Late Holocene cooling and increased zonal asymmetry in the mid-latitude North Atlantic

Abstract. Sea Surface Temperature reconstructions derived from alkenone biomarker (SST-alk) reveal a cooling trend in the North Atlantic during the late Holocene (the last 5,000 years), contrary to the warming simulated by transient climate models driven by 20 ppm increase in greenhouse gas concentrations. It has been suggested that the apparent cooling in paleo-records may reflect the evolution of summer temperatures, a seasonal signal biased by the preferential growth of haptophyte algae during warm months. Here, we investigate the spatial pattern of SST-alk changes and show that late Holocene cooling is characterized by an increased zonal SST gradient in the mid-latitude North Atlantic, with greater cooling in the west than in the east. Multiple proxies indicate that this increase in zonal asymmetry is associated with reorganizations of the inter-gyre ocean circulation. We find that transient simulations, such as TraCE-21k, do not reproduce the zonally asymmetric cooling and the inferred changes in inter-gyre circulation from the mid- to late Holocene. This misrepresentation of spatial and temporal variability likely explains the data-model discrepancy in the mid-latitude North Atlantic.