The influence of El Niño and Indian Ocean Dipole on the extreme decline in the water level of Lake Toba, Indonesia

Lake Toba, the world's largest tectonic-volcanic lake and part of the UNESCO Global Geopark, has experienced several episodes of extreme water level subsidence over the past four decades. This phenomenon of extreme decline has significant ecological, social, and economic implications, requiring a comprehensive understanding of its controlling factors. This study evaluates the relative contributions of the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) to rainfall variability and extreme water-level subsidence events during the period 1984–2020. The approaches used include percentile-based extreme threshold analysis, stochastic analysis through Fokker–Planck equations, rainfall anomaly analysis, climate composite, Standardized Precipitation Index (SPI), lake water balance, and wavelet transform coherence (WTC). The results show that the extreme decline in lake water levels is not controlled by a single factor, but by a dynamic interaction between global climate teleconnectivity and local hydrological regulation. The 1987, 1998, and 2016 episodes were strongly associated with intense El Niño and, in 1998, were amplified by positive IODs, resulting in significant rainfall deficits, droughts, and water balance deficits. In contrast, the 1990 episode reflected more of a combination of regional drought, hydrological deficits, and possible outflow management influences. WTC analysis shows that the relationship between rainfall and ONI is stronger and more persistent than that between rainfall and IOD, while water balance is the most dominant controller of lake water-level fluctuations. These findings confirm that the risk of extreme water-level decline in Lake Toba is non-stationary and requires an adaptive, risk-based, and integrated water management strategy, informed by climate teleconnection information, to support the sustainability of lake water resources.