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Fredrick Semazzi, Beverly Burns, Neng-Huen Lin & Jae-Kyung Schemm
Journal of Climate, October 1996
In this case study the role of global SST anomaly forcing in promoting the extreme climatic conditions that prevailed in Africa during the years of 1950 and 1973 is examined. In 1950 abundant rainfall was observed over tropical Africa, particularly over the Sahel and Southern Africa. By contrast, in 1973, this rainfall anomaly pattern was characterized by the opposite phase, with most of the continent experiencing severe droughts.
The primary research vehicle in this investigation is the standard version of the NCAR CCMI GCM with horizontal resolution of rhomboidal spectral truncation at wavenumber 15 (R15). Two separate 10-yr simulations based on the 1950 and 1973 observed sea surface temperature (SST) have been performed. The empirical orthogonal functions method is employed to isolate the annual cycle harmonies in the data and also to remove statistical noise. The filtered seasonal rainfall fields for the model and the observations are compared to investigate the response of the African continental climate to the 1950 and 1973 SST climatologies.
CCM1 successfully simulates the primary features of the seasonal mean climate conditions and anomalies over the Sahel and Southern Africa. The authors attribute this to the ability of the model to simulate the annual harmonic oscillation realistically. Over equatorial Africa, where the semiannual oscillation is observed to be relatively more important than it is at the higher latitudes of the continent, the model simulation is not as successful. This occurs because of the deficient simulation of the semiannual harmonic oscillation by the model. A weaker annual cycle comprising the annual mean (nonoscillating component) and the annual harmonic oscillation in the 1973 run relative to the 1950 experiment provides a viable explanation for the synchronous climatic anomaly conditions that prevailed in northern and southern Africa during these two years. Investigation of the relative role of the GCM's internal variability and the SST externally forced variability during the rainy season over tropical Africa yields valuable insight into the reasons for the observed anomalous climatic behavior. Over the Sahel and Southern Africa, where the annual harmonic oscillation is relatively large, externally forced SST variability dominates over internal variability in explaining the drier conditions in 1973 relative to 1950.