The warming of the global ocean remains an unabated consequence of human-driven climate change (IPCC et al., 2021). The progressive rise in atmospheric greenhouse gas concentrations causes an extra downward heat flux of about 0.6-0.8 W m −2 through the sea surface (Desbruyères et al., 2017;Johnson et al., 2016), with a likely acceleration in recent years (Von Schuckmann et al., 2020) and several ramifications on sea level rise (Tebaldi et al., 2021), ocean stratification and mixing processes (Sallée et al., 2021), or ocean deoxygenation and carbon sequestration (Keeling et al., 2009). This global warming rate is often inferred from in situ observations of ocean heat content, which became well constrained in the mid-2000s owing to the completed implementation of the global network of 0-2,000 m Argo profiling platforms (Riser et al., 2016). A significant source of uncertainty on global and regional OHC increase remains however linked to the comparatively poor systematic and homogeneous sampling of the deep ocean below 2,000 (Garry et al., 2019;Purkey & Johnson, 2010). This issue contributed to motivate the deep extension of the Argo array since the mid 2010s, the Deep-Argo program (Johnson et al., 2015;Roemmich et al., 2019). Key regions for deep ocean heat storage variability were targeted to conduct Deep-Argo pilot experiments. One of them is the Subpolar North Atlantic Ocean (SPNA) where a cold and dense water mass-the North Atlantic Deep Water (NADW)-constantly propagates interannual to multi-decadal thermohaline and biogeochemical

<p>Sustained shipboard hydrography surveys along the A25-Ovide section (2002 – 2018) are combined with data from a regional pilot array of Deep Argo floats (2016 – 2021) to estimate the decadal variability and linear trends in the temperature of overflow-derived waters in the Irminger Sea. Removing local or remote dynamical influences (heave) enables to identify a new statistically-significant trend reversal in Iceland Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) core temperatures (spice). The latter took place in 2014 and interrupted a long-term warming of those water masses that was prevailing since the late 1990’s. Deep-Argo floats further reveal an overall acceleration of this cooling since 2014, with a mean rate of change estimated at -18 m°C yr<sup>-1</sup> during 2016 – 2021, as well as a boundary-intensified pattern of change. This, along with the absence of apparent reversal in the Nordic Seas and with DSOW warming and cooling twice as fast as ISOW, points out the entrainment of subpolar intermediate signals within the overflow plumes near the Greenland-Iceland-Scotland sills as a most likely driver.</p>