Assessment of the Impacts of Tropical Cyclone Fantala to Tanzania Coastal Line: Case Study of Zanzibar

The study investigated the impacts of tropical cyclone (TC) Fantala (11 to 27 April, 2016) to the coastal areas of Tanzania, Zanzibar in particular. Daily reanalysis data consisting of wind speed, sea level pressure (SLP), sea surface temperatures (SSTs) anomaly, and relative humidity from the National Centres for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) were used to analyze the variation in strength of Fantala as it was approaching the Tanzania coastal line. In addition observed rainfall from Tanzania Meteorological Authority (TMA) at Zanzibar office, Global Forecasting System (GFS) rainfall estimates and satellite images were used to visualize the impacts of tropical cyclone Fantala to Zanzibar. The results revealed that, TC Fantala was associated with deepening/decreasing in SLP (from 1012 1010 mb) around the north-western Madagascar and coastal Tanzania, whereas the mean SSTs was greater than 28 ̊C and an SSTs anomaly ranged from 0 to 2.3 ̊C. The vertical wind shear which ridged at Mozambican Channel and over north-eastern Madagascar was high enough (12 15 ms) to support the intensifying of Fantala. The thermodynamic and dynamic conditions of Fantala influenced heavy rainfall of greater than 170 mm over most stations in Zanzibar. Moreover, Fantala disrupted the temporal variability of 2016 March to May (MAM) seasonal rainfall. Besides, more than 420 people were homeless, at least 3330 houses were destroyed, and about 2 people died. As for mainland Tanzania Fantala resulted in a death of 12 people in Kilimanjaro and Arusha, more than 315 houses were washed away by flooding leading to 13,933 people being homeless. Conclusively the study calls for an extensive research work based on examining and forecasting the TCs rainfall impacts and their contribution during the two rainfall seasons of OND and MAM in Tanzania. How to cite this paper: Kai, K.H., Ngwali, M.K. and Faki, M.M. (2021) Assessment of the Impacts of Tropical Cyclone Fantala to Tanzania Coastal Line: Case Study of Zanzibar. Atmospheric and Climate Sciences, 11, 245-266. https://doi.org/10.4236/acs.2021.112015 Received: October 27, 2020 Accepted: February 23, 2021 Published: February 26, 2021 Copyright © 2021 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
Tropical cyclones (TCs) are among the most devastating natural atmospheric phenomenon which results in damages to property, infrastructure, marine species and even the loss of lives [1] [2] [3]. The Southwestern Indian Ocean (SWIO) basin is among the tropical basins most prone to TCs [4]. These areas have strongest TCs in January and highest frequency of TCs from December to March (DJFM) [5] [6].
In the Western Indian Ocean (WIO), the TCs often lead to catastrophic environmental and socio-economic impacts [7] [8]. For instance, over Mozambique, TC Eline in 2002 TC season, was associated with enhanced flooding with the estimated death toll of up to 700 people, and the livelihoods of about 2 million people were affected [9]. As for the East African (EA) coastal waters, historical records show that, it is not often for the TCs to pass near EA coastal waters [10], and the only cyclones are that occurred at Lindi in 1958 [11] and the other ones occurred in Zanzibar and Bagamoyo since 1872, whereas the last was that occurred in Zanzibar in 1994 [12]. These tropical cyclones had just approached the EA coastal waters, and brought significant impacts to the coast of Tanzania, with devastating impacts in Zanzibar.
The tropical cyclone Fantala on its tropical depression (TD) stage was forecasted to pass EA water and probably hit the Tanzania coastline. However, it turned (recurred) back to the northeastern tip of Madagascar and re-intensified to be severe tropical cyclone Fantala. On 17 th to 18 th April, 2016, Fantala was intensified to be a category 4 tropical cyclone and hence resulted in heavy rainfall, strong winds and waves with devastating impacts to Zanzibar. Since TCs and TSs season over the SWIO is coincided with the rainfall seasons of October to December (OND) and March to May (MAM) of the EA and Tanzania in particular [6]. Indeed, TCs are among the predictors used by IGAD Climate Prediction and Analysis Center (ICPAC) to foresee the likelihood of the OND and MAM seasonal rainfall over the IGAD region. Thus, the strength of TCs is of crucial issues of concern to socio-economic livelihoods of people over this area. Hence analysing the strength, socio-economic and livelihoods impact of Fantala on Tanzania and Zanzibar is of great importance. Currently the impacts of Fantala to EA and Zanzibar in particular are either not yet known or not well documented. Thus, this study aimed to examine the dynamics of Fantala and investigate the strength and magnitude of its impacts on Zanzibar.

