The impact of vertical mixing schemes on the position of the ITCZ in the eastern tropical Pacific

The presence of a double Intertropical Convergence Zone (ITCZ) in the tropical Pacific is a persistent feature of global coupled ocean-atmosphere models that gives rise to excessive precipitation south of the equator. The ITCZ position is extremely sensitive to changes in the magnitude and distribution of the Sea Surface Temperature (SST) in the tropical band, due to the strong coupling between SST and convective precipitation.  The complexity of the air-sea interactions makes it hard to disentangle the different mechanisms at play to identify the main driving processes behind this ubiquitous bias. Here, we use a coupled ocean-atmosphere regional model, the Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System, to investigate the impact that different parametrizations of the oceanic vertical mixing have on the water column dynamic, SST and subsequently the convective precipitation distribution in the eastern tropical Pacific. The model includes an atmospheric component, the Weather Research and Forecast Model (WRF), and an oceanic component, the Regional Ocean Modeling System (ROMS). The same atmospheric setup, with a resolution of 20km, has been forced with observed SSTs and with two ocean parameterizations. Different temperature gradients and oceanic stratification give rise to a double ITCZ or to a southward shift of the maximum precipitation band. Particularly in late winter and spring, a surface warming of a few degrees south of the equator around 5°S affects the distribution of the sea level pressure. The consequent changes in the surface wind pattern impact the usually asymmetric behavior of the trade winds, the south easterlies are no longer able to cross the equator and converge in the ITCZ in the northern hemisphere.