This study was to determine the spatiotemporal characteristics of historical and projected drought events throughout Isiolo County, Kenya, through using self-calibrating Palmer drought severity index (scPDSI). The scPDSI is a complex and robust drought index that applies the water balance model by incorporating the role played by evapotranspiration and soil properties on drought analysis therefore making it appropriate to identify the linkages between meteorological, agricultural and hydrological droughts. The historical scPDSI was computed at a monthly timescale using a 39-year long monthly mean precipitation data from Climate Hazard Group Infrared Precipitation with Station (CHIRPS) and monthly average temperature data from the Climate Research Unit (CRU). The climatological (1950-1996) available water holding capacity (AWHC) of the soil was obtained from Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC) for biochemical dynamics at a spatial resolution of 1o x 1o. The projected scPDSI under Representative Concentration Pathways (RCPs) was computed using bias corrected monthly temperature and precipitation model output data from Coordinated Regional Climate Downscaling Experiment (CORDEX). The datasets were extracted for ten grid points in the County. The scPDSI was used to assess the historical and projected duration, severity, and intensity of droughts. The major significant historical and projected drought events and their characteristics were clearly identified using the run theory. ScPDSI runs have shown that more severe drought events dominated the period between 1980 and 2000. ScPDSI had the longest dry event duration of 61 months and a severity of 126.412 with adverse effects on the eastern locations. The projected drought events identified Mar 2046 –Mar 2048 under RCP4.5 to be the most severe drought lasting for 25 months with severity of 59.292 while under RCP8.5 run Nov 2042 – Nov 2046 is identified as the most severe, 114.362 with the duration of water stress anticipated to last for 49 months. To examine the spatial variability of the drought events in the County, the Empirical Orthogonal Analysis (EOF) was applied to the historical and projected scPDSI time series. The EOF results indicated that the two leading eigen vectors accounted for over 85% of the spatial variability for both historical and projected droughts under the RCPs. Subsequently, the Mann-Kendall (MK) test was applied to the projected scPDSI, temperature and precipitation timeseries in order to determine the local expected drought trends. The MK test of the identified significant increase in trend for temperatures under RCP8.5 and precipitation under RCP4.5 towards the end of the last decade under the study period considered. Both scenarios showed a decline in trends of drought events in Isiolo County from 2020-2050.
Convectively coupled equatorial Kelvin waves (CCEKWs) are those types of equatorially trapped disturbances that propagate eastward and are among the most common intra-seasonal oscillations in the tropics. There exists two-way feedback between the inter-tropical convergence zone (ITCZ) and these equatorially trapped disturbances. Outgoing Longwave Radiation (OLR) was utilized as a proxy for deep convection. For CCEKWs, the modes are located over the West Atlantic, equatorial West Africa, and the Indian Ocean. The influence of other circulations and climate dynamics is studied for finding other drivers of climate within East Africa. The results show a positive relationship between Indian and Atlantic Oceans Sea Surface Temperatures and March-May rainfall over equatorial East Africa over the period of 1980 to 2010. This influence is driven by the Walker circulation and anomalous moisture influx enhanced by winds. Composite analysis reveals strong lower-tropospheric westerlies during the active phase of the CCKWs activities over Equatorial East Africa. The winds are in the opposite direction with the upper-tropospheric winds, which are easterlies. Singular Value Decomposition shows a strong coupling interaction between rainfall over equatorial East Africa and CCKWs. This study concludes that Kelvin waves are not the main factors that influence rainfall during the rainy season. Previous studies show that the main influencing factors are ITCZ, El-Nino Southern Oscillation (ENSO), and tropical anticyclones that borders the African continent. However, CCKWs are a significant factor during the dry seasons.
Increased frequencies and intensities of extreme weather events have negatively impacted climate-sensitive socio-economic sectors in Kenya and larger Equatorial East Africa (EEA). Madden–Julian oscillation (MJO) influence intra-seasonal weather variability over Kenya although less attention has been given to its effect on extreme weather events such as droughts and floods, which have increased in frequency and intensity. Outgoing Longwave Radiation (OLR) was used in this work as proxy data for rainfall to study the geographical distribution and circulation anomalies associated with MJOs and their impacts on extreme weather events. Extreme weather events are identified using the self-calibrating Palmer Drought Severity Index (sc-PDSI), based on sc-PDSI, 2013/2014 and 2017/2018 as the drought and flood years, respectively. The background power spectral analysis reveals that MJOs are more dominant during the March–May (MAM) season than other seasons. The variance analysis depicted that the maximum power of MJO-filtered OLR is cantered within the tropical Indian Ocean, maritime continent and the tropical Pacific Ocean. Upper tropospheric (200 hPa) wind signatures give a clear Matsuno-Gill-type circulation compared to the lower tropospheric wind flows. Thus, the signatures can be used to develop a dynamic MJO index for prediction purposes. There exists a weak direct relationship between MJO and sc-PDSI; however, the influence may result from its modulation of atmospheric circulation as illustrated by the wind and velocity potential patterns before and after the passage of the convective MJO phase. Supplementary Information The online version contains supplementary material available at 10.1007/s00703-022-00948-9.
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