Middle to late Holocene cooling and increased zonal asymmetry in the mid-latitude North Atlantic

Sea Surface Temperature reconstructions derived from alkenone biomarker (SST-alk) reveal a cooling trend in the North Atlantic during the late Holocene (the last 5000 years), contrary to the warming simulated by transient climate models driven by 20 ppm increase in greenhouse gas concentrations. In this study, we present new SST-alk time series from the inter-gyre region of the North Atlantic. Our results, together with existing data, indicate that late Holocene cooling is marked by an increasing zonal SST gradient across the mid-latitude North Atlantic, with more pronounced cooling in the west than in the east. Complementary planktonic foraminifera assemblage data, interpreted alongside other circulation proxies, suggest that a late Holocene reorganization of inter-gyre ocean circulation as a likely driver of this zonal asymmetry. Finally, we compare these proxy-based reconstructions with transient climate model simulations (TraCE-21k), and find that models do not reproduce the observed zonally asymmetric cooling and the inferred circulation changes. This misrepresentation of spatial and temporal variability likely explains the data-model discrepancy in the mid-latitude North Atlantic.