An Assessment of the Seasonal Rainfall and Its Societal Implications in Zanzibar Islands during the Season of October to December, 2019

The assessment of the performance of the October to December (OND), 2019 rainfall season in Zanzibar (Unguja and Pemba) with reference to local downscaled Tanzania Meteorological Authority (TMA) forecast, and regional (Intergovernmental Authority on Development Climate Prediction and Ap-plication Center (IGAD-ICPAC) weather forecasts were assessed by comparing the long term average of OND rainfall data and previous OND rainfall seasons of 2016, 2017 and 2018 as well as extreme positive Indian Ocean Dipole (IOD) during OND seasons of 1961, 1994, 1997, 2006 and 2019 for Zanzibar. The study assessed zonal (u) and meridional (v) winds at 850 and 200 mb, monthly and dekadal sea surface temperature (SST); the Madden Julien Oscillations (MJO) forecast reports and the ocean heat content data. Both gridded and observed datasets were processed into dekadal, monthly and seasonal averages and then analysed. The results revealed that, based on the observations, above normal rainfall of 936 and 908 mm were reported at stations of Kisauni (Unguja) and Karume airport (Pemba) during 2019 OND season. This amount was the first and second ever recorded for the extreme positive IOD during OND seasons of 1961, 1994, 1997, 2006 and 2019, and also the first for the previous higher OND rainfall seasons of 2016, 2017 and 2018 which was highly variable. Moreover, mean OND rainfall in Zanzibar. Indeed, the results show that the presence of the MJO during OND and the enhanced positive IOD was among the causes for the observed wetness of the 2019 OND in Zanzibar and most parts of the country. Moreover, the dominant easterly, southeasterly and northwesterly onshore winds during 2019 OND also contributed to heavy rainfall. The monthly rainfall variability among stations had the highest amount of rainfall which ranges from 400 to 500 mm which was observed during October in Kisauni and Karume airport, while the lowest amount ranging from 150 to 180 mm was observed during November in Matangatuani and the surround-ing stations. Based on the comparison of the forecasted reports issued by ICPAC and TMA, the results revealed that irrespective of not considering the likelihood of occurrence of MJO and strong positive IOD both forecasts has performed well with that of ICPAC being leading. Conclusively, since the 2019 OND season has been uniquely characterized by the presence of MJO and IOD polarities it would be worthful to consider the two as input parameters during the OND rainfall forecast over the region.


