Assessing the independent and combined impacts of ENSO and IOD on rainfall and paddy production in Sri Lanka during the main cultivating season

Abstract

The El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are among the dominant large-scale climate modes affecting rainfall in south-Asia including Sri Lanka. The Maha season (October to February) is the principal paddy cultivation season in Sri Lanka, and its performance is strongly influenced by inter-annual climate variability. However, the independent and combined influences of ENSO and IOD on seasonal rainfall anomalies and paddy production remain insufficiently quantified. Hence, this study examined the impacts of ENSO and IOD on Maha season rainfall, particularly during the Second Inter-monsoon (SIM; October-November), and the associated paddy production in Sri Lanka. Daily rainfall data from 41 meteorological stations, the Ocean Niño Index (ONI) and Dipole Mode Index (DMI) for identifying ENSO and IOD events, respectively, and district-wise paddy production data from 1979 to 2020 were used. Statistical analyses included one-way ANOVA, cross-tabulation with chi-square tests, and one-sample t-tests (p=0.05). El Niño years associated with IOD-positive years exhibited approximately 85% of higher positive deviations in mean cumulative rainfall compared to independent El Niño years (without association with IOD years). Correspondingly, Maha season paddy production increased during El Niño years associated with IOD-positive years than during independent El Niño years in all selected paddy growing districts. Both La Niña associated with IOD-negative years and independent La Niña years (without association with IOD years) exhibited reduced SIM rainfall, with no substantial differences between them. Accordingly, Maha season paddy production showed distinct negative anomalies. The IOD-positive years associated with El Niño years significantly enhanced SIM rainfall. The only independent IOD-positive event (without association with ENSO years) during the study period, which occurred in the 2019/2020 Maha season, showed an apparent increase in SIM rainfall and resulted in the highest recorded total paddy production of 3.19 million metric tons. In contrast, both IOD-negative years associated with La Niña and independent IOD-negative years (without association with ENSO years) have led to reduction in paddy production due to suppressed SIM rainfall.  However, the occurrence of independent IOD-negative events during SIM were very rare to draw robust conclusions. Overall, the findings demonstrate a clear synergistic influence of ENSO and IOD on the rainfall variability of the main cultivation season (Maha) and paddy production in Sri Lanka, underscoring its importance of seasonal agricultural planning and policy formulations.