The Space-time characteristics of minimum and maximum temperature values over the tropical Eastern Africa region

Abstract

This study examined the space-time characteristics of minimum and maximum temperature values over the Tropical Eastern Africa region in order to delineate any significant changes in the traditional space-time temperature characteristics over the region. The temporal characteristics investigated in the study included the inter-annual patterns of the trend, seasonal and cyclical variations at the individual stations. Both graphical and statistical methods were used in the trend analysis. In the graphical approach, visual examination of the inter-annual trend was used whilst analysis of variance (ANOVA) and non-parametric methods were used in the statistical approach. The recurrences of low/high inter-annual temperature values were examined using the maximum entropy method (MEM) of spectral analysis. The last part of the study attempted to provide some insight into the potential causes of the observed inter-annual variability in the temperature characteristics. EINino/ Southern oscillation (ENSO), the Quasi-Biennial Oscillation (QBO), cloudiness,

and OLR data were used in the study to quantify the regional/global scale systems that may be associated with the observed temperature characteristics. Relationships between these systems and the observed temperature characteristics were investigated using Simple correlation analysis. X2 -tests were used in case of grouped minimum and maximum temperature observations. The data used in the study was from 53 stations with record lengths of 24 and 53 years. All the data were quality-controlled before they were used in any analysis. Results of the quality-control analysis declared most of the temperature records within Tropical Eastern Africa to be homogeneous as most of the mass curves were linear. Instrumental records world-wide have been affected by changes in instrument types,

changes in observational schedules and methods, and changes m the station environment. The inter-annual minimum and maximum temperature patterns showed distinct decadal variability signals in the observed minimum and maximum temperature records. The 1980's decade was generally the warmest in record at most of the locations. The late 1970's and the early 1990's were also relatively warm periods in most areas. These patterns were however reversed at maritime and lake areas. The warming/cooling observed at some locations m recent years introduced significant linear trends when the records were subjected to trend analysis. The observed minimum and maximum temperature trend patterns however showed a lot of geographical and seasonal variations. Some locations did not indicate any significant trend signals. Recurrences of extremely high/low values were the commonest feature of the inter-annual variability patterns of the minimum and temperature records. Results of the spectral analysis showed recurrences of extremely high/low temperature values centred

around four major spectral peaks namely, 2-29 years, 3-4 years, 4.5-6.5 years, and 10- 12.5 years. Few spectral peaks also appeared with longer periods of recurrence. The long-period cycles were however not statistically significant at most of the locations. The 2-2.9 years spectral peak was associated with the QBO whilst the 3-4 and 4.5-6.5 years peaks were associated with the inter-annual variability in ENSO characteristics. The 10-12.5 years cycle was associated with the solar variability cycle. These quasi-periodic oscillations have also been observed in many other Climatic parameters over the region They however explained a very small proportion of the total inter-annual temperature variation. Results of the Simple correlation analysis indicated that both morning and afternoon cloudiness were negatively correlated with the observed minimum and

maximum temperature values and the subsequent temperature range. Maximum temperature values were also found to be significantly correlated with QBO phases. Statistically significant correlations were also found between the minimum and maximum temperature values and the Southern Oscillation Index (SOl), especially for time-lagged temperature values. It may be concluded from this study that the period starting from the 1970's to the early 1990' s was relatively warm at a number of locations in the study region. Relative cooling or no temperature change at all was observed at most maritime locations within the same period. Large variations were however evident in the spacetime characteristics of the warming/cooling trends. The observed warming/cooling trends may have been reflective of inter-decadal temperature variability. To determine whether these trends were actually signals of new inter-decadal variability in the minimum and maximum temperature records, the temperature observations of the 1990's will be central. Although no comprehensive analysis was carried out to delineate urbanisation effects in the study, preliminary examination showed that a larger scale phenomenon may be responsible for the observed minimum and maximum temperature trends. A more elaborate study will however be required to delineate with certainty the role played by urbanisation in the observed warming/cooling signals. The findings of this study are of utmost importance especially at this time when much political, scientific, and socio-economic attention has been focused on human-induced Climate issues. Temperature is an important parameter in the determination of the space-time distribution of natural resources, and life in general. These findings will therefore be useful to planners and managers of food, agriculture, water, socioeconomic, and many other natural resources that are Climate-dependent. The findings will complement the current global search for models that can incorporate climate change and variability at the regional and local levels.