The Role of Overturning Zonal Circulations in Determining the Seasonality of East African Precipitation

The role of overturning zonal circulations in determining the seasonality of East African precipitation

Siyu Zhao, Kerry Cook, and Edward Vizy

Department of Geosciences, The University of Texas at Austin

Equatorial East Africa has a unique semiarid/arid climate and a bimodal precipitation annual cycle, with much of its rainfall delivered during March-May and October-December. The population is heavily dependent on these rainy seasons. Upper-level convergence occurs year-round, with varying intensity that can suppress or permit convection. Previous studies associate this upper-level convergence, and the resulting subsidence, with an Indian Ocean overturning zonal circulation. This study investigates the existence, seasonality, and variability of overturning zonal circulations near East Africa in reanalyses, and explores their influence on East African precipitation.

A two-dimensional decomposition of the atmospheric circulation is used to identify zonal and meridional overturning components, and zonal circulation systems are found to be more important in maintaining subsidence over East Africa throughout the year. The streamfunction shows two zonal circulations that may be relevant for East Africa. One is largely over the Indian Ocean with an upward branch over the Maritime Continent; it occurs throughout the year and becomes strongest during June-September. The other zonal circulation has a rising branch over the Congo Basin, and it is strongest during both Apr-May and Oct-Dec.

By hypothesizing that there are years with strong or weak zonal circulations, we correlate the 500-hPa vertical-p velocity averaged over Equatorial East Africa with regional 500-hPa vertical p-velocity for Jan-Mar, Apr-May, Jun-Sep, and Oct-Dec. Correlations between the vertical velocities over Equatorial East Africa and the Maritime Continent indicate a clear teleconnection during Oct-Dec, which becomes weaker but remains over 95% confidence level during Jan-Mar in ERAI, NCEP2, MERRA2, and ERA5 and during Apr-May in MERRA2 and ERA5. While no significant correlation exists in Jun-Sep, the streamfunction representing this zonal circulation is strongest during those months. This difference may be associated with the development of the Tropical Easterly Jet in Jun-Sep, which breaks down the correlation among the branches of the zonal circulation but amplifies the streamfunction. Meanwhile, the correlation between vertical velocities over the Congo Basin and East Africa is not strong or consistent among the reanalyses.

Indices are derived from vertical velocity and the zonal component of the divergent wind to represent the strength of the zonal circulations. Composites of seasons with strong (weak) zonal circulation over the Indian Ocean have less (more) rainfall over Equatorial East Africa during Oct-Dec. Moist static energy and moisture budget analyses are used to understand how heat and moisture advection interact with the upper-tropospheric mass convergence to influence rainfall.