Mode waters occupy only 20% of the upper 2000 m of the ocean, yet they strongly shape ocean heat storage and oxygen uptake. Whether these reservoirs will persist under continued warming remains unclear. Based on Argo observations, direct measurements from bottle and CTD casts, ocean reanalysis products, and model simulations, here we report signs of a collapse in the North Pacific Central Mode Water since the early 2020s, leading to a rapid decline in the subsurface oxygen maximum at Station ALOHA, the monitoring site for the Hawaii Ocean Time-series program, 50 km north of Oahua. Over recent decades, weakened westerlies in the Central Mode Water formation region aggravate surface warming and stratification, consequently suppressing Central Mode Water renewal. Typically, a strong El Niño strengthens the westerlies, promoting vigorous Central Mode Water formation. However, the strong 2023–2024 El Niño failed to reinvigorate the Central Mode Water because the atmospheric teleconnections were shifted unusually eastward, well outside the Central Mode Water formation region. Similar anomalies are projected in warming climate, and our findings imply inhibited surface-thermocline exchanges due to failed mode water ventilation, with far-reaching impacts on climate, biogeochemical cycles, and marine ecosystems.

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