ENSO Neutral Phase Dynamics from Climatological and Ecological Perspectives: Patterns, Trends, and Implications over Four Decades (1980-2019)

The ENSO neutral phase is crucial because it provides a more stable global climate scenario that can serve as a basis for assessing the impacts of extreme ENSO variability and long-term climate change. This study used secondary data from the neutral phase between 1980 and 2019. Data were analyzed using descriptive statistics and hierarchical cluster analyses. The results show that weak (0-0.2) and strong(0.2-0.4, positive and negative) neutral phases most frequently occur during the early transition period and the east monsoon, with a fluctuating decadal trend that tends to stabilize with a downward trend in the last two decades. The west monsoon was the period with the least occurrence, whereas the east monsoon and early transition periods were the periods in which neutral phases, both weak and moderate, occurred most frequently. Decadal patterns and trends indicate that in the 1980s, negative anomalies were more common, and the 1990s were dominated by weak to strong positive anomalies. Meanwhile, from 2000 to 2019, the frequency tended to decrease and become more stable, with smaller deviations between the two halves of the decade. The pattern shows that each increase in frequency in one half of the decade is usually followed by a decrease in the next half, reflecting a typical up-and-down oscillation pattern.