Introduction
Zanzibar is among the East African country which largely depends on rain fed agricultural activities for economic growth and as a way to alleviate poverty. The variability of rainfall has a great impact on the agricultural planning and water resources management [1]. Climatically, Zanzibar lies under the tropical climate with two regimes of rainfall patterns i.e. long rains season of March to May (MAM) and short rains season of October to December (OND) and characterized with heavier precipitation records than on the mainland coast due to moisture convergence in sea breezes [2]. The long rain season in the islands is characterized by abundant rainfall with good temporal and spatial coverage, while during the short rain season are characterized by limited rainfall with poor spatial and temporal coverage [3] [4] [5]. Normally the short rains are associated with the northeast monsoon flow, while the long rains are dominated by southerly to southwesterly and southeasterly monsoon flow [4] [5] [6]. The seasonal and annual rainfall observed in the world (Zanzibar in particular) has resulted from Ocean/atmosphere interaction [7] [8] through exchanging mass (including gases, aerosols, sea spray, and water), momentum (through wind stress and evaporation), and energy (including heat) at the sea water and air interface i.e. atmosphere impacts the ocean mainly through the exchange of momentum, net surface heat flux and freshwater flux. For instance, reference [9] has noted that almost half of the absorbed solar energy at the sea surface is released back to the atmosphere in form of OLR. Also, reference [5] noted that the monthly and sea-  [9]. The short rainfall season over the East Africa (EA) is normally affected by the influence of the positive polarity of the Indian Ocean Dipole (IOD) [10] [11] which leads to heavy rainfall over the coast of EA. IOD is an anomalous SST gradient between the western equatorial Indian Ocean (50˚E -70˚E and 10˚S -10˚N) and the southeastern equatorial Indian Ocean (90˚E -110˚E and 10˚S -0˚N). Besides, studies including [12] have shown that OND have a larger degree of interannual variability relative to climatology than have the MAM rains. Other phenomenon, which impacts OND rainfalls in Zanzibar, is the El Niño (warm ENSO) and La Nina (cold ENSO) events [13]. During the warm phase (El Niño), Zanzibar appears to be associated with above average rainfall while during La Niña Zanzibar and entire northern coastal of Tanzanian are associated with below average rainfall [13].
On the other hand, from November to March (DJFM) the climate of Zanzibar is influenced by the Southwestern Indian Ocean (SWIO) Tropical Cyclones (TCs) which either enhance or decline rainfall over the region [5] [6] [14] [15]. For instance, tropical cyclone Izilda (28 th March, 2009) and Idai (4 th -21 st March, 2019) dried the entire coastal strip [5] [16], while tropical cyclone Bondo (15 th -28 th December, 2006) and Fobane (5 th -15 th February, 2014) enhance the rainfall over the entire country [5]. Over the SWIO basin TCs/TSs has a mean frequency of 4.5/2.8 per season [5] [17] and the mean daily rainfall associated to TCs/TSs over Tanzania ranged from are 2.0 mm/day to 6.5 mm/day in Tanga and Mtwara/Mbeya [5]. The seasonal long-term average maximum temperature of Zanzibar is about 33.3˚C during December to February (DJF) and its average minimum temperature is about 29.5˚C during June to September [5] [18]. As for November, the long-term average maximum and minimum temperature is 31.0˚C and 22.7˚C for Unguja and 30.9˚C and 23.1˚C for Pemba, while the average total rainfall is about 202.5 mm at Unguja and 169.29 mm at Pemba. As for November, the long term average maximum and minimum temperature is 31.0˚C and 22.7˚C for Unguja and 30.9˚C and 23.1˚C for Pemba, while the average total rainfall is about 202.5 mm and 169.29 mm at Unguja and Pemba [18].

Regional and Local Forecasts for the 2019 OND Seasonal Rainfall
The regional IGAD Climate Prediction and Analysis Center (ICPAC) revealed the consensus forecast for the 2019 OND rainfall season that, Tanzania and its island states (Unguja and Pemba) to be characterized by normal to above normal rainfall (Figure 1(b)). The forecast was characterized with higher chance of wetter conditions in most of the equatorial and southern sectors including Tan-  [19]. Also, the forecast indicated on the increased likelihood of drier than average rainfall during September over southeastern and north-eastern Ethiopia, eastern Kenya, southern Somalia, western Tanzania and central Sudan [19]. Besides, earlier than normal onset of OND rains was forecasted across the western sector including Tanzania, Burundi, Rwanda Uganda, and western Kenya. Moreover, the likelihood of episodic rainfall events leading to flash floods over areas with increased likelihood of near to below normal rainfall was foreseen. Temperature out breaks was forecasted to be above average particularly, over eastern Tanzania, eastern Kenya, Ethiopia, Somalia, Eritrea and Sudan indicating increased likelihood of warmer conditions [19].
On the other hand, Tanzania Meteorological Authority (TMA) downscaled rainfall forecast for 2019 OND (Figure 1(a)), The results showed that Zanzibar (Unguja and Pemba) was forecasted to have normal to below normal rainfall characterised by poor spatial coverage [20], with the onset and cessation of second week of October and fourth week of December, 2019. The forecast revealed the presence of long dry spells over the northern coast areas and normal rains on the second week of November [20]. The TMA downscaled forecast was based on existence of above normal SST over the central Indian Ocean and Southwestern Indian Ocean and northwestern areas over the Somali coast during current OND which were expected to bring the offshore winds from the coastal. Also, the below normal SSTs over the southeast Atlantic Ocean (near Angola coast) were expected to decline rainfall over the northern coast (including Zanzibar) and the northeastern high lands of Tanzania, and enhance the westerly flow from Congo air mass which resulted into rainfall activities over western areas of the country [20].

