| dc.description.abstract | The linkage of the Subtropical Indian Ocean Dipole (SIOD) with the October-December rainfall
season over East Africa was investigated. The data used in the research spanned for the period
1961-2013 and were monthly datasets of rainfall, sea surface temperatures (SSTs) and wind.
Climate Research Unit (CRU) rainfall data, Hadley Centre SST from the UK Met Office and
NCEP-NCAR wind were used.
Methods applied involved standardization of both rainfall and SST data, a procedure
recommended where observations are influenced by diverse factors and need to be compared.
The temporal and spatial patterns of rainfall studied were interannual variability and trends and
using both smoothed and unsmoothed time series. The trends were computed using the least
square regression coefficients and their significance evaluated using the ANOVA method. The
relationships between SIOD with both the October-December East Africa rainfall and the Indian
Ocean Dipole (IOD) were determined using the Pearson correlation coefficients. The Student ttest
was used to test the significance of the correlations. Based on the evolution patterns of
SIOD, composited anomalous distribution of rainfall, SSTs and wind were studied during
significant SIOD events.
The results show that the variability of rainfall is highest in arid lands on the eastern region of
East Africa and lowest on the western sector. The trends also show a regional bias, with the
western region (much of Uganda) reporting positive and significant trends based on the
smoothed data. The trend in the October-December SIOD index was evidently negative. Further
analysis showed that in the recent decades, the eastern pole of SIOD has been reporting higher
magnitudes of positive anomalies than the western, hence depicting a sustained non-homogenous
oceanic warming.
The instantaneous SIOD-IOD are insignificant over January to September, but significant
negative correlations reported in the October-December period. The correlations were positive at
lag four and turned negative in approach to lag zero when peak negative values were reported.
Significant correlations were usually reported in lag four and zero. Hence it was observed that
there exists substantial changes in the relationship between these periods.
The case studies of SIOD showed that rainfall anomalies tend to be slightly enhanced especially
in the equatorial region during all-year persisting positive SIOD events. This may be related the
weakening of the Mascarine High hence maintaining the Intertropical Convergence Zone (ITCZ)
within the equatorial region over East Africa. The SIOD events involving phase changes of
positive to negative during the June-August period also reported enhanced rainfall. The wind
patterns at 850mb showed that these events are associated with the intensification and westward
displacement of the Mascarine anticyclone, hence promoting the flow of moist equatorial
easterlies.
This research has established that SIOD has both oceanic and atmospheric connections that
influence the East Africa October - December rainfall season. The oceanic impact is in the
SIOD-IOD contrasting interactions especially during the OND season. On the other hand, the
atmospheric relations lies on its influence to the intensity and position of the Mascarine High.
Hence the monitoring of both of these indices may prove useful for diagnostic studies in over
East Africa especially during the short rains season. It is also suggested that further studies may
be carried out for the December - February and March - May rainfall seasons, which fall well
within the SIOD evolution period. | en_US |
