Variability of the Turkana Low‐Level Jet in Reanalysis and Models: Implications for Rainfall.

The complex topography of East Africa poses challenges for accurate modeling of regional climate. The Turkana Channel in northwestern Kenya is an important feature because of a persistent low‐level jet (LLJ) that blows through it, which has complex interactions with local and regional rainfall. We establish the annual cycle and interannual variability of the LLJ in the ERA5, MERRA‐2, and JRA‐55 reanalyses. The jet is strongest during wet seasons in the surrounding region. Results suggest a statistically significant weakening of the LLJ over the last 30–40 years in two out of the three reanalyses. We propose an explanation based on the jet's relationship with regional warming patterns and zonal surface pressure gradients, which link the jet to larger‐scale climate dynamics including the Walker Circulation. If these changes continue in the future, there may be significant implications for rainfall including increases in northwest Kenya and decreases further inland. However, the global models used to produce climate projections vary in their simulations of the LLJ in part because they represented topography. Consequently, it is not possible to assess how future LLJ changes will affect regional climate using CMIP5 models alone. Differences between the reanalyses preclude their direct use for model evaluation. Improving the processes by which topographical observations are mapped onto model grids could lead to improvements in the simulation of the East African climate. A field campaign to measure the LLJ directly could resolve uncertainties in the literature, help constrain reanalyses, and determine which models have the most realistic LLJ representation. Plain Language Summary: The Turkana Low‐Level Jet is an important physical feature of the East African climate. It is present throughout the year and is both a factor in the aridity of the channel through which it blows and a source of moisture for rainfall in the wider region. We investigate how the jet varies throughout the year and across longer timescales, and suggest that it may have been getting weaker over the last 30–40 years. We suggest that this is due to connections between the jet and large‐scale climate dynamics, such as the Walker Circulation. However, because the jet is represented differently in different datasets, and climate models struggle to simulate it, we can't be sure of the impact these changes might have in the future. One potential way forward is to measure the jet directly in a field campaign. Key Points: The Turkana Low‐Level Jet may be slowing down on decadal timescales. However, reanalysis simulations of the trend and annual cycles differThe jet slowdown is a plausible consequence of changes to zonal surface pressure gradients, linking the jet to the Walker CirculationCoupled Model Intercomparison Project 5 representations of the jet are resolution‐dependent [ABSTRACT FROM AUTHOR]