EVALUATE THE SUB SEASONAL TO SEASONAL RAINFALL VARIABILITY AND ITS POTENTIAL DRIVERS DURING JUNE TO SEPTEMBER (JJAS) SEASON OVER OROMIA

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.