Browsing by Author "Mahongo, Shigalla B."

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    Analysis of Rainfall Variations and Trends in Coastal Tanzania
    (Western Indian Ocean Journal of Marine Science, 2013-09) Mahongo, Shigalla B.; Francis, Julius
    Rainfall in coastal Tanzania has been investigated for their variability and trends from seven key weather stations during the last 50 years (1960-2009). Results indicate that the island of Mafia receives the highest amount of rainfall (1879 mm/yr) while Kilwa Masoko receives the lowest (1029 mm/yr). Generally, precipitation in the islands is heavier than on the mainland coast due to moisture convergence in sea breezes. The monthly series indicate predominance of annual and semi-annual oscillations in the northern sector, while in the southern coast the annual signal is the most dominant. Partial correlation analysis revealed a significant influence of the El-Niño Southern Oscillation (ENSO) on annual rainfall. However, the effects of the Pacific Decadal Oscillation (PDO) and the Indian Ocean Dipole (IOD) are relatively smaller. In the seasonal timescale, the effects of the large scale climatic phenomena are relatively smaller during the rainfall and Northeast (NE) monsoon seasons, but are significantly larger during the Southeast (SE) monsoon. Linear regressions of the monthly rainfall series portray a general downfall trend at each station, but all the trends are insignificant. The inter-annual and decadal variations show that the year 2003 and the last decade (2000-2009) were generally the driest in the last half century.
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    Coastal resources and their use. In: Eastern Africa Atlas of Coastal Resources
    (UNEP, 2001-01) Francis, Julius; Mahongo, Shigalla B.; Semesi, A.; Muhando, Christopher A; Mgaya, Yunus D.; Daffa, J.; Ngoile, Magnus
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    Eastern Africa Atlas of Coastal Resources: Tanzania
    (2001) Francis, Julius; Mahongo, Shigalla B.; Semesi, A. K.; Muhando, Christopher A.; Mgaya, Yunus D.; Daffa, J.; Ngoile, Magnus
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    Investigating the Effect of Winds and Storms on Shoreline Erosion along the Coast of Tanzania
    (David publishing, 2013) Mahongo, Shigalla B.; Shaghude, Yohanna W.
    Abstract: Wind data spanning 28 years (1977-2004) from four meteorological stations (Tanga, Zanzibar, Dar es Salaam and Mtwara) were analyzed to elucidate the effect of winds on coastal erosion along the coast of Tanzania. The investigation entailed wavelet analysis of the time series of winds which were related with past erosion events at each site. The study also analyzed qualitatively the incidences of tropical storms in relation to coastal erosion events along the coast. The results revealed that most of the critical erosion incidents in the past few decades were associated with periods of occurrence of extreme wind speeds and tropical storms. Furthermore, the results indicated that the wind speeds and the frequency and intensity of tropical storms had generally been increasing during the past three decades. The findings were consistent to those recently reported for the coast of Dar es Salaam. The increasing trends of winds and storms are considered to have direct impact on the wave climate with increased risk of shoreline wave-induced erosion that is already affecting relatively many parts of Tanzania
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    Modelling the dynamics of the Tanzanian coastal waters
    (Tanzania Fisheries Research Institute, 2013) Mahongo, Shigalla B.; Shaghude, Yohanna W.
    A numerical modeling study was carried out using the Regional Ocean Modeling System (ROMS) for the Tanzanian coast to investigate the seasonal dynamics of water circulation, temperature and salinity. The model results indicated the presence of an eddy on the surface that had been developed during the Northeast (NE) monsoon and which has not been documented previously. The study also revealed that, the core of the East African Coastal Current (EACC) passes adjacent to the coast, just off the three major islands of Pemba, Zanzibar, and Mafia. There are localized patches of strong currents parallel and adjacent to the mainland coast, with magnitudes that are influenced by the coastline configuration, bottom topography and the extent of exposure to the main stream of the EACC. The current speeds along the coast of Tanzania are lowest in February and March, and highest in July, August, and November but generally not exceeding 1 ms-1. Surface salinities generally vary between 34.8 and 35.5, whereas surface temperatures range from a minimum of 25.0°C to a maximum of 30.2°C. The modelled salinity and temperature profiles are similar to those observed from field observations of previous investigations
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    Monthly Variations in Sea Level at the Island of Zanzibar
    (Western Indian Ocean Journal of Marine Science, 2010-01) Mahongo, Shigalla B.; Francis, Julius
    Meteorological and tide gauge data were used to analyze correlations between climatic parameters and variations in mean sea level at Zanzibar for the period 1985-2004. This involved spectral and multiple regression analysis of the monthly variables, as well as harmonic analysis of hourly sea level. Air pressure and rainfall remained relatively constant during the 20-year study period, but there were trends in sea level, northeast winds, southeast winds and air temperature. Monthly variations in mean sea level, composed predominantly of semi-annual, annual and 4-year oscillations, were represented by the steric effect proxies of rainfall and air temperature (45%), southeast and northeast monsoon winds (41%), and air pressure (5%). The trend in sea level (9%) appeared to be mainly correlated with northeast winds. The annual cycle in sea level (36%) was represented to a certain degree by rainfall (11%), air temperature (10%), southeast winds (8%) and northeast winds (7%). The semi-annual component (28%) was best represented by southeast winds (15%), with the remaining 13% of the variability being equally represented by rainfall, northeast winds and air pressure. The 4-year oscillations, which accounted for about 27% of the variation in sea level, were mainly represented by air temperature (12%), rainfall (8%) and southeast winds (6%). There is a strong likelihood that physical processes other than meteorology and tides influenced the observed variations in sea level, especially in the 4-year cycle.
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    Wind patterns of coastal Tanzania: Their variability and trends
    (Western Indian Ocean Journal of Marine Science, 2012-10) Mahongo, Shigalla B.; Francis, Julius; Osima, S. E.
    Patterns in Tanzanian coastal winds were investigated in terms of their variability at the weather stations of Tanga, Zanzibar, Dar es Salaam and Mtwara. Three-hourly data collected over a 30-year period (1977-2006) were used for the study. Statistical analyses included regressions, correlations, spectral analysis, wavelet analysis and partial correlations. Among the four stations, Mtwara proved to have the strongest winds, and Dar es Salaam the weakest. The study confirmed that NE winds prevail from November to March along the coast of Tanzania and SE winds from April to October. The monthly means were mainly composed of annual and semi-annual signals, as well as low frequency oscillations of different wavelength. Generally, the wind speeds were significantly correlated with the El-Niño Southern Oscillation and the Pacific Decadal Oscillation, while at Mtwara the winds were also correlated with the Indian Ocean Dipole. These correlations were higher during the SE Monsoon than during the NE Monsoon. Trends in the monthly mean and maximum wind speeds indicated that the winds have generally strengthened over the past three decades, the corresponding rates of increase being about 0.04-0.07 and 0.03-0.08 m.s-1.y-1respectively, except for the monthly maximum speeds at Mtwara which declined at a rate of ~0.02. The maximum wind speed at Dar es Salaam remained almost unchanged.