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Observed formation of easterly waves over northeast Africa

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Abstract

This study explores the thermodynamic and kinematic features of easterly waves over northeast Africa in July–September season 2005–2015. A daily African easterly wave (AEW) index is formulated from transient satellite rainfall and reanalysis vorticity, and the ten most intense cases are studied by composite analysis. Surface moisture is advected from central Africa towards the Red Sea during AEW formation. The anomalous 600 hPa wind circulation is comprized of a cyclonic-south anticyclonic-north rotor pair and accentuated easterly jet along 17N. Composite convection is initiated over Ethiopia and subsequently intensifies following interaction with a zonal circulation located downstream. Composite AEW temperature anomalies reveal a cool lower–warm upper layer heating profile. 2–8 day variance of satellite OLR reaches a maximum over the southern Arabian Peninsula, suggesting an upstream role for surface heating and the Somali Jet. The large scale environment is analyzed by regression of the AEW index onto daily fields of rainfall, surface air pressure and temperature in July–September season (N = 1004). The rainfall regression reflects a westward propagating AEW wave-train of higher values on 13N and lower values on 7N with a longitude spacing of ~25°. The air pressure and temperature regression features a N–S dipole indicating an anomalous northward ITCZ. A low pressure signal west of the Maritime Continent coupled with a warm zone across the South Indian Ocean coincides with AEW formation over the eastern Sahel.

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Acknowledgements

The research benefited from South African Dept of Education support. The providers of online data and analysis systems are acknowledged:/iridl.ldeo.columbia.edu/,/climexp.knmi.nl/,/apdrc.soest.hawaii.edu/,/giovanni.gsfc.nasa.gov/. A. Mekonnen of North Carolina A&T State Univ and G. Kiladis of NOAA-ESRL provided useful advice.

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Authors and Affiliations

  1. University of Zululand, KwaDlangezwa, 3886, South Africa

    Mark R. Jury

  2. Physics Department, University of Puerto Rico Mayagüez, Mayagüez, 00681, Puerto Rico

    Mark R. Jury

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  1. Mark R. Jury
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Correspondence to Mark R. Jury.

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Responsible Editor: M. T. Prtenjak.

Appendix

Appendix

See Table 3.

Table 3 Example of spreadsheet layout to calculate the AEW index: all averaged over 10-15N, longitudes bold, index outcome (row sum) italic, columns 30–40E (10–20E) are shifted down (up) by 1 day
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Jury, M.R. Observed formation of easterly waves over northeast Africa. Meteorol Atmos Phys 130, 279–293 (2018). https://doi.org/10.1007/s00703-017-0520-2

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  • DOI: https://doi.org/10.1007/s00703-017-0520-2

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