A New Hydroclimatic Signature: Negative NAO Dominance Under Weak La Niña Conditions During the 2025–2026 Northwest Africa Floods

ABSTRACT

The winter of 2025–2026 in Morocco exhibited a rapid hydroclimatic transition from prolonged drought to widespread flooding, representing an emerging hydroclimatic signature across Northwest Africa. Seasonal precipitation exceeded long‐term averages by 35.09%, triggering severe flash flooding in December 2025 and extensive riverine inundation across northern basins in February 2026. Analysis of large‐scale circulation from the ERA5‐Land reanalysis reveals a pronounced southward displacement of the North Atlantic jet stream, with core wind speeds exceeding 75 m s ⁻¹ at 250 hPa. Concurrent sea‐level pressure anomalies show a weakened Azores High (−11 hPa) and a relatively shallow Iceland Low (+8 hPa), indicating a negative phase of the North Atlantic Oscillation. Simultaneously, sea surface temperature anomalies in the Niño 3.4 region reached approximately −1°C, consistent with weak La Niña conditions. The coexistence of a negative NAO phase and weak La Niña conditions coincided with broader atmospheric circulation anomalies over the North Atlantic sector. However, statistical analyses indicate that NAO variability represented the dominant control on precipitation over Morocco. These circulation anomalies favoured repeated moisture‐rich Atlantic storms and atmospheric‐river–like transport towards Morocco. Satellite observations from Copernicus Sentinel 1 confirm extensive floodplain inundation along the lower Sebou River during peak flows. The results demonstrate that the negative phase of the North Atlantic Oscillation was the primary atmospheric driver of the 2025–2026 floods, while weak La Niña conditions represented a secondary background influence on the large‐scale circulation. Storm sequencing and orographic enhancement further amplified runoff generation and flood magnitude across Moroccan river basins.