Characterization of the Turkana Low-level Jet Stream and the Associated Rainfall Using Cmip6 Models and Era5 Reanalysis

Abstract

The Turkana low-level jet stream (TJ) is important to climatic conditions over northern Kenya and East Africa. The representation of the TJ in climate models varies due to the TJ interaction with Turkana channel that is influenced by model resolution and influences the model representation of the regional climate. This study compares features of the TJ in data from Coupled Model Intercomparison Project Phase 6 (CMIP6) model simulations with European Center for Medium-range Weather Forecasting atmospheric Re-Analysis version 5 (ERA5). The study presents analysis of spatial structure and annual cycle of the TJ, and relates strength of the TJ with topographic formation of the Turkana channel rainfall in the models and reanalysis (RA). Considering study period between 1981 and 2014, models reveal climatological wind speeds that match those of the reanalysis from the ERA5 at the jet entrance (13 m/s) but lower magnitudes of wind speed and vertical shears compared to ERA5 within the Turkana channel. The models with slowest wind speeds, have a flattened Turkana channel and fail to exhibit the terrain constriction at 37°E which otherwise aids in accelerating winds to form a jet core. Furthermore, they fail to represent the narrowing of the channel as in ERA5, thereby forming blocking walls in the channel, forcing vertical ascent and mixing, and weakening shear. This boosting of ascent motion promotes rainfall formation and enhances wet anomalies at the exit of the TJ when the jet stream is weaker. By applying a new narrowing index, we demonstrate the need to improve topography details in the CMIP6 models, particularly those with resolution coarser than 1.5°, in order to properly simulate the TJ and the observed rainfall over the northwestern areas of eastern Africa.