Shipboard hydrography along the A25-Ovide section (2002 – 2018) is combined with a high-resolution mooring array (2014 – 2020) and a regional fleet of Deep-Argo floats (2016 – 2021) to describe temperature changes of overflow-derived water masses in the Irminger Sea. Removing dynamical influences enables to identify a new statistically-significant trend reversal in Iceland Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) core temperatures in the mid-2010s. A basin-wide cooling trend of -16 ± 6 m°C yr-1 during 2016 – 2021 – but reaching as strong as -44 ± 13 m°C yr-1 for DSOW in recent years – is found to interrupt a warming phase that was prevailing since the late 1990’s. The absence of an apparent reversal in the Nordic Seas and the faster changes detected in DSOW compared to ISOW point out the entrainment of subpolar signals within the overflows near the Greenland-Iceland-Scotland sills as a most likely driver.

Plain Language Summary

The North Atlantic Deep Water is one of the most voluminous water masses of the global ocean. Its deepest constituent – the Lower North Atlantic Deep Water (LNADW) – forms in the Nordic Seas before cascading into the North Atlantic at the Greenland-Iceland-Scotland sills and progressing south towards the Southern Ocean. The temperature and salinity of LNADW are known to obey decadal trends in response to forcing in its source regions as well as subsequent mixing with surrounding and overlying water masses during its Atlantic journey. Here, repeated measurements from oceanographic vessels, continuous monitoring with moored instrumentations, and autonomous Deep-Argo floats in the Irminger Sea (east of Greenland) during 2002 – 2021 are used to reveal a new warming-to-cooling reversal of LNADW in 2014. This signal, which presumably originates in the entrainment of upper and intermediate ocean variability near the Greenland-Iceland-Scotland sills, will progressively travel southward within the lower branch of the Atlantic Meridional Overturning Circulation.