Study Site
Zanzibar is among the United Republic of Tanzania

Datasets
For seeking the solutions to the aforementioned specific objectives, the study  (u) and meridional (v) and surface winds to assess the flow of weather systems during October to December, 2019. The extended reconstructed sea surface temperature (SSTs) anomalies were used to show the monthly positional variation of strong positive polarity of the Indian Ocean Dipole (IOD) for each month of the OND, 2019 season, also the dekadal SST variability product (and reports) was freely downloaded from the operational SST Anomaly Charts of the NOAA/NESDIS. Moreover, the MJO monitoring products were freely downloaded from Australian Bureau of Meteorology (BOM) and the Outgoing Longwave Radiation (OLR) variability for this period was obtained from NOAA CPC and NCEP. The OLR was produced from the interpolated data provided by the NOAA/OAR/ESRL and was used to forecast the Madden Julian Oscillation (MJO) status using CA model such that the OLR is often used as a way to identify tall, thick, convective rain clouds (CPC, 2019). Also, the NOAA High resolution (0.25˚ × 0.25˚) optimal interpolated SST Version 2 anomalies [23] were used. Moreover, the ocean heat content data was also used; this data was used to understand which areas of the SWIO region have higher potential to ocean evaporation. Moreover, monthly and daily observed rainfall and maximum and minimum temperature records from most stations in Zanzibar was acquired from TMA offices of Unguja and Pemba airports, as well as Matangatuani agro meteorological station in Pemba. The datasets were used to feature the dekadal wind flow patterns during the 2019 OND season.

Methods
The zonal (u) and meridional (v) winds data was calculated into ten days average (dekad) for the months of October to December, 2019 and the vector maps were displayed and analysed to observe strong and weak convergence areas and the mean direction of the wind at 850, 500 and 200 mb in different land (Tanzania and her neighborhoods) and oceanic (SWIO)) areas. The dekadal and monthly variability of SSTs anomalies along the SWIO region defined by 28.5˚E to 100˚E and 5˚N to 40˚S were calculated and displayed for observing the dekadal and monthly variations of the strong positive polarity of IOD. Also the dekadal variability of operational SSTs anomaly charts produced by NOAA CPC was used to observe the variation of SSTs along the SWIO region. Moreover, the monthly variability of the ocean heat content was also calculated to identify areas and direction of high heat content (low pressure and high evaporation) and low heat content (high pressure and low evaporation areas). The forecasted maps and reports of MJO and OLR developed by NOAA CPC for different phases during October to December, 2019 were used. Additionally, TMA station observation data of rainfall, maximum and minimum temperatures as well as gridded 10 m wind during October to December, 2019 were collected and calculated into dekadal, monthly and seasonal averages and then compared with the long-term average data. Using these averages, the station percentage contributions based on long term total or mean records were calculated. These data were used to

Weather System Which Lead to Uniqueness of 2019 OND Season
The results show the existence of MJO and strong positive IOD over the equatorial Indian Ocean and over the SWIO region, which enhanced the strength and the distribution of the OND rainfall for the first two weeks of October and weakened in the third week, and slightly strengthened in the fourth weak. The constructed analog of MJO using OLR (

