Evaluating Reanalysis Reliability under Compound Climate Extremes for Energy Resilience in the Maritime Continent

Future energy systems in the Maritime Continent are expected to be increasingly dominated by solar power as part of the broader decarbonization and energy transition agenda, with substantial growth in solar potential projected across Indonesia. While overall resource availability is likely to remain high due to Indonesia’s equatorial location, climate change may increase spatial and seasonal variability in surface solar radiation through shifts in cloudiness and atmospheric circulation, underscoring the need for climate-informed energy planning. 

However, the robustness of the reanalysis products under intensifying hydro-climatic extremes remains insufficiently assessed. This study evaluates the performance of the ERA5-Land reanalysis in reproducing surface solar irradiance relative to observations from 23 ground-based stations over the period 2019–2025, using the 2020–2022 triple-dip La Niña event as a natural stress test. Results are further contextualized within observed multi-decadal climate trends spanning 1981–2024. 

The evaluation reveals a systematic clear-sky bias in ERA5-Land that is strongly dependent on the atmospheric regime. While the reanalysis captures the phase of the diurnal irradiance cycle reasonably well under moderate conditions, its performance degrades markedly during high-impact weather regimes. During the deep convective phases of the 2021 La Niña, in situ observations show pronounced attenuation of surface solar irradiance associated with persistent cloud cover, whereas ERA5-Land frequently maintains elevated irradiance estimates. This behavior points to limitations in the representation of cloud optical properties, especially for thick stratiform cloud decks characteristic of the Asian Winter Monsoon. As a result, ERA-5 reproduces rainfall occurrence but underestimates the magnitude of associated solar dimming, leading to a systematic overestimation of solar resource availability during periods of heightened system vulnerability, which may translate into biased generation forecasts, inadequate reserve allocation, and increased operational risk for solar-dominated power systems. 

Other characteristics of climate data that are particularly relevant for future energy systems include emerging climate trends, especially those reflected in extreme climate indices. Analysis of ground stations indicates widespread asymmetric warming, with minimum temperatures increasing more rapidly than maximum temperatures, alongside a statistically significant intensification of wet extremes (RX1DAY) and changes in dry spell characteristics. The increasing prevalence of hydro-climatic extremes implies that the atmospheric regimes under which reanalysis performance is weakest are likely to become more frequent. 

Overall, this study identifies a critical resilience gap in renewable energy resource assessment for the Maritime Continent. Reliance on unadjusted reanalysis data may lead to systematic underestimation of solar power drought risk. We argue that future energy planning should move beyond uniform bias correction and adopt regime-aware approaches that explicitly account for limitations in the representation of cloud-radiative processes under extreme monsoonal conditions.