The continuous shoaling of wintertime mixed layer depth near East Greenland using an observation-based reconstructive datasets

Variability of mixed layer depth (MLD) in the Irminger Sea is critical to deep convection, deep water formation, and the Atlantic Meridional Overturning Circulation (MOC). Previous studies about MLD in the central Irminger Sea have shown that atmospheric surface forcing dominates interannual variability of MLD. However, the decadal-scale MLD variability in the East Greenland (EG) region (i.e., the western boundary of Irminger Sea) remains incompletely understood. Based on a multidecadal observation-based reconstructed dataset covering 1993–2022, this study identifies a robust three-decade MLD shoaling in the EG region, with an overall trend of −139.1 m·decade −1 . This shoaling exhibits a clear regime shift around the late 2000 s: a rapid shoaling rate of −331.2 m·decade −1 in Period Ⅰ (first half, 1993–2007), followed by a marked slowdown to −76.5 m·decade −1 in Period Ⅱ (second half, 2008–2022). To identify the dominant driver of this MLD shoaling, we assess the relative contributions of five candidate processes: surface heat flux, surface freshwater flux, wind mixing, horizontal advection and sea-ice. Surface buoyancy fluxes exert a negligible net effect, as the shoaling impact of declining freshwater flux is compensated by heat flux. Sea-ice melt contributes only marginally due to its limited coverage. Instead, horizontal advection transports upstream water masses from the north and interior Irminger Sea, whose stratification favors mixed-layer shoaling at varying rates. Amplified by local coupled turbulent mixing processes, this drives the three-decade MLD shoaling and its late-2000 s regime shift. This long-term MLD shoaling in this key deep convection hotspot may have important implications for the long-term variability of the MOC, and hence for the climate system.