Abstract
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 Application 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, these values were second ever-recorded highest OND rainfall since 1916 to 2019 where the first one was observed in 1961. Furthermore, the results revealed that 2019 OND seasonal rainfall had the highest amount of contribution based on historical climatology. For instance, the 2019 OND rainfall for Kisauni, Pemba airport and Matangatuani contributed to 198%, 303% and 231% of the long term (1987-2016) 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 surrounding 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.
Highlights
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 results show the existence of Madden Julian Oscillation (MJO) and strong positive Indian Ocean Dipole (IOD) over the equatorial Indian Ocean and over the Southwestern Indian Ocean (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 Outgoing Longwave Radiation (OLR) (Figure 2 right panel) depicts a more progressive MJO event with enhanced convection shifting over the maritime continent, and suppressed convection developed across Africa and the western Indian Ocean (IO) on the second week of October, 2019, while for November, 2019 the MJO was in its weak phase over the Indian Ocean
Summary
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. 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]. Reference [5] noted that the monthly and seasonal OLR over coastal Tanzania, from 1982-2011/12 was characterized by OLR deep convection cells of less than 200 - 250 Wm−2, whereas during Tropical Cyclone (TC) the deep convection cells falls to less than 120 Wm−2 and a net change of ±15 Wm−2 over coastal Tanzania indicating that ocean releases more energy to evaporation, resulting into deep convective clouds over areas characterised by deep OLR values [9]
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