Tracking phase-specific vegetation responses to extreme climate anomalies in Tropical Asia using Himawari-8/9 AHI

Tropical Asia is influenced by large-scale climate modes, notably the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), yet their impacts on vegetation in tropical Asia remain uncertain. A key challenge is the scarcity of clear-sky observations from satellites in this persistently cloudy region. The Advanced Himawari Imager (AHI) onboard Himawari-8/9 geostationary satellites provides observations every 10 minutes, substantially increasing clear-sky samplings and enabling more robust monitoring of vegetation responses to climate anomalies. Here, we focus on two recent large climate anomalies, the positive IOD (pIOD) event in 2019 and the compound pIOD and El Niño event in 2023/24, to characterize how vegetation responses evolve within events.

Based on ENSO and IOD indices, we defined the event periods as May 2019–November 2019 for the 2019 pIOD event, and April 2023–March 2024 for the 2023/24 compound pIOD and El Niño event. We used the two-band Enhanced Vegetation Index (EVI2) derived from Himawari-8/9 AHI surface reflectance (Li et al., 2025; Zhang et al., 2025) for 2016–2024, and monthly climate variables (i.e., shortwave radiation (SWR) and vapor pressure deficit (VPD)) over the same period. Monthly anomalies were computed relative to the 2016–2024 average. To capture intra-event evolution, we further assessed anomalies by sub-seasonal phases within each event.

We found that the anomalies in EVI2 showed clear phase-dependent responses during these events. For example, EVI2 decreased over continental Southeast Asia in phase 1 (April–June 2023) of the 2023/24 compound pIOD and El Niño, but increased in the same region in phase 3 (September–December 2023). These changes in EVI2 were consistently associated with a trade-off between atmospheric dryness (higher VPD) and enhanced light availability (higher SWR). Our results highlight that vegetation dynamics during extreme climate anomalies are strongly modulated by phase-specific light-dryness regimes, while the causal impact is unclear when examining these pIOD and El Niño events as a whole.