The Misrepresentation of Southern African Easterly Jet in Models and Its Implication for Aerosol, Clouds, and Precipitation Distributions

The Southern African easterly jet (AEJ-S) is an important mid-tropospheric feature critical to understanding the tropical convective system over central Africa and the aerosol-cloud interactions over the southeast Atlantic Ocean. However, it remains unclear how well models represent AEJ-S and its influence on aerosol transport, clouds, and precipitation distribution. Here, we use ground- and satellite-based observations and reanalysis datasets to assess the representation of AEJ-S in the Coupled Model Intercomparison Project Phase 6 (CMIP6) models between September and October during the peak of mid-tropospheric winds, aerosol transport, clouds, and precipitation. We find that most CMIP6 models have difficulty accurately simulating the strength, position, and spatial distribution of AEJ-S. Specifically, AEJ-S is relatively weaker and at a slightly lower altitude in the ensemble of CMIP6 models than represented by observation and reanalysis datasets. To assess the influence of the misrepresented AEJ-S on CMIP6-simulated aerosol, clouds, and precipitation distributions, we performed composite analyses using models with low and high biases based on the estimates of their mid-tropospheric easterly wind speed. We find that the misrepresentation of AEJ-S in CMIP6 models is associated with the overestimation of clouds and precipitation over central Africa, the underestimation of clouds over the southeast Atlantic Ocean, and the limitation of aerosol over the continent or deviation from the typical zonal aerosol transport over the Atlantic Ocean. Because aerosols, clouds, and precipitation are important components of the regional climate system, we conclude that accurate representation of AEJ-S is essential over central Africa and the southeast Atlantic Ocean.