TITLE:
Concurrent Impacts of ENSO and the Dipole Mode Index on Wet and Dry MAM Rainfall Variability in Zanzibar, Tanzania (1981-2024)
AUTHORS:
Mohammed Yussuf Bakar, Zhao Yu, Daudi Ndabagenga, Madundo Albert, Mhenzi Fredy, Silla Abdoul, Msafiri Mtupili, Miraji Khamis
KEYWORDS:
MAM Rainfall, ENSO, Indian Ocean Dipole, Climate Variability, Zanzibar
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.14 No.1,
January
22,
2026
ABSTRACT: This study investigates the concurrent impacts of the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), represented by the Dipole Mode Index (DMI), on March-April-May (MAM) rainfall variability in Zanzibar, Tanzania, during the period 1981-2024. Analysis of Empirical Orthogonal Function (EOF), Concurrent ENSO-DMI phases and atmospheric circulations of wet and dry years during the study period were examined. The results reveal that the first leading mode (EOF1) of rainfall variability accounts for 88% of the total variance and exhibits a spatially coherent pattern across both Unguja and Pemba islands. The associated principal component (PC1) exhibits a strong positive spatial correlation with sea surface temperature anomalies in the Niño 3.4 region (r = 0.7) and a weak positive spatial correlation within the DMI region (r = 0.3), indicating the influence of both the tropical Pacific and Indian Oceans on seasonal rainfall variability. Further analysis of the concurrent ENSO-DMI impacts on wet and dry years shows that, during wet MAM seasons, the concurrent frequency of El Niño and positive IOD phases dominates at 50%, while La Niña and positive IOD co-occur in 33.3%. In contrast, dry years are largely dominated by the concurrence of La Niña and positive IOD, accounting for 71.5% of dry events, which exceeds the contribution from La Niña combined with negative IOD (28.5%). Further analysis of composite atmospheric circulation analyses indicate that enhanced rainfall is linked to low-level cyclonic circulation, moisture convergence, and upward motion. In contrast, the dry conditions correspond to anticyclonic flow, moisture divergence, and subsidence. These findings demonstrate that MAM rainfall variability in Zanzibar is more modulated more by El Niño-positive IOD (La Niña-positive IOD) phases during wet (dry) years, together with regional atmospheric dynamics, underscoring the importance of considering multiple climate drivers for improved seasonal forecasting and climate adaptation planning.