The Eocene (56–34 million years ago) is characterised by declining sea surface temperatures (SSTs) in the low latitudes (~4°C) and high southern latitudes (~8-11°C), in accord with decreasing CO₂ estimates. However, in the mid-to-high northern latitudes there is no evidence for surface water cooling, suggesting thermal decoupling between northern and southern hemispheres and additional non-CO₂ controls. To explore this further, we present a multi-proxy (Mg/Ca, δ¹⁸O, TEX86) SST record from the western North Atlantic (~36°N paleolatitude). Our data confirm a long-term decline in SSTs of ~5°C between the early (~53 Ma) and the middle (~42 Ma) Eocene, supporting declining atmospheric CO₂ as the primary mechanism of Eocene cooling. However, from the middle Eocene onwards, North Atlantic zonal temperature gradients are decoupled, which we attribute to the incursion of warmer waters into the eastern North Atlantic and the inception of Northern Component Water across the early-middle Eocene transition.