Abstract <p>Rainfall during boreal fall (September-November: SON) is crucial for water resources and flood risk in Sri Lanka (SL), yet it shows significant interannual variability driven by large-scale tropical climate modes, particularly the Indian Ocean Dipole (IOD) and the El Niño–Southern Oscillation (ENSO). Despite their recognised importance, the distinct influences of IOD & ENSO on mean and extreme rainfall over SL remain insufficiently understood. This study examines the combined and independent influences of IOD and ENSO on mean rainfall (Rmean) and heavy rainfall days (R10mm) during SON for 1981–2023 using partial correlation and regression analyses to isolate their respective effects, together with Generalised Extreme Value (GEV) theory to assess the statistical representation of R10mm. The analyses reveal that IOD has dominant independent influences on Southwest SL rainfall, ENSO predominantly affects West SL, and co-occurring IOD-ENSO affects both regions. These relationships come from condition-dependent atmospheric mechanisms, with combined positive IOD–ENSO conditions associated with stronger circulation, moisture transport and convergence, and convective activity than the independent forcing of either mode. Both Rmean and R10mm show increasing tendencies during the study period, particularly in the southwest and west SL. The GEV analysis further shows that non-stationary models, including climate drivers, better represent R10mm in SL than stationary models. These findings improve understanding of SL regional rainfall variability, which is associated with climate drivers, and provide useful insights for seasonal prediction, flood risk assessment, and climate resilience planning in SL.</p> Graphical Abstract <p>This graphical abstract illustrates the physical mechanisms through which the Indian Ocean Dipole (IOD) and the El Niño–Southern Oscillation (ENSO) regulate boreal fall (September-November: SON) mean rainfall (Rmean) and heavy rainfall days (R10mm) over Sri Lanka (SL). The schematic explains how the positive phases of IOD (+IOD) and ENSO (El Niño) shape regional Rmean and R10mm over SL by influencing the large-scale atmospheric circulation, moisture transport, and convective activity. Arrow and cloud sizes qualitatively represent the relative strength of atmospheric forcing and rainfall intensity. The larger arrows and clouds indicate stronger circulation anomalies, enhanced moisture convergence, and heavier rainfall. When positive IOD (+IOD: Dipole Mode Index, DMI <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\ge \)</EquationSource> </InlineEquation> +0.4) and El Niño (Niño3.4 index <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\ge \)</EquationSource> </InlineEquation> +0.5) conditions co-occur, their effects reinforce each other, leading to the strongest low-level moisture transport toward SL. This combined condition enhances the vertically integrated moisture convergence (VIMC) and atmospheric specific humidity (SH), reduces outgoing longwave radiation (OLR), and intensifies deep convection over SL. This combined forcing produces the most significant increases in both Rmean and R10mm over Southwest and West SL. During the independent +IOD events (DMI <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\ge \)</EquationSource> </InlineEquation> +0.4), circulation and moisture anomalies are comparatively weaker than the combined condition, which is spatially confined, and primarily enhances convection and rainfall over Southwest SL. In contrast, independent El Niño events (Niño3.4 index <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\ge \)</EquationSource> </InlineEquation> +0.5) produce comparatively weaker circulation anomalies than independent +IOD events, and mainly influence West SL. Negative phases of IOD (-IOD: DMI <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\le \)</EquationSource> </InlineEquation> -0.4) and ENSO (La Niña: Niño3.4 index <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\le \)</EquationSource> </InlineEquation> -0.5) are not indicated in the graphical abstract because they generally have a link with opposite circulation and moisture transports, which lead to reduced convection and rainfall over these SL regions. Therefore, the graphical abstract only shows the dominant rainfall-enhancing mechanisms associated with +IOD and El Niño conditions, which are most relevant for understanding SL rainfall variability and extremes during boreal fall.</p>

Distinct Roles of IOD and ENSO in Shaping Mean and Extreme Rainfall over Sri Lanka During Boreal Fall

期刊论文
发表日期：

2026年5月12日

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Tishan Thambipillai

摘要

Abstract

Rainfall during boreal fall (September-November: SON) is crucial for water resources and flood risk in Sri Lanka (SL), yet it shows significant interannual variability driven by large-scale tropical climate modes, particularly the Indian Ocean Dipole (IOD) and the El Niño–Southern Oscillation (ENSO). Despite their recognised importance, the distinct influences of IOD & ENSO on mean and extreme rainfall over SL remain insufficiently understood. This study examines the combined and independent influences of IOD and ENSO on mean rainfall (Rmean) and heavy rainfall days (R10mm) during SON for 1981–2023 using partial correlation and regression analyses to isolate their respective effects, together with Generalised Extreme Value (GEV) theory to assess the statistical representation of R10mm. The analyses reveal that IOD has dominant independent influences on Southwest SL rainfall, ENSO predominantly affects West SL, and co-occurring IOD-ENSO affects both regions. These relationships come from condition-dependent atmospheric mechanisms, with combined positive IOD–ENSO conditions associated with stronger circulation, moisture transport and convergence, and convective activity than the independent forcing of either mode. Both Rmean and R10mm show increasing tendencies during the study period, particularly in the southwest and west SL. The GEV analysis further shows that non-stationary models, including climate drivers, better represent R10mm in SL than stationary models. These findings improve understanding of SL regional rainfall variability, which is associated with climate drivers, and provide useful insights for seasonal prediction, flood risk assessment, and climate resilience planning in SL.

Graphical Abstract

This graphical abstract illustrates the physical mechanisms through which the Indian Ocean Dipole (IOD) and the El Niño–Southern Oscillation (ENSO) regulate boreal fall (September-November: SON) mean rainfall (Rmean) and heavy rainfall days (R10mm) over Sri Lanka (SL). The schematic explains how the positive phases of IOD (+IOD) and ENSO (El Niño) shape regional Rmean and R10mm over SL by influencing the large-scale atmospheric circulation, moisture transport, and convective activity. Arrow and cloud sizes qualitatively represent the relative strength of atmospheric forcing and rainfall intensity. The larger arrows and clouds indicate stronger circulation anomalies, enhanced moisture convergence, and heavier rainfall. When positive IOD (+IOD: Dipole Mode Index, DMI \(\ge \) +0.4) and El Niño (Niño3.4 index \(\ge \) +0.5) conditions co-occur, their effects reinforce each other, leading to the strongest low-level moisture transport toward SL. This combined condition enhances the vertically integrated moisture convergence (VIMC) and atmospheric specific humidity (SH), reduces outgoing longwave radiation (OLR), and intensifies deep convection over SL. This combined forcing produces the most significant increases in both Rmean and R10mm over Southwest and West SL. During the independent +IOD events (DMI \(\ge \) +0.4), circulation and moisture anomalies are comparatively weaker than the combined condition, which is spatially confined, and primarily enhances convection and rainfall over Southwest SL. In contrast, independent El Niño events (Niño3.4 index \(\ge \) +0.5) produce comparatively weaker circulation anomalies than independent +IOD events, and mainly influence West SL. Negative phases of IOD (-IOD: DMI \(\le \) -0.4) and ENSO (La Niña: Niño3.4 index \(\le \) -0.5) are not indicated in the graphical abstract because they generally have a link with opposite circulation and moisture transports, which lead to reduced convection and rainfall over these SL regions. Therefore, the graphical abstract only shows the dominant rainfall-enhancing mechanisms associated with +IOD and El Niño conditions, which are most relevant for understanding SL rainfall variability and extremes during boreal fall.