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Surface-driven amplification of MJO circulation anomalies across East Africa and its influence on the Turkana jet

Talib, Joshua, Taylor, Christopher M., Harris, Bethan L. and Wainwright, Caroline M. ORCID: 2023. Surface-driven amplification of MJO circulation anomalies across East Africa and its influence on the Turkana jet. Quarterly Journal of the Royal Meteorological Society 149 (754) , pp. 1890-1912. 10.1002/qj.4487

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In semi-arid environments, rainfall-driven soil moisture fluctuations exert a strong influence on surface turbulent fluxes. Intraseasonal rainfall variability can therefore impact low-level atmospheric temperatures and influence regional circulations. Using satellite observations and an atmospheric reanalysis, this work investigates whether rainfall variability induced by the Madden-Julian Oscillation (MJO) triggers land-atmosphere feedbacks across East Africa. We identify that during the East African wet seasons, March to May and October to December, surface fluxes are sensitive to MJO-induced precipitation variations across low-lying regions of South Sudan, and highland regions of Uganda and south-west Kenya. For example, during MJO phases 6 to 8, when rainfall is suppressed, surface temperatures and sensible heat fluxes increase, whilst evapotranspiration decreases. Spatial variations in the surface flux response to rainfall variability feed back onto the atmosphere through amplifying MJO-associated pressure anomalies. During dry MJO events for instance, surface warming across the exit region of the Turkana channel increases the low-tropospheric along-channel pressure gradient and intensifies the Turkana jet. We conclude that average surface-driven temperature fluctuations during a single day are responsible for approximately 12% of MJO-associated variability of the Turkana jet speed. However, we expect that the accumulation of heat over multiple days to the west of the East African highlands further amplifies anomalies in the pressure gradient and jet intensity. Modelling experiments are required to quantify the accumulated impact of the surface forcing. Surface-driven Turkana jet variations influence the East African moisture budget and affect the intensity and inland propagation of coastal convection. Not only is this the first study to investigate the importance of intraseasonal land-atmosphere feedbacks across East Africa, but it is also the first to show that Turkana jet characteristics are partly driven by surface conditions. This work motivates an investigation into whether sub-seasonal forecast models fully harness the predictability from surface-induced jet variations

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Environmental Sciences
Publisher: Wiley
ISSN: 0035-9009
Date of First Compliant Deposit: 22 May 2023
Date of Acceptance: 10 May 2023
Last Modified: 31 Aug 2023 18:31

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