A Moist Static Energy Budget Perspective on Precipitation Changes during the Mid-Holocene African Humid Period

The Mid-Holocene (roughly 7,000-5,000 years before present) was a time of profound changes in the landscape of northern Africa. Variations in orbital forcing led to higher insolation of the northern hemisphere during summer, which triggered a so-called African Humid Period. Climate proxies indicate an intensified West African Monsoon during this period and a northward extension of precipitation, but climate models underestimate both the expansion and accumulation of rainfall during monsoon season. Causes of these shortcomings could be the insufficient representation of feedbacks between rainfall, soils and vegetation, but it could also be – in alternative or in addition – the inadequate parameterization of convective processes. To investigate the influence of the representation of convection, Jungandreas et al. (2021) performed simulations with ICON-NWP for Mid-Holocene northern Africa, applying present day soil conditions and using different horizontal model resolutions, ranging from 40 km with parameterized convection to 5 km with resolved convection. In the JAS mean, the simulations with parameterized convection produce more precipitation and a further northward expansion of precipitation, than the simulations with resolved convection. These results show that the effects of soil feedbacks and the representation of convection do not interact linearly. Therefore, we investigate these simulations more closely, using the moist static energy budget to analyze the dynamics of the tropical atmosphere. Furthermore, we are conducting simulations with a simplified model setup with an idealized tropical continent, following the TRACMIP protocol (Voigt et al., 2016). These simulations include runs with parameterized and with resolved convection and are performed with the new ICON-Earth System Model (ICON-ESM V1.0) (Jungclaus et al., 2021). The idealized continent excludes soil feedbacks, so this approach allows us to isolate the dynamical effects of resolved convection. Utilizing the moist static energy budget, our results will add understanding of fundamental dynamical processes related to precipitation during the mid-holocene African Humid Period.

References

Jungandreas, L., Hohenegger, C. and Claussen, M. (2021), ‘Influence of the representation of convection on the mid-holocene west african monsoon’, Climate of the Past 17(4), 1665–1684. DOI: https://doi.org/10.5194/cp-17-1665-2021.

Jungclaus, J. H. et al. (2021), ‘The icon earth system model version 1.0’, Journal of Advances in Modeling Earth Systems. Preprint. DOI: https://doi.org/10.1002/2016MS000748.

Voigt, A. et al. (2016), ‘The tropical rain belts with an annual cycle andclaussen a continent model intercomparison project: Tracmip’, Journal of Advances in Modeling Earth Systems 8(4), 1868–1891. DOI: https://doi.org/10.1002/2016MS000748.