Ocean temperature response to idealized Gleissberg and de Vries solar cycles in a comprehensive climate model

The ~90-year Gleissberg and ~200-year de Vries cycles have been identified as two distinctive quasi-periodic components of Holocene solar activity. Evidence exists for the impact of such multi-decadal to centennial-scale variability in total solar irradiance (TSI) on climate, but concerning the ocean, this evidence is mainly restricted to the surface response. Here we use a comprehensive global climate model to study the impact of idealized solar forcing, representing the Gleissberg and de Vries cycles, on global ocean potential temperature at different depth levels, after a recent proxy record indicates a signal of TSI anomalies in the northeastern Atlantic at mid-depth. Potential impacts of TSI anomalies on deeper oceanic levels are climatically relevant due to their possible effect on ocean circulation by altering water mass characteristics. Simulated solar anomalies are shown to penetrate the ocean down to at least deep-water levels. Despite the fact that the two forcing periods differ only by a factor of ~2, the spatial pattern of response is significantly distinctive between the experiments, suggesting different mechanisms for solar signal propagation. These are related to advection by North Atlantic Deep Water flow (200-year forcing), and barotropic adjustment in the South Atlantic in response to a latitudinal shift of the westerly wind belt (90-year forcing).

Age model, raw data and interpolated raw data from proxy records of sediment cores GeoB6007-1 and GeoB6007-2

Here we present a 1200 yr long benthic foraminiferal Mg/Ca based temperature and oxygen isotope record from a ~900 m deep sediment core off northwest Africa to show that atmosphere-ocean interactions in the eastern subpolar gyre are transferred at central water depth into the eastern boundary of the subtropical gyre. Further we link the variability of the NAO (over the past 165 yrs) and solar irradiance (Late Holocene) and their control on subpolar mode water formation to the multidecadal variability observed at mid-depth in the eastern subtropical gyre. Our results show that eastern North Atlantic central waters cooled by up to ~0.8± 0.7 °C and densities decreased by Sigma theta=0.3±0.2 during positive NAO years and during minima in solar irradiance during the Late Holocene. The presented records demonstrate the sensitivity of central water formation to enhanced atmospheric forcing and ice/freshwater fluxes into the eastern subpolar gyre and the importance of central water circulation for cross-gyre climate signal propagation during the Late Holocene.