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).

Oceanography during TYDEMAN cruise DCM (DeepChlorMax) to the Equatorial Atlantic

The cruise with RV Tydeman was devoted to study permanently stratified plankton systems in the (sub)tropical ocean, which are characterised by a deep chlorophyll peak between 80 and 150 m. To minimise lateral effects by horizontal transport of nutrients and organic matter from river outflow and upwelling regions, stations were selected in the middle of the North Atlantic Ocean between the continents of America and Africa. (5 - 35° N and 50 - 15° W). Here the vertical distributions of light and nutrients control the abundance and growth of autotrophic algae in the thermically stratified water column. This phytoplankton is numerically dominated by the prokaryotic picoplankters Synechococcus spp. and Prochlorococcus spp., which are smaller than 2 ?m. The productivity of the 100 to 150 m deep euphotic zone can be high, because a high heterotrophic/autotrophic biomass ratio induces a rapid regeneration of nutrients and inorganic carbon. Primary grazers are mainly micro-organisms such as heterotrophic nannoflagellates and ciliates, which feed on the small algae and on bacteria. Heterotrophic bacteria can outnumber the autotrophic algae, because their number is related to the substrate pools of dissolved and particulate dead organic matter. These DOC and detritus pools reach equilibrium at a concentration, where the rate of their production (proportional to algal biomass) equals their mineralisation and sinking rate (proportional to the concentration and weight of POC and detritus). At a relatively low value of the weight-specific loss rates, the equilibrium concentration of these carbon pools and their load of bacteria can be high. The bacterial productivity is proportional to the mineralisation rate, which in a steady state can never be higher than the rate of primary production. Hence the ratio in turnover rate of bacteria and autotrophs tends to be reciprocally proportional to their biomass ratio.