Radiative driving of shallow return flows from the ITCZ

Abstract

AbstractThe tropical overturning circulation over the central‐east Pacific not only extends through the troposphere but also includes a shallow component outflowing from the ITCZ near the planetary boundary layer (PBL) top, the Shallow Meridional Circulation (SMC). Idealized simulations with the Weather Research and Forecasting (WRF) model, which qualitatively reproduce this two‐component circulation, are used to investigate the mechanism driving the SMC by altering model parameterization schemes (PBL, cumulus and radiation), the solar heating rate, and the SST gradient. A radiative‐driving model for subsidence is tested and found to quantitatively predict the altitude and strength of the SMC in all experiments. This includes experiments with a changed SST gradient, in which the changes in temperature profile and clouds near PBL top in the subsiding region altered radiative cooling so as to make the SMC vary in proportion to the SST gradient. These results indicate that the SMC is strongly controlled by radiative cooling rather than independently by SST patterns or convective processes, at least in this model setup. Experiments with altered solar heating of the atmosphere affected the strength of the SMC strongly, while changes in cumulus parameterization had a weaker effect. Parameterization changes influenced the SMC mainly by altering shallow cloud amount and humidity in subsidence regions, but vertical gradients of temperature also affect the radiative driving of the SMC in at least one case. The results highlight that atmospheric radiative heating is of prime importance for determining the character of deep and shallow overturning.