Abstract:
The June to September rainfall season, locally known as Kiremt, contributes 60.3% over the
western, central, eastern, and southern highlands of the Oromia region. In this study, we evaluate
sub-seasonal to seasonal rainfall variability and potential drivers understood to minimize the
incidence of extreme events such as floods and droughts. The study at hand utilized a variety of
meteorological data sources in order to analyze the potential drivers of rainfall patterns. This
study included data from the stations, ENACT, CHRIPS, SST, MSLP, and GPH, which provided
essential information for the study. The methods used for this study were the coefficient of
variation, precipitation concentration index, standardize rainfall anomaly, correlation coefficient,
and composite analysis. The western Oromia region has less rainfall variability (less than 20%
CV), the central parts of the region have moderate rainfall variability (between 20% and 30%
CV), and other regions have high rainfall variability. The PCI in western Oromia, exhibits a
uniform distribution of rainfall, with moderate distribution over the eastern and southern high
land. The rainfall anomaly in Oromia's western, central, eastern, and southern high lands
experienced severe drought in 1991, 1995, 2002, 2003, 2015, and 2017, while 1998, 2006, 2010,
and 2020 were the wettest years due the strong El-Niño, La-Niña, positive and negative IOD. The
mean sea surface temperature increases, which enhances rain-fall over the region. On 850 hpa,
the velocity potential is positive during the wettest year that forms the convergence area, while it
is negative during the direst year that leads to divergence. The potential driver’s impacts on the
Oromia rainfall during Kiremt season ENSO region (Ni-ño3.4, Niño4, Niño3, Niño 1+2) and PDO
are negatively correlated, while IOD and AMO are positively correlated with Oromia rainfall.
During strong EL-Niño, there was a deficit of rain-fall over western, central, and eastern Oromia
and an increase over southern Oromia and La Nina, with excess rainfall over the Kiremt
beneficiary area. The positive phase of IOD would increase rainfall, while the negative phase
would decrease rainfall across the region. Since this study comprehensive and useful information
on the spatio-temporal variability of potential drivers from anomalous global SST pattern to
Oromia rainfall variability, which could be used to enhance sub-season and seasonal rainfall
forecast skill.