The Outcome of October, November and December, 2019 Rainfall
In assessing the pertained weather for October, 2019 the outcome of the aforementioned weather systems presented in Figure 8  Similar to October, the November rainfall for all three dekads was higher at Unguja than at Pemba Figure 8 (middle panel). Moreover, Figure 8    The results of rainfall distribution presented in Figure 8 (bottom panel) shows that the three dekads of December, 2019 had rainfall ranging from 20 to 190 mm at all investigated stations, and during the first dekad rainfall in Unguja was higher compared to Pemba, while during the third one Karume Airport had the highest rainfall of about 186 mm. Similar result holds for the distribution of the monthly total rainfall, which ranged from 107 to 275 mm with the highest downpour records in Paje and Karume airport. Unlike November, December, 2019 rainfall distribution was not affected by east west orientation, and it was 4.2%, 5.2% and 10.3% of the December long term (1987-2016) total rainfall or 126%, 156% and 301% of the December long term mean for Kisauni, Matangatuani and Karume Airport, respectively. The presented results indicate that 2019 OND rainfall was stronger compared to the previous OND records.
Generally, the analysis of the dekadal and monthly rainfall observations presented in all panels of Figure 8 for Zanzibar revealed that unlike most previous OND seasons, the 2019 OND season was marked as the second in history and being characterized by unique features of heavy rainfall and good spatial distribution country wide. Also, of the three months (October, November and December) the highest rainfall was observed in October and lowest during December.

Maximum and Minimum Temperature Analysis
The results of maximum and minimum temperature based on the climatology  show that the November, 2019 mean maximum and minimum temperatures for Unguja was higher by 0.07˚C and 1.88˚C to that of climatology, while the minimum temperature for Matangatuani was higher by 1.14˚C than that of climatology. These results indicate that the current climate behaves to be warmer than the past. Also, the November, 2019 maximum and minimum temperature anomalies presented in Figure 9 (top panel) revealed the minimum temperatures are increasing at the highest rates than the maximum temperatures with that of Pemba being steeper than the rest. For instance, the minimum temperature of Pemba and Unguja show an increasing trend of 0.08˚C and 0.06˚C while that of maximum is 0.04˚C (Figure 9 top panel) indicating an increased frequency of warmer nights compared to the pristine days. Based on the same climatology, the December, 2019 mean maximum and minimum temperatures for Unguja was higher by 0.04˚C and 1.23˚C to that of the reference climatology, while the minimum temperature for Matangatuani Pemba was higher by 0.8˚C than that of the reference climatology. These results indicate that the current climate experience warmer nights compared to the past (1987-2016). Based on the same reference climatology, the December, 2019 maximum and minimum temperature anomalies Figure 9 (lower panel) revealed that minimum temperatures are increasing at the highest rates than the maximum with that of Pemba being steeper (0.06˚C) and 0.02˚C at Matangatuani and Karume airport, while that of Unguja was 0.03˚C/day. As for maximum temperature their trends were positive 0.002˚C for Unguja and negative, and −0.02˚C in Pemba.

The Performance of the 2019 OND Rainfall
The analysis of the performance of the 2019 OND rainfall was based on 1) TMA and 2) ICPAC forecasts for long term OND rainfall Atmospheric and Climate Sciences vect more moisture from adjoin oceanic area towards land which enhances rainfall. These onshore linearly converged winds relation to OND rainfall agrees with the previous studies [19], which noted that zonal winds show the strongest and most consistent relationships with the OND rainfall over East African; and the great role of zonal winds during wet OND periods. As for monthly rainfall comparison during the season, Figure 9 shows that October was having the highest contribution of rainfall amount ranging between 400 -500 mm, while November and December had low contribution of about 150 -280 mm. Moreover, Figure 9 ( On comparing the OND rainfall forecast outcomes for Zanzibar and northern coast of Tanzania the results show that irrespective of not considering the likelihood of the occurrence of the MJO and strong positive IOD, but the ICPAC consensus forecast for the northern coast (including the Zanzibar islands) was more accurate compared to the downscaled TMA forecast.

