From global to regional: Advancing the simulation of the Moroccan climate with a variable resolution GCM

Morocco -as part of both the Mediterranean and North African region- has long been recognized as a major climate change hotspot where precipitation is projected to decline significantly. This can threaten the stability of many climate-sensitive sectors including water and agriculture. Effective management of such sectors requires a better understanding and assessment of climate variability and change in the regional context.

Downscaling approaches are needed to bridge the gap between the coarse resolution of the Global Climate Models (GCMs) and the scales suitable for finer climate assessments and impact or adaptation studies. This is classically done through limited area Regional Climate Models (RCM) driven by large-scale fields from the global models. While these models can improve the representation of many processes including mesoscale circulation and orographic effects, they also suffer from weaknesses that can significantly alter the reliability of climate change projections. The potential inconsistencies between the physical parameterizations of the RCMs and its forcing GCMs, the incomplete description of some climate forcings, as well as some methodological choices may impact the results in a non-negligible way. It is also difficult to distinguish between the impact of a better description of small scales and the impact of systematic biases inherited from the forcing models.

In this work, we examine the feasibility of using a variable resolution, global, general-circulation model LMDZ (Laboratoire de Météorologie Dynamique, Z stands for zoom) in a coupled configuration (atmospheric/land-surface component of the IPSL climate model) using telescopic zooming and enhanced resolution (approx. 35 km) over North Africa to better reveal regional aspects of the distribution of the precipitation over Morocco and their response to global warming. The simulations produced with this configuration are compared to a hierarchy of simulations, including intermediate resolution global simulations (50km) and low-resolution AMIP simulations produced in the framework of the CMIP6 exercise. The simulations are evaluated against various sets of observations (stations and satellite-based datasets). Our results show clear improvements related to the increased resolution and the ability of the model to capture the main large-scale circulation patterns of interest for Morocco. In addition, the model clearly illustrates the impact of weather regimes on precipitation and temperature mean and extreme events. The next step will consist in using this configuration to produce and analyze downscaled climate projections, to better understand the mechanisms of regional climate change and quantify the uncertainties.