No abstract

No abstract

No abstract

Increasing anthropogenic greenhouse gas (GHG) concentrations cause a positive Earth energy imbalance (EEI), with resulting surplus heat in the climate system increasing ocean heat content (OHC) 1-3 (Fig. 1a, F1). Unprecedented oceanic warming has been observed since at least the 1950s, reaching record values from 2012-2021 (rEFs. 3,4 ). This oceanic warming has been pervasive, spreading from the surface to the abyssal layers (each responding differently; Box 1), and with the long-term oHC trend accelerating [5][6][7][8] .Owing to the large thermal inertia of the ocean, subsurface warming represents the slow response to external influences, in particular to GHG forcing (Box 1). In response to past and current carbon emissions, future ocean warming is therefore committed for many centuries 5,9 , and is related to the current acceleration of ocean warming 10 (Fig. 1a). For instance, under representative concentration pathway (RCP) 8.5, it is projected that the total upper-2,000-m OHC increase from 2017-2100 will be ~5-7 times that observed from 1970 to 2017 (rEF. 6 ). The irreversibility of this ocean warming on centennial timescales creates additional requirements for climate policy, particularly considering the widespread impacts 10,11 .Indeed, ocean warming has a multifaceted role in the Earth system via its links to the energy, water and carbon

Observed and predicted increases in Southern Ocean winds are thought to upwell deep ocean carbon and increase atmospheric CO 2 . However, Southern Ocean dynamics affect biogeochemistry and circulation pathways on a global scale. Using idealized Massachusetts Institute of Technology General Circulation Model (MITgcm) simulations, we demonstrate that an increase in Southern Ocean winds reduces the carbon sink in the North Atlantic subpolar gyre. The increase in atmospheric CO 2 due to the reduction of the North Atlantic carbon sink is shown to be of the same magnitude as the increase in atmospheric CO 2 due to Southern Ocean outgassing. The mechanism can be described as follows: The increase in Southern Ocean winds leads to an increase in upper ocean northward nutrient transport. Biological productivity is therefore enhanced in the tropics, which alters the chemistry of the subthermocline waters that are ultimately upwelled in the subpolar gyre. The results demonstrate the influence of Southern Ocean winds on the North Atlantic carbon sink and show that the effect of Southern Ocean winds on atmospheric CO 2 is likely twice as large as previously thought in past, present, and future climates.

Simulation of coupled carbon-climate requires representation of ocean carbon cycling, but the computational burden of simulating the dozens of prognostic tracers in state-of-the-art biogeochemistry ecosystem models can be prohibitive. We describe a six-tracer biogeochemistry module of steady-state phytoplankton and zooplankton dynamics in Biogeochemistry with Light, Iron, Nutrients and Gas (BLING version 2) with particular emphasis on enhancements relative to the previous version and evaluate its implementation in Geophysical Fluid Dynamics Laboratory's (GFDL) fourth-generation climate model (CM4.0) with ¼°ocean. Major geographical and vertical patterns in chlorophyll, phosphorus, alkalinity, inorganic and organic carbon, and oxygen are well represented. Major biases in BLINGv2 include overly intensified production in high-productivity regions at the expense of productivity in the oligotrophic oceans, overly zonal structure in tropical phosphorus, and intensified hypoxia in the eastern ocean basins as is typical in climate models. Overall, while BLINGv2 structural limitations prevent sophisticated application to plankton physiology, ecology, or biodiversity, its ability to represent major organic, inorganic, and solubility pumps makes it suitable for many coupled carbon-climate and biogeochemistry studies including eddy interactions in the ocean interior. We further overview the biogeochemistry and circulation mechanisms that shape carbon uptake over the historical period. As an initial analysis of model historical and idealized response, we show that CM4.0 takes up slightly more anthropogenic carbon than previous models in part due to enhanced ventilation in the absence of an eddy parameterization. The CM4.0 biogeochemistry response to CO 2 doubling highlights a mix of large declines and moderate increases consistent with previous models. Plain Language Summary Cutting edge climate model development efforts often do not include representation of the carbon cycle and biogeochemical tracers due to the large computational expense of these additional tracers. During GFDL CM4.0 development, an effort was made to include a simple representation of these processes to explore ocean biogeochemistry at eddying resolution. This simple ocean biogeochemistry builds on the previous BLINGv1 effort (Galbraith et al., 2010,

The Southern Ocean is the largest sink of anthropogenic carbon in the present-day climate. Here, Southern Ocean pCO 2 and its dependence on wind forcing are investigated using an equilibrium mixed layer carbon budget. This budget is used to derive an expression for Southern Ocean pCO 2 sensitivity to wind stress. Southern Ocean pCO 2 is found to vary as the square root of area-mean wind stress, arising from the dominance of vertical mixing over other processes such as lateral Ekman transport. The expression for p\hbox {CO}_{2} is validated using idealised coarse-resolution ocean numerical experiments. Additionally, we show that increased (decreased) stratification through surface warming reduces (increases) the sensitivity of the Southern Ocean pCO 2 to wind stress. The scaling is then used to estimate the wind-stress induced changes of atmospheric pCO 2 in CMIP5 models using only a handful of parameters. The scaling is further used to model the anthropogenic carbon sink, showing a long-term reversal of the Southern Ocean sink for large wind stress strength.

No abstract