Societal Impacts of the 2019 OND Rainfall
The 2019 OND, rainfall was exceptional in intensity, spatial and temporal distribution. The two existing rainfall bringing mechanisms of MJO and strong positive polarity of IOD enhanced the October rainfall and resulted into number of catastrophic events. For instance, in Zanzibar town the floods associated with these rains forced significant number of people to migrate their houses, some local bridges were washed away and transportation was for some times stranded. Two peoples were reported dead with several injured at Mikunguni (area where the construction of underground water tunnel is going on). Moreover, number of ponds like Ziwa Maboga, Bint Amran among others, were flooded and hence hindering many socioeconomic activities. Some of the areas that were severely impacted in urban and town areas of Zanzibar are Ziwa Mboga, Amani sebuleni; Jang'ombe, Bint Amran among others were flooded, hence forcing the societies living nearby these areas to leave their houses. The unpublished records of the impacted societies in urban and town areas of Zanzibar collected from the Disaster Commission of the Revolutionary Government of Zanzibar (R o GZ) revealed that about 500 houses were affected, and 300 families were forced to leave Irrespective of having twine cyclones early December, but these severe events did not have strong catastrophic events, over Zanzibar but sporadic events of heavy rain and strong winds which brought significant number of damages (e.g. washing away of bridges in Morogoro, Dodoma among others) which were recorded over the country.

Discussion
Normally the northern coastal areas including Zanzibar and the northeastern highlands of Tanzania are characterized by two rainfall regimes of long rains during MAM and short rain during OND [3]. Most OND seasonal rainfall over most areas of EA is characterized by strong variability with poorly spatial distribution, weak intensity, and heavy down pool over few areas as well as late onset and early cessation. Unlike most OND seasons, the 2019 OND had been unique in spatial distribution, intensity and onset and cessation. The season was associated with well-distributed rainfall in most areas in Zanzibar and Tanzania [26] where their references noted that EA OND rainfall is highly correlated with the Indian Ocean Zonal Mode (IOZM), which was derived, from the IOD indices. Furthermore, the results agreed well with the previous study, which revealed that unstable moisture advection over the eastern boundary of EA, is only during MAM and OND rainfall seasons [27]. Besides, the study finds that, the performance of 2019 OND season was enhanced by the existence of the MJO during its onset as agreed by [13]. Reference [13] noted that an early onset of MAM/OND, that early MJO peak enhances extreme wetness/dryness, while the late MJO peak corresponds to moderate dryness/wetness. Also, the high performance of 2019 OND season was enhanced by the occurrence of the early December, 2019 twine tropical cyclones activities enhanced the performance of the 2019 OND rainfall in Zanzibar and Tanzania at large. The study results had been well agreed with reference [28] which noted that days with a westerly moisture flow, bringing the Congo airmass, enhance rainfall by up to 100% above the daily mean, and these days are more likely during phases 3 and 4 of the MJO and when there are one or more tropical cyclones. The presented results had revealed that for the last short rain seasons the regional ICPAC and TMA forecast reports were significant correct based on the fact that what has been forecasted has been highly observed. trend of 0.08˚C/day and 0.06˚C/day respectively, while that of maximum is 0.04˚C/day indicating an increased frequency of warmer nights compared to the pristine days. Indeed, the results have shown that due to strong negative association between the OND rainfall and temperature, the increased rainfall reduces the strength of both maximum and minimum temperatures intensity over most areas in Zanzibar.

Conclusions
The study concludes that: 1) The onset of the OND, 2019 season was peculiar to its characteristics and impacts due to weather systems (e.g. MJO and positive IOD) controlling its onset and its performance, and the season resembled that of 2006, 2008 among other, though they were not strong as the 2019 OND season.
2) The need of special consideration of research work should be conducted to understand the influence of the MJO mixed with IOD, tropical cyclones and thermodynamics of the southern Indian Ocean adjacent to Southwestern Indian Ocean (SWIO) region to the spatial and temporal distribution of OND rainfall over Tanzania.
3) The operational forecasting and research works should deeply engage with finding other inputs (factors or predictors) which can enhance the performance of the OND rainfall in Tanzania based on the onset, intensity and distribution (spatial and temporal).