Abstract:
Precipitation over the Upper Blue Nile Basin (UBNB) contributes most of
the annual rainfall in Ethiopia and plays an important role in agriculture and
thus the economy. In this study we applied various wavelet analysis methods
to explore possible in
uences of large-scale climatic patterns on monthly precipitation
by taking 20 Raingauge stations across the UBNB for the last 32
years (1987-2018) and their teleconnections to three prominent climate indices
namely; Ni~no4, Indian Ocean Dipole (IOD) and Indian Summer Monsoon Index
(ISMI). Rainfall Anomaly Index (RAI) is calculated using precipitation
data to identify the dry and wet periods. Then the temporal patterns of
precipitation are detected by wavelet and global coherence. Subsequently,
wavelet coherence (WTC) between precipitation and oceanic indices (ENSO,
IOD, ISMI) is applied to identify the dominant driving factors causing precipitation
variability. The WTC results reveals a common dominant oscillation
at 8-16 months for all stations over UBNB. The results suggest that ENSO
has dominant coherence at 8-16 months periodicity during 1991-1994, 19992002
and 2005-2009, whereas IOD have noticeable e ects at 4-8 months time
scales. ISMI is also responsible for the variability of summer precipitation
with the e ective periods of 4-6 months over the basin. Meanwhile, partial
wavelet coherence analysis (PWCA) is used to investigate the standalone relationship
between the climate indices and precipitation after removing the
e ect of interdependency. Finally, the spatial variation of summer (June
to September) precipitation during the incidence of IOD and ENSO years
conclude that the co-occurring of La Ni~na and positive IOD brings excess
precipitation via the modulation of monsoon rainfall while El Ni~no and negative
IOD phases cause rainfall de cit over the entire basin.