# ENSO phase transition enables prediction of winter North Atlantic Oscillation one year ahead

## TL;DR

This study shows that predicting winter North Atlantic Oscillation becomes more accurate one year in advance when El Niño–Southern Oscillation transitions occur.

## Contribution

The study identifies ENSO phase transitions as a key factor in improving one-year-ahead NAO predictability through atmospheric angular momentum dynamics.

## Key findings

- NAO prediction skill improves significantly during ENSO phase transition years.
- Atmospheric angular momentum dynamics link ENSO transitions to NAO predictability.
- Large ensemble sizes enhance prediction skill during ENSO transitions.

## Abstract

The winter North Atlantic Oscillation (NAO) is a dominant mode of climate variability affecting temperature and precipitation across the Northern Hemisphere, yet its prediction at seasonal-to-decadal (S2D) lead times remains challenging. Here, using multi-year hindcasts from a multi-model ensemble initialized on 1 November for 1962–2019, we show that NAO skill one year ahead improves significantly when the El Niño–Southern Oscillation (ENSO) undergoes a phase transition next year. This improvement is linked to the northward propagation of anomalous atmospheric angular momentum, which dynamically organizes the NAO and is captured in reanalysis and models. During ENSO transition years, prediction skill increases with ensemble size, and when more than 10 members are used, the forecasts display the signal-to-noise paradox. These findings highlight the potential for enhanced one-year NAO predictability when ENSO transitions are present and large ensemble sizes are used in S2D prediction systems, given the skillful prediction of ENSO phase transitions at one-year lead times by multi-model ensembles.

This study shows that El Niño–Southern Oscillation phase transition enhances the predictability of the winter North Atlantic Oscillation one year ahead through atmospheric angular momentum dynamics, improving long-range climate forecasts.