Global climate models have difficulties to simulate the northward extension of the monsoonal precipitation over north Africa during the mid-Holocene as revealed by proxy data. A common feature of these models is that they usually operate on too coarse grids to explicitly resolve convection, but convection is the most essential mechanism leading to precipitation in the West African monsoon region. Here, we investigate how the representation of tropical deep convection in the ICON climate model affects the meridional distribution of monsoonal precipitation during the mid-Holocene, by comparing regional
simulations of the summer monsoon season (July to September, JAS) with parameterized (40km-P) and resolved deep convection (5km-E).
Simulations with parameterized convection produce generally more rain and extend precipitation further north than simulations with resolved deep convection.
The most striking difference between these simulations is the difference in the probability distribution function of precipitation and its resulting interactions with the land surface. The convective parametrization produces light and large-scale precipitation, keeping the soils moist and supporting the development of convection. In contrast, less frequent but locally intense precipitation events lead to high amounts of runoff in simulations with resolved deep convection. Hence, the stronger runoff inhibits the moistening of the soil during the monsoon season and limits the amount of water available to evaporation. This is true for simulations with present-day land surface cover and for simulations where we prescribe a higher mid-Holocene-like vegetation cover.
However, in experiments with the same constant soil moisture field, precipitation expands equally far north in resolved deep convection and parameterized convection simulations. This highlights the importance of the type of rainfall in modulating land-atmosphere feedbacks, instead of only considering the amount of rainfall.