Relationships Between QBO in the Lower Equatorial Stratospheric Zonal Winds and East African Seasonal Rainfall |
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Authors: | M Indeje F H M Semazzi |
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Institution: | (1) Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina, |
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Abstract: | Summary Teleconnections between the seasonal rainfall anomalies of March through May (“long-rains”) over eastern Africa (Uganda,
Kenya and Tanzania) and the lower equatorial stratospheric (30-mb) zonal winds for the 32-year period 1964–1995 are examined
using statistical methods. The analysis is based on the application of the simple correlation method and QBO/rainfall composite
analysis. A statistical study of spatial correlation patterns is made in an effort to understand the climatic associations
between the equatorial stratospheric zonal wind and regional rainfall at the interannual scale. The aim of this analysis is
to establish whether this global signal can be employed as predictor variable in the long-range forecasts. The study is part
of an ongoing investigation, which aims at designing a comprehensive and objective, multi-variate-forecast system of seasonal
rainfall over eastern Africa. The correlation parameters include simultaneous (zero lag), and the non-zero lag correlations.
The statistical significance of the correlation coefficient r] is tested based on the Monte Carlo t-statistical method, and
the standard correlation tables.
Our results indicate significant positive simultaneous and non-zero lag correlations between rainfall over parts of East Africa
and lower equatorial stratospheric zonal wind during the months of March–May and June–August. Significantly high correlations
are concentrated over the western regions of eastern Africa with peak values of (+ 0.8) observed over these areas. These associations
have been observed to be more prominent during lag than in the simultaneous correlations. Strong month to month lag coherence
is observed after June prior to the onset of the March to May seasonal rainfall and persists for more than 4 months. Correlation
indices for the eight homogeneous rainfall regions over eastern Africa which are derived from our Empirical Orthogonal Function/Cluster
analysis shows a clear annual cycle with significant relationships between QBO and seasonal rainfall occurring during boreal
summer (June–August). The season with the weakest relationship is December–February. It is however, noted that although the
coherence between QBO-Index and rainfall during the long-rains is significantly high, there are some wet/dry years for which
the relationship between the long rains and the lower equatorial zonal wind are not significant (for example in 1966, 1973
and 1983). These years have been associated with strong and prolonged ENSO events. Preliminary comparison of the QBO-Index
and the newly found Indian Ocean dipole mode index (DMI) indicates that the two climate variables may be significantly related.
Of the six high dipole mode events in the Indian Ocean that were observed in 1961, 1967, 1972, 1982, 1994 and 1997, all except
1967 coincided with the easterly phase of the QBO-Index and below normal rainfall over western highlands of eastern Africa.
Contingency analyses indicate 60 percent likelihood for the occurrence of above normal rainfall during the westerly phase
of the QBO and 63 percent likelihood of below normal rainfall during the east phase of the QBO. Our correlation analysis results
indicate that about 36 percent of the variability of the long-rains season over eastern Africa are associated with the QBO-Index.
Our results further show that the tendency of the lower equatorial stratosphe ric zonal wind prior to the season is a good
indicator of the performance of the long rains of eastern Africa. A positive OND minus JJA QBO trend is a good indicator for
the non-occurrence of drought over eastern Africa. Similarly, a negative trend is a good indicator for the non-occurrence
of high rainfall over the region. The identified characteristics and domain of influence of the QBO signal in different regions
of East Africa suggests that this global oscillator may offer useful input to objective multi-variate rainfall prediction
models for eastern Africa.
Received June 4, 1999 Revised November 25, 1999 |
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