The spatio-temporal spectrum of meridional and vertical water vapor transports in the tropics and subtropics

We investigate the spatio-temporal spectrum of meridional and vertical water vapor transports in the tropics and subtropics and their contribution to the general circulation using reanalysis data. The transports are divided into planetary scale (zonal wavenumbers 1-3), baroclinic scale (zonal wavenumbers 4-7), stationary and transient contributions. The analysis reveals significant poleward water vapor transport in the tropical and subtropical lower troposphere by slow westward propagating planetary scale waves during summer and winter in both hemispheres. There is also significant poleward water vapor transport by eastward propagating baroclinic scale waves in the lower troposphere, however the planetary-scale transport penetrates deeper into the tropics (by approximately 10 degrees). The meridional latent heat transports associated with the water vapor transports are significantly stronger and extend further equatorward than the sensible heat transports, which are dominated by eastward propagating baroclinic scale waves. The meridional water vapor transports by the planetary and baroclinic scales waves coincide with significant vertical water vapor transports from the surface to the upper troposphere. As for the meridional latent heat transports, the vertical latent heat transports are much stronger and extend further equatorward than the vertical sensible heat transports. During winter the vertical sensible heat transports and the latent heat transports have opposite signs.

The contribution of the various water vapor transports to the atmospheric general circulation on moist isentropes is subsequently assessed using the statistical transformed Eulerian mean (STEM) formulation developed by Pauluis, Shaw and Laliberte (2011). The STEM formulation represents a generalization of the transformed Eulerian mean (TEM) formulation in that it uses both the mean value and the eddy-induced variance of a scalar when converting the eddy-induced meridional transport of the scalar into a meridional circulation. The meridional water vapor transport associated with the westward propagating planetary-scale waves leads to a significant meridional circulation in the tropics and subtropics in both hemispheres while the transport associated with the eastward propagating baroclinic scale waves leads to a meridional circulation which extends from the subtropics to the polar region. When the two transports are combined they produce a meridional circulation that extends from the deep tropics to the polar regions in both hemispheres. This meridional circulation induced by planetary and baroclinic scale water vapor transports is not captured by the standard TEM formulation because it only includes eddy-induced sensible heat transports. During summer the meridional circulation induced by water vapor transports dominates over the relatively weak circulation induced by sensible heat transports. This dominance explains the large difference between the circulations on dry and moist isentropes during Northern Hemisphere summer.