[Paper Review] Climate network and complexity based ENSO forecast for 2026

Josef Ludescher, Jun Meng+3 more|IIASA PURE (International Institute of Applied Systems Analysis)|Feb 16, 2026

Climate variability and models0 citations

TL;DR

The paper combines a climate network approach and a System Sample Entropy (SysSampEn) complexity method to forecast ENSO for 2026. The two methods diverge, with a neutral ENSO event more likely overall, and a weaker El Niño if it occurs, supported by ERA5 data and a type forecast favoring Eastern Pacific El Niño.

ABSTRACT

The El Niño Southern Oscillation (ENSO) is the dominant driver of interannual global climate variability and can lead to extreme weather events such as droughts or flooding. Recently, we have developed several statistical approaches for early ENSO forecasting, in particular, its El Niño phase. The climate network-based approach allows forecasting the onset of an El Niño event or its absence about 1 year ahead [1]. The complexity-based approach allows additionally to forecast the magnitude of an upcoming El Niño event in the calendar year before the onset [2]. Additionally, we have developed methods for forecasting the type (Eastern Pacific or Central Pacific) of an El Niño [3] and for probabilistic forecasting of La Niña and neutral events [4], also by the end of the calendar year before the event. Here we present the forecasts of these methods for 2026. The climate network and the complexity-based approach do not provide concurring signals for this year. The combined forecast indicates that a neutral event is more likely than an El Niño. If an El Niño develops in 2026, the complexity-based approach predicts a weaker event with a magnitude of $0.84\pm0.36$°C.

Motivation & Objective

• Motivate early ENSO forecasting due to its global climate impacts and utility for mitigation.
• Present a climate network-based forecast for El Niño onset/absence one calendar year ahead.
• Present a SysSampEn-based forecast for El Niño magnitude and onset in the prior year.
• Forecast El Niño type and probabilities for La Niña/neutral outcomes.
• Discuss data choices (NCEP/NCAR and ERA5) and compare combined forecasts with official CPC forecasts.

Proposed method

• Climate network approach: compute mean link strength S(t) from cross-correlations between Niño3.4 region and other tropical Pacific nodes to forecast El Niño onset in the following year; threshold crossing triggers alarm.
• SysSampEn approach: compute System Sample Entropy from ERA5 daily near-surface temperatures in Niño3.4; use linear regression linking y-1 SysSampEn to El Niño magnitude in year y.
• Use version (ii) of the network algorithm, requiring ONI < 0.5°C for the rest of the year to count alarms; thresholds learned 1950–1980.
• Forecast El Niño type using ΔT_WP-CP index (EP vs CP) to interpret likelihood of type if onset occurs.
• Combine forecasts with Laplace’s rule of succession to quantify joint probabilities and compare with NOAA CPC forecast.
• Apply ERA5-based network checks to compare with NCEP-based results and discuss partial crossing interpretations.

Experimental results

Research questions

• RQ1 What is the predicted probability of an El Niño onset in 2026 according to the climate network and SysSampEn methods?
• RQ2 What is the predicted magnitude of an El Niño in 2026 if it occurs, and how certain is that magnitude?
• RQ3 What is the predicted El Niño type (Eastern Pacific vs Central Pacific) for 2026 if onset occurs?
• RQ4 What are the probabilities for La Niña and neutral outcomes in boreal winter 2026/27 when El Niño does not occur?
• RQ5 How do ERA5-based forecasts compare with NCEP-based forecasts and with official CPC forecasts?

Key findings

• The climate network approach predicts the absence of an El Niño in 2026 with 91.4% probability (for onset absence).
• The SysSampEn complexity method predicts a weak El Niño in 2026 with magnitude 0.84 ± 0.36°C and 71.4% probability of onset.
• A combined interpretation using Laplace’s rule of succession yields a 37.5% probability of an El Niño onset in 2026 (given past divergence).
• ERA5-based results tend to align more with the SysSampEn forecast, suggesting a weak El Niño if it occurs; ERAs suggest higher El Niño probability than the NCEP-based result.
• When an El Niño onset occurs, the EP (Eastern Pacific) type is favored with about 76.9% probability based on ΔT_WP-CP and related indicators.
• For non-El Niño years, the interannual ONI relationship forecast indicates a 29.1% probability of La Niña and 70.9% probability of neutral, conditioned on no El Niño.