Wind Patterns of Coastal Tanzania: Their Variability and Trends

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-Ni o 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.