Dynamics of the Shallow Return Flow in ITCZ Regions

The generally accepted view of the meridional circulation in the tropical Eastern Pacific is one of a deep overturning cell driven by deep convective heating in the intertropical convergence zone (ITCZ), similar to the zonal mean Hadley circulation. However, recent observations of the atmosphere from the tropical Eastern Pacific have called this view into question. In several independent data sets, a significant meridional return flow out of the ITCZ region was observed, not only at high altitudes, but also at low altitudes, just above the atmospheric boundary layer (ABL). This "shallow return flow" is a new finding and has potentially significant impacts on climate in the Eastern Pacific.

Fundamentally, the shallow return flow can be seen as a large scale "sea-breeze" circulation driven by oceanic temperature gradients when deep convection is absent in the ITCZ region. A simple model of this circulation can predict the thickness and altitude of the return flow. A shallow return flow is also seen in an idealized (but three-dimensional, with full physics) numerical simulation of the Hadley circulation. In these simulations, the moisture transport out of the ITCZ region is actually greather in the shallow return flow than in the high altitude return flow associated with the deep overturning.