Data and Methods
The study used the daily reanalysis data of wind speed, sea level pressure (SLP), sea surface temperatures (SSTs) anomaly, and relative humidity from the Na-  [14], and which affects the genesis and intensification of TCs [15] [16] [17] was derived using the [18] and [3] where, U 200 , U 850 and V 200 , V 850 are the zonal and meridional wind fields at 200 and 850 mb, respectively.
The geographic region of 39˚E -100˚E and 40˚S -5˚N was used to display the daily variation of the derived VWS 28 for the 10 days average before, during and after Fantala. This was done to see the dates and areas of which Fantala was being intensified or diminished based on the fact that, TCs strength is inversely related to VWS 28 [13]. Apart from the VWS 28 strength, the westerly (−VWS 28 ) and easterly (+VWS 28 ) flow variation of the VWS 28 was also observed and analyzed.  Lastly the cost incurred during Fantala was also estimated.

Dynamic and Thermodynamics of Tropical Cyclone Fantala
The results of the analysis of the dynamics and impacts of Fantala to coastal Tanzania Zanzibar in particular show that, the 10 days average SSTs anomaly before Fantala (Figure 1(a)) had higher SSTs anomalies ranged from 0 -2˚C, over the SWIO region, but the 16 days average SST anomaly during Fantala ( Figure 1(b)) showed that, the highest SST anomaly which ranged from 0 -2.5˚C over the SWIO was located at the region defined by 0˚ -5˚S and 50˚E -55˚E. These results indicate that during Fantala most SWIO areas were asso- Atmospheric and Climate Sciences ciated with higher SSTs anomaly, and it should be noted that, higher SSTs anomalies play a vital role in cyclone genesis and intensification as supported by [5] [9] among others. Moreover, the study results of higher SSTs anomalies during Fantala is well agreed by [20] who noted that, higher SSTs acts as a critical parameter necessary to fuel intensification of thunderstorms that may grow into a cyclone. Besides, the results in Figure 1(b) showed the lowest SST anomaly ranged from 0 -1˚C at northern tip of Madagascar. This lowest SSTs anomaly could be explained by upwelling of sea water caused by the spinning of the cyclone at the ocean surface. As for the daily mean variation of SSTs during Fantala (figures were not shown) the results revealed a great increase in daily mean SSTs with a range of 28˚C -32˚C northward of 20˚S, indicating that the area tracked by Fantala had highest SSTs levels (ocean warmth) resulting to higher evaporation potentials as supported by [21] who noted that the warmer SSTs favoring deep convection. Also [22] noted that SST in the source regions is strongly correlated with precipitation in the rainy season, and that oceanic area is defined as a significant source of precipitation when on average more than 20% of the total evaporation, and at least 250 mm/yr of evaporation ends up as continental precipitation. Thus, our finding and that of [21] and [22] indicate that evaporation from the oceans is the primary source of water vapor in the atmosphere i.e. the warmer the water, the greater the evaporation. Furthermore, the results of the 10 days SSTs anomalies after Fantala (Figure 1 Figure 3(a) was mimicked by that of Figure 3(b), where at low levels (surface to 700 mb) moisture column was increasing with height, and limited moisture content was seen at the middle of the atmosphere (i.e. 700 -450 mb), whereas at the top of the atmosphere the moisture was increase with height. As for the vertical moisture distribution on 17 th April, 2016 at points A (red curve) and B (green curve) Figure 3(c), revealed that at both points the moisture was decreasing with height from 925 to 850 mb with A having higher Rh relative to B. Moreover, results in Figure 3(c) show that, at 850 to 700 mb the Rh was slightly increasing with height with further increase from 700 to 600 mb, above this level the Rh for point A was further increased with height from 400 mb onwards, but that of B had constantly lower Rh. The higher moisture distribution at lower level and low moisture at mid level followed by higher moisture at upper level ( Figure 3) indicate the rising motion at the lower levels followed by sinking motion middle levels, and this midlevel drier conditions tends to retard the formation of intense convection which results in TCs [23] associated with the radiative cooling [24]. This moisture variation with both time and height can also explain the variation of strength of Fantala with time as well as the variation of the rainfall impacts as you approaches the coastal areas.
The results of the mean sea level pressure (slp) 10 days before, during and after Fantala Figures 4(a)-(c) revealed that, 10 days before the cyclone ( Figure  4(a)) the coastal areas and the entire Tanzania was under the influence 1012 mb ridge, but during the cyclone a significant fall pressure was notice. For instance, both the northern tip of Madagascar and coastal Tanzanian and her hinterlands was under the influence of the low pressure trough of 1010 mb Figure 4(b) & Figure 4(c). This low pressure trough allow the surface to midlevel/upper level convergence/divergence of air which results in rising (low level)/sinking (upper level) motion [24]. Moreover, the results of low pressure during the TCs presented in Figure 4(b) & Figure 4(c) are supported by [25] [26] that, TCs pressure relationship had predictive utility i.e. the TCs strength are inversely related to pressures strengths. Furthermore, the 10 days average SLP after the storm show that, the ridge which was existed before the cyclone has recovered and hence affect the weather over most parts of the coastal line. This affecting of weather condition was influence by surface divergence associated with upper level confluence.
The results of the distribution of average wind circulation at 850 mb for 10 days before, 16 days during and 10 days after Fantala, presented in ( Figure 5) shows that the wind circulation for 10 days average before Fantala ( Figure 5(a)) was slight along the coastal areas of Tanzania, and was showing weak circulation at north eastern side of Madagascar, but with strong speed at the northern tip of Madagascar. Likewise, weak cyclonic circulation was observed at north eastern side of Madagascar (at about 8˚S -17˚S and 55˚E -75˚E) indicating the development of low level depression/storm around that area. Over coastal Tanzania and hinterlands Figure 5(a) reveals a low level winds were more easterly, but with increasing speed.
The results in Figure 5(a) revealed a weak wind circulation at the north eastern     [30], and [18], whom they conclude that the average VWS 28 threshold which supports TCs development, intensification and dissipation to decaying varies between 10 and 15 ms −1 (depending on the basin) while references [27]; [28] and [30] had gone further and stated that, VWS 28 of less than 10 ms −1 favors rapid intensification and that greater than 10 ms −1 favors decay, thus the results in Figure 6 show that as days go on, the Furthermore, the results in Figure 6(d) revealed that, the 10 ms −1 VWS 28

