The potential predictability of March-April-May rainfall over East Africa using ITCZ indices
Njeri, Joseph K
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The East African rainfall regime is highly variable and predictability on seasonal time scales has always been a challenging endeavour. Several predictors have been assembled in the past for the purposes of seasonal rainfall prediction over East Africa. These include, the MaddenJulian-Oscillation (MJO), Indian Ocean Dipole (IOD), Quasi Biennial Oscillation (QBO), Sea Surface Temperature Anomalies (SSTA) and Sea Surface Temperature (SST) gradients. In this study, the predictability potential of seasonal rainfall anomalies using ITCZ indices was investigated. Utilizing the National Centre of Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) resources of standard pressure level winds for the period 1961-2010 together with the interpolated Outgoing Long Wave radiation datasets from the National Oceanic and Atmospheric Administration (NOAA) for the period 1976-2010, the location as well as the magnitude/strength of the Inter-tropical Convergence Zone (ITCZ) was assessed. The relationship between the location of ITCZ and seasonal rainfall during the MarchMay season was studied using each of the indices. Meridional and vertical shear of the zonal wind were computed from the NCEP/NCAR winds on a monthly basis each serving as an index of ITCZ. The Outgoing Long-wave Radiation (OLR) data were used as proxy for cloudiness representing the ITCZ band. It was found that the latitudinal index of ITCZ defined by Outgoing Long-wave Radiation (OLR), vertical shear of zonal wind and meridional shear of zonal wind indices were good descriptors of ITCZ location and strength. In Particular, the Outgoing Long-wave Radiation indices closely mimicked the general pattern of the position of the axis of ITCZ defined from wind patterns. Examination of the inter-annual variability of ITCZ showed that the locations . During December, the ITCZ preferred the southern part of its mean location during an El Niño year and to the north of its mean location during a La Nina year. The ITCZ was found to be highly variable during the month of March. Correlation analysis was carried out between the location of ITCZ and the various indices, i.e. the meridional shear of the zonal wind indices, the Outgoing Long-wave Radiation indices and the vertical shear of the zonal wind indices on a monthly time scale. The Outgoing long wave radiation produced higher correlations with monthly rainfall at stations than the other two indices. However, strong correlations were confined to lag 0. The correlations between the meridional shear indices and monthly rainfall were rather weak but passed the 95% confidence test. Moreover, correlations seemed to be higher just before the season and would diminish further away from the season. OLR indices were found to account for at most 28% of the variance of March and May rainfall when ITCZ was located at 12.5˚S in February while the meridional shear of zonal wind indices accounted for 15% of the variance in March and April when the ITCZ was located at 7.5˚S in January. The vertical shear of zonal wind indices accounted for 10% of rainfall variability in March. Variance explained during the months of April and May was even lower. Meridional shifts in ITCZ were also examined with view to establishing tele-connections between them and monthly rainfall anomalies. OLR indices showed very strong and significant correlations at lag 0. As for the meridional shear of the zonal wind indices, most correlations were non-significant during December and January. The vertical shear indices showed rather low but significant correlations at both lags 1 and 0 ly rainfall prediction. Regions immediately to the west of Lake Victoria, eastern and coastal parts of Kenya as well as those to the north west of Tanzania showed skillful models during the months of March with coefficients of determination of 42.5%, 51.2%, 35.9%, 40.7% and 46.0% respectively. Western Uganda and southern Tanzania showed skilful models during the month of April while Northern and Western Uganda as well as northern Kenya showed skillful models during the month of May. In all the cases, the analysis of variance indicated that all the regression coefficients were significant at the 95% confidence level. March models showed that the 3 categories namely, below normal, near normal and above normal had equal chances of being predicted. Below normal category was more reliable. In April, the above normal category was the most attainable. During an anomalously wet year, ITCZ structure was found to be well defined and preferred to sit within the confines of the equatorial parts of the region i.e. the band restricted itself to between 7.5˚S and 5˚N while during an anomalously dry year, the ITCZ structure was noisy and seemed to be characterized by presence of other synoptic systems. It was also observed to oscillate over a wider latitudinal band (between 15˚S and 10˚N), weakening particularly when approaching the equatorial region during the dry year considered. Notably, there was a difficulty in separating the Near Equatorial Trough (NET) from the real ITCZ during a dry year. Its strength was also found to be weak during the transitional months of March, April and May in a typical dry year.
CitationMaster of Science in Meteorology,
University of NairobiMeteorology