Foraminiferal isotopic evidence of abrupt mid-20 th Century Onset of Hydrographic Instability in Nordic Seas Inflow Waters

The flow of warm Atlantic waters into the Nordic Seas largely determines the transport of ocean heat to the Arctic and is a prominent feature of the Atlantic Meridional Overturning Circulation (AMOC). Here we provide a ~250-year-long (1750 to 1992 AD), annually-to subannually-resolved record of Nordic Sea inflow water characteristics inferred from changes in δ 18 O of planktic foraminiferal carbonate. The new record is reflective of upper ocean temperatures across the Atlantic Water Zone of the Nordic Seas and reveals a previously unrecognized increase in temperature instability ~AD 1950 that appears to have impacted rates of Greenland Ice Sheet melting, Arctic sea-ice extent and Arctic Surface Air Temperature, delaying the regional response to Anthropogenic Global Warming by several decades. While the relationship between the sudden change in hydrographic conditions and AMOC strength and stability is not yet clear, the change in inflow characteristics ~AD 1950 was clearly imprinted on deep waters overflowing the Nordic Sea Basin. Fig. 5: GS13 δ 18 O, Labrador Sea, Denmark Strait temperatures and salinities. (A) As in Figs. 2-4. (B) 1-and 5-yr smoothed GS13 δ 18 O (blue lines) and Central Labrador Sea instrumental SSS from the compilation of Riverdin et al. 50 (red line) and Labrador Sea SSS as simulated in response to “Greenland/Arctic” atmospheric blocking from Ionita et al. 62 (brown lines). (C) GS13 δ 18 O (as in B) along with mean potential temperature of Denmark Strait overflow waters since 1949 AD after Eldevik et al. 45. Open circles denote missing years in the instrumental record. Blue horizontal bars indicate the timing of the Great Salinity Anomalies after Belkin 47. Red/white transition in background color indicates the start of instabilities in the hydrographic characteristics of Nordic Seas inflow waters.