Ocean physics
Ocean physical mixing processes can be divided into mesoscale eddy-induced mixing, which is essentially isopycnal, and small-scale, diapycnal mixing largely due to internal gravity wave breaking. The former can be directly represented in high-resolution models but is parameterised in coarse resolution models, while the latter will be parameterised in all models for the foreseeable future. Work on mixing will therefore be divided into two parts; firstly a small number of high-resolution simulations of glacial and interglacial modes will be compared to assess the effects of eddy mixing processes that can be directly simulated. Secondly, parameterising small-scale mixing in terms of local stratification and topographic roughness, we will use GENIE to assess the potential for changes in stratification and mixing to cause significant glacial-interglacial changes in deep-ocean CO2 sequestration.
The rate of diapycnal mixing of CO2 across density surfaces is a critical factor in the G-IG carbon storage question. Attempts to quantify this mixing in models are hampered by spurious diapycnal leakage which occurs in z-coordinate ocean models (while isopycnal models suffer comparable problems with conservation and boundary processes). This leakage is not in general reduced in high resolution models which include explicit eddies . We will therefore focus our attempts to quantify and control diapycnal leakage on GENIE, starting with idealised source-sink distributions in simple domains and using established methods to localise leakage in density space. By steadily increasing domain complexity, we will determine how leakage is spatially distributed and estimate the degree of leakage in domains with realistic geometry. We will investigate methods to control diapycnal leakage through a reformulation of the advective fluxes following the methods used to successfully control diffusive fluxes. If this works, it will yield a parameterisation that could be used in higher-resolution models. If not, we will have to control leakage by removing factors that exacerbate it.
Assessing the potential for changes in Southern Ocean carbon storage depends on good simulation of Southern Ocean (S.O.) overturning circulation. New analysis and models for changes in S.O. circulation have been proposed . The overturning circulation depends on the balance of processes including eddy fluxes, Ekman fluxes, diapycnal mixing, buoyancy forcing, convection and bottom boundary currents, and it is clear therefore that it must alter between glacial and interglacial time as temperatures, ice coverage and possibly wind patterns change. The substantial significance of this for atmospheric CO2 concentrations is being explored by us . Appropriately extended but still relatively efficient versions of GENIE exist with enhanced, variable resolution in polar regions, refined convection and eddy mixing parameterisation, and a bottom boundary layer representation. Using a relatively large number of long integrations, widely spaced in terms of parameters and forcing, we will assess the hypothesis that changes in these processes could lead to large variations in G-IG carbon storage.
We will consider the combined effects of physical and biogeochemical mechanisms for CO2 change, especially when these are modulated by seasonal forcing and together with WP3, we will seek to simulate the 80–100ppm drawdown of CO2 at the LGM in GENIE by prescribing ice sheets (ICE5G) and orbital forcing, with dust fluxes from WP6 and/or previous work. If successful we will proceed to model other glacial/interglacial transitions and cycles with interactive ice-sheets, and thereby test whether we have achieved an adequate understanding of their causes and mechanisms. These will be extended back in time as necessary data become available. In particular, we hope to consider the causes of the lower values of interglacial CO2 in MIS 13/15.
While there are no direct diagnostics for the physical mechanisms described here, many of the proxies being synthesised in WP1 and 2 are affected by them and they interact strongly with the biogeochemical mechanisms described in WP3. Proxies of sea ice extent, ocean temperature, stratification and nutrient utilization must all be consistent with the conditions modelled in the Southern Ocean. Sensitivity experiments with the models will be conducted to examine which possible mixing/circulation schemes are consistent with the proxy records.
Gathering together the various strands of this sub-project we will use systematic, automated data assimilation techniques to incorporate data of special relevance to S.O. circulation into the S.O.-focused model version we have created. Using refined estimates of mixing, circulation and diapycnal leakage parameters and associated error estimates, the assimilation framework will then be used to determine the likely overall contribution of S.O. physical processes to G-IG carbon storage change, and to test the relevant proposed hypotheses concerning mixing, ventilation and sea-ice processes. To isolate the effects of physical mechanisms for CO2 change we will seek to simulate the 50ppm drawdown of CO2 in early glacial time (e.g. stages 5a-d) (when there are no large changes in dust inputs or export production) by forcing GENIE with appropriate boundary conditions.