The Gulf Stream is a major conduit of warm surface water from the tropics to the subpolar North Atlantic. Here we observe and simulate a submesoscale (<20 km) mechanism by which the Gulf Stream exchanges water with subpolar water to the north. Along isopycnals, the front has a sharp compensated temperature-salinity contrast, with distinct mixed water between the two water masses 2 and 4 km wide. This mixed water does not increase downstream despite substantial energy available for mixing. A series of streamers detrain this water at the crest of meanders. Subpolar water replaces the mixed water and resharpens the front. The water mass exchange accounts for a northward flux of salt of 0.5–2.5 psu m² s⁻¹, (large-scale diffusivity O (100 m² s⁻¹)). This is similar to bulk-scale flux estimates of 1.2 psu m² s⁻¹ and supplies fresher water to the Gulf Stream required for the production of 18° subtropical mode water.Keywords: ocean mixing, eddies, Gulf Stream, submesoscale mixin
