Coupling between large-scale eddies, the width and vertical structure of ITCZs

An analysis of atmospheric energy transport in 22 years (1980-2001) of ERA40 re-analysis is presented. In the analyzed budgets, there is a large cancellation between divergences of dry static and latent energy such that the total energy divergence is positive over all tropical oceanic regions except for the east Pacific cold tongue, consistent with previous studies. In the central-eastern Pacific ITCZ, energy import by the mean circulation is balanced by energy export by sub-monthly eddies, while the west Pacific and Indian oceans are characterized by mean advective export and a small eddy contribution. The import of energy by the mean circulation into the central and eastern Pacific ITCZ is a significant finding as it implies that the mean circulation exhibits a form of moist static instability where an increase in the surface fluxes or decrease in radiative fluxes cannot be balanced by a stronger circulation.

The relationship between the eddies and the mean circulation is examined. Large-scale moisture diffusion is correlated with eddy moisture export on (500 km)² spatial scales. The eddies, hypothesized to be due to dry intrusions into the mid-upper troposphere, are linked to the depth of mean convection in large-scale convergence zones with larger eddy export associated with shallower circulations due to the eddies' systematic drying effect. These relationships suggest that due to enhanced eddy activity, thinner ITCZs will be characterized by shallower circulations then wider ITCZs, consistent with the observed systematic variation in the depth of the mean circulation across the Pacific.

A simple model, capable of skillfully simulating the observed tropical circulation, is used to further explore eddy-mean circulation interactions in the deep tropics.