Intraseasonal to Interannual Modulation of Diurnal Precipitation Distribution Over Eastern Africa

The modulation of the rainfall diurnal cycle at intraseasonal and interannual time scales is examined using gridded rainfall data over Eastern Africa and the nearby Indian Ocean for the period 1998-2014. Our focus is on the October-December season which is strongly impacted by both the Madden-Julian Oscillation (MJO) and the Indian Ocean Dipole Mode (IODM). The effect of the MJO, which is not synchronized across the whole region, is mainly through changes in the amplitude of the diurnal cycle, especially an enhancement of the late afternoon peak in active phases of the MJO. The rainfall phase only shows minor variations between the active and quiescent phases of the MJO, except over the drier regions and the Indian Ocean where the overall diurnal rainfall signal is more uneven. The effect of the positive phase of the IODM (warm western Indian Ocean) on Eastern Africa rainfall is found at any time of the day, but the enhancement is more prominent during the wettest part of the day (generally the afternoon). Substantial changes in the diurnal phase of the rains are found in some land areas away from the coast or from mountain ranges, where the dominant afternoon peak during the negative IODM phase is replaced by a nighttime maximum during the positive phase. Cross-sections and a cluster analysis of diurnal rainfall patterns over Kenya and southern Somalia suggest that this feature may be associated with the development of longer-lived rainfall systems propagating inland from the coast during the IODM positive phase.