Observed Relationship between the Turkana Low-Level Jet and Boreal Summer Convection

The Turkana low-level jet is a circulation feature over northern Kenya that is not spatially well-resolved in many observational datasets. Here, high-resolution satellite-derived precipitation datasets (TRMM, IMERG, and CMORPH) and atmospheric reanalyses (ERA5 and MERRA2) are analyzed to better understand the controls on jet strength and the relationship between the jet and boreal summer rainfall variability in the jet exit region over South Sudan. The height of the South Sudan wet season, August, is analyzed for 2000 – 2017.

Climatologically, rainfall peaks over the jet exit region during the early morning hours, coinciding with the jet maximum. The strong jet enhances low-level wind convergence and briefly shifts the low-level zonal moisture gradient westward. Over the western South Sudan, rainfall peaks in the late afternoon/evening in association with daytime heating.

The correlation between the daily Turkana Jet strength and precipitation indicates a significant negative relationship over eastern South Sudan, and over the Ethiopian and East African Highlands. Composite analysis of strong and weak jet periods reveal that the atmospheric conditions associated with a strong jet include enhanced low-level ridging along the East African coast south of the equator resulting in a stronger height gradient in the Turkana Channel, as well drier conditions over the channel and adjacent highlands. These conditions support a stronger jet that is further enhanced by katabatic flow due to the stronger nighttime cooling over the highlands, as well as enhanced dry air advection over the jet exit region. Thus, while low-level convergence may increase, convection weakens over eastern South Sudan due to the reduced atmospheric moisture content and weaker instability. The opposite occurs for the weak jet case.