The Rainfall Impacts of Fantala on Tanzanian Coastal Line
The  Figure 9 shows that, the spatial distribution of rainfall over all investigated stations and for all four seasons was quite good, but the temporal rainfall distribution was very poor in MAM, 2016 with large amount of rainfall records in April and very small amount in March and May. Additionally, the results in Figure 9 revealed that, the rainfall strengths for the 2013-2015 was higher at Makunduchi (Figure 9(b)) and Kilombero (Figure 9(c)) as compared to Zanzibar airport (Figure 9 as well as 850 mb moisture distribution presented in Figure 2(a) and Figure   2(b) and Figure 3, respectively. The results presented in Figure 10

Discussions
Tropical cyclone (TC) has been among the natural weather phenomenon which brings adverse impacts to both Tanzanian coastal lines and its hinterlands.
Though the impacts of TCs to the coastal areas of EA depend on its position, strength and season [35]    search and rescue. Also the need of having plans and algorithms to greatly quantify the total financial losses due to these extreme weather events is not avoided.
Indeed, the forecasting institutions should have to develop their capability to accurately foreseen the TCs counts, strengths and their rainfall contribution to the EA region for combating the disaster prevention and reduction mechanisms.

Conclusion
The study has shown that both the dynamic and thermodynamic potentials of the tropical cyclone result in significant strength of the TC and hence increase the intensity of the impacts to the socio-economic livelihoods and even deaths.
Moreover, the study has shown that TCs based on its position and track affect the rainfall pattern of the specific season (e.g. OND and MAM rainfall seasons).
Hence since strong TCs including Fantala had great impacts on the Tanzania coasts, there is a need to extensively examine the TCs rainfall impacts and their contribution during our OND and MAM seasonal forecasts. Also extensive modeling studies on TCs frequency, tracks and landfalls are of significant importance. Indeed, coast benefit analysis programmes of the associated TCs losses to Tanzania and Zanzibar in particular should be launched.