The dynamics of the shallow circulation and its associated moisture transport

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 at low altitudes, just above the atmospheric boundary layer. This "shallow circlation" is a new finding and has potentially significant impacts on climate in the Eastern Pacific.

Fundamentally, the shallow circulation 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 altitude and thickness of the "shallow return flow," which is the upper half of the shallow circulation.

The Weather Research and Forecast Model (WRF) is used to simulate an idealized Hadley circulation driven by moist convection in a tropical channel. The shallow circulation is reproduced, with remarkable similarity to the circulation observed in the East Pacific. The simulations confirm that the return flow is strongest when deep convection is absent in the ITCZ, and weakest when deep convection is strong. The model also shows that moisture transport out of the ITCZ is far greater in the shallow return flow than in the high altitude return flow associated with the deep overturning, and that a budget for water transport in and out of the ITCZ region is grossly incomplete without it. Much of the moisture carried out of the ITCZ in the shallow return flow is recycled into the boundary layer, and this moisture appears to slightly enhance the boundary layer stratocumulus clouds in the descent regions of the Hadley circulation.