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<p>The Southwest Indian Ocean (SWIO) is an area accounting for about 11% of global tropical cyclone (TC) activities, shaping regional weather patterns across southern Africa. The present study examined the characteristics of SWIO tropical cyclone frequency (TCF) and its relationship to East African rainfall, focusing on how preceding January-February-March (JFM) TCF influences subsequent austral autumn March-April-May (MAM) rainfall using statistical techniques. Furthermore, the potential for TCF to improve the predictability of rainfall is explored using the regression statistical model. The study covers 1981–2021, employing tropical cyclone data from IBTrACS v4, monthly gridded precipitation from CRU TS4.05, atmospheric circulation fields from the ERA5 reanalysis, and sea surface temperature data from NOAA ERSST v5. Lead–lag correlation analysis between the JFM TCF index and East African rainfall, around the equatorial area, identifies statistically significant correlations during MAM that persist after El Niño-Southern Oscillation (ENSO) removal, indicating that the observed TCF-rainfall linkage is not primarily driven by ENSO. This suggests that an increased occurrence of TC over the SWIO is associated with increased rainfall over the East Africa study region. Based on the physical mechanisms, we found that the prevalence of TC induces upper-level anticyclonic circulation anomalies centered over east Madagascar. The northern flank of this anticyclone excites southeasterly winds traversing towards the coast of the East Africa region, penetrating inland to reach the equatorial East Africa region. The presence of anticyclonic circulation, coupled with the surface-level southerlies from the South Indian Ocean through the Mozambique Channel and coastal East Africa, is associated with upper-level divergence, which indicates strong convection and, eventually, rainfall over the study region. Using leave-ten-out cross-validation, the joint SST-TC indices yield the predictive skill of <i>r</i> = 0.73 (r² = 0.52; <i>n</i> = 41), significant at the 95% confidence level, indicating that 52% of the interannual rainfall variance is explained by the combined SST anomalies, while excluding TCF reduces the skill to <i>r</i> = 0.56 (r² = 0.31), indicating the added contribution of tropical cyclone activity to interannual rainfall variability. These findings could contribute to accurately taking action in advance in dealing with probable rainfall events in the context of the TC and enhanced decision-making for weather agencies.</p> Graphical Abstract <p></p> <p>This study examined the cross-seasonal impacts of austral summer tropical cyclones (TCs) over the Southwest Indian Ocean (SWIO) on rainfall patterns in East Africa during the subsequent austral autumn, using gauge-interpolated CRU precipitation, ERA5 reanalysis, TCs from IBTrACS, and NOAA ERSST data from 1981 to 2021. Statistical techniques, including correlation, regression and composite analysis, were applied to investigate the linkage and associated mechanism between Tropical Cyclone Frequency (TCF) and rainfall. Additionally, an empirical linear regression model was constructed using predictors of austral summer TCF and SST indices to predict East African rainfall in the following austral autumn. The findings suggest that increased tropical cyclone activity over the SWIO can trigger large-scale atmospheric circulation, resulting in wetter conditions in East Africa. Indeed, the regression model incorporating the TCs revealed an improved prediction skill for rainfall, suggesting that the TCs might serve as important precursory signals for rainfall conditions over East Africa. These findings offer a new insight into the interaction between extratropical and tropical climatic systems and provide a potential signal for seasonal rainfall predictions, which could support improved planning and disaster preparedness in the region.</p>

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Seasonal Teleconnections Between Summer Tropical Cyclones in the Southwest Indian Ocean and the Following Autumn Rainfall over East Africa

  • 期刊论文

  • 发表日期：

    2026年5月12日

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  • Dickson Mbigi,
  • Paul T.S. Limbu

摘要

The Southwest Indian Ocean (SWIO) is an area accounting for about 11% of global tropical cyclone (TC) activities, shaping regional weather patterns across southern Africa. The present study examined the characteristics of SWIO tropical cyclone frequency (TCF) and its relationship to East African rainfall, focusing on how preceding January-February-March (JFM) TCF influences subsequent austral autumn March-April-May (MAM) rainfall using statistical techniques. Furthermore, the potential for TCF to improve the predictability of rainfall is explored using the regression statistical model. The study covers 1981–2021, employing tropical cyclone data from IBTrACS v4, monthly gridded precipitation from CRU TS4.05, atmospheric circulation fields from the ERA5 reanalysis, and sea surface temperature data from NOAA ERSST v5. Lead–lag correlation analysis between the JFM TCF index and East African rainfall, around the equatorial area, identifies statistically significant correlations during MAM that persist after El Niño-Southern Oscillation (ENSO) removal, indicating that the observed TCF-rainfall linkage is not primarily driven by ENSO. This suggests that an increased occurrence of TC over the SWIO is associated with increased rainfall over the East Africa study region. Based on the physical mechanisms, we found that the prevalence of TC induces upper-level anticyclonic circulation anomalies centered over east Madagascar. The northern flank of this anticyclone excites southeasterly winds traversing towards the coast of the East Africa region, penetrating inland to reach the equatorial East Africa region. The presence of anticyclonic circulation, coupled with the surface-level southerlies from the South Indian Ocean through the Mozambique Channel and coastal East Africa, is associated with upper-level divergence, which indicates strong convection and, eventually, rainfall over the study region. Using leave-ten-out cross-validation, the joint SST-TC indices yield the predictive skill of r = 0.73 (r² = 0.52; n = 41), significant at the 95% confidence level, indicating that 52% of the interannual rainfall variance is explained by the combined SST anomalies, while excluding TCF reduces the skill to r = 0.56 (r² = 0.31), indicating the added contribution of tropical cyclone activity to interannual rainfall variability. These findings could contribute to accurately taking action in advance in dealing with probable rainfall events in the context of the TC and enhanced decision-making for weather agencies.

Graphical Abstract

This study examined the cross-seasonal impacts of austral summer tropical cyclones (TCs) over the Southwest Indian Ocean (SWIO) on rainfall patterns in East Africa during the subsequent austral autumn, using gauge-interpolated CRU precipitation, ERA5 reanalysis, TCs from IBTrACS, and NOAA ERSST data from 1981 to 2021. Statistical techniques, including correlation, regression and composite analysis, were applied to investigate the linkage and associated mechanism between Tropical Cyclone Frequency (TCF) and rainfall. Additionally, an empirical linear regression model was constructed using predictors of austral summer TCF and SST indices to predict East African rainfall in the following austral autumn. The findings suggest that increased tropical cyclone activity over the SWIO can trigger large-scale atmospheric circulation, resulting in wetter conditions in East Africa. Indeed, the regression model incorporating the TCs revealed an improved prediction skill for rainfall, suggesting that the TCs might serve as important precursory signals for rainfall conditions over East Africa. These findings offer a new insight into the interaction between extratropical and tropical climatic systems and provide a potential signal for seasonal rainfall predictions, which could support improved planning and disaster preparedness in the region.