Both analyses show a global overturning circulation with high entropy air flowing from the equatorial regions to the Poles, and a return flow at lower entropy. The most significant difference lies in that, in the midlatitudes, the total mass transport in moist isentropic coordinates is approximately three times larger than the transport in liquid water coordinates. The more intense circulation in moist isentropic coordinates can be partially explained by the fact that the difference in moist entropy between the poleward and equatorward flow is much smaller than the difference in liquid water entropy. Hence, a similar total entropy transport would require a larger mass transport when measured in moist isentropic coordinates.

The difference between liquid water entropy and moist entropy is very small in the upper troposphere where water content is small. This implies that the enhanced circulation in moist isentropic coordinates corresponds to an additional poleward flow in the lower troposphere. This is confirmed by analyzing the meriodonal mass transport in terms of its joint distribution of liquid water entropy and moist entropy. The main finding here is the presence of a large poleward flow of air with high moist entropy (or equivalent potential temperature) and low liquid water entropy (or potential temperature). The moist entropy corresponds to that of the tropopause level measured a few degrees poleward, and these air parcels are expected to rise to the upper tropopause in the midlatitude regions. In contrast, the liquid water entropy corresponds to near surface values, indicating that this poleward flow takes place near the Earth's surface. This also explains the vanishing of this poleward flow when the circulation is averaged on potential temperature surfaces. The poleward mass transport in this low level flow of warm moist air is comparable to the upper tropospheric branch of the overturning circulation. This leads to the conclusion that a large fraction of the upper tropospheric air at high latitudes must have risen in the stormtracks rather than in the equatorial regions, and that local release of latent heat plays a key role in maintaining the stratification of the midlatitudes.

References:

Czaja, A. and J. Marshal, 2006: The Partitioning of Poleward Heat Transport between the Atmosphere and Ocean. 63, 1498–1511.

Held, I. M. and T. Schneider, 1999: The surface branch of the zonally averaged mass transport circulation in the troposphere. J. Atmos. Sci., 56, 1688–1697.