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Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports (ORCHESTRA)- Work Package 3

Work Package 3 addresses the Exchange between the Southern Ocean and the global ocean and the research will be conducted by the National Oceanography Centre (NOC), the British Antarctic Survey (BAS), the Plymouth Marine Laboratory (PML), the British Geological Survey (BGS) and the Met Office.

To address Q3, WP3 will conduct budget analyses using the hydrographic/tracer sections (T1) to diagnose the three-dimensional velocity field of the waters entering, leaving and recirculating within the South Atlantic sector of the Southern Ocean. This method generates optimally-constrained velocity fields by invoking conservation of properties in specific layers. It will be informed by directly-measured velocities on the cruises, and the rate of mixing between layers constrained using turbulent mixing estimates from vertical microstructure profilers. Combination with the measured tracer fields will enable transports of key properties (including heat and carbon) to be derived, for each of the different layers, and in totality.

An overall budget will be constructed for a box enclosing the entire South Atlantic sector of the Southern Ocean, enabling the diagnosis of its net role in the global heat and carbon cycles. The location of the ANDREX II section subdivides this box into a northern region enclosing the ACC and a southern region enclosing the Weddell Gyre. The analyses will thus separately quantify the fluxes associated with the upper and lower limbs of the Southern Ocean overturning.

Each of the ORCHESTRA hydrographic/tracer sections has been occupied in the last 5-10 years with full suites of physical/biogeochemical measurements. Using these, a diagnosis of the changes in the storage of heat and carbon occurring over a quasi-decadal interval, and interpretation in terms of known changes in climatic forcing and increasing atmospheric CO 2 concentrations will be made. The temporal representativeness of the sections will be assessed using profiling float and glider data (T2, T3), mapped as per WP2 above. Additionally, data from the profiling floats will be used to produce sections at 10-day intervals and 0.25° spacing along the hydrographic section lines, and thereby examining changes in upper ocean properties and transports on seasonal to decadal timescales.

The impact of physical variability on the changing distributions of biogeochemical parameters (including anthropogenic carbon) will be quantified using statistical relationships between them and coincident temperature and salinity observations. Where available (e.g. from the international Southern Ocean Carbon and Climate Observations and Modelling (SOCCOM) programme), oxygen will be used as an independent predictor. For the mixed layer, publicly-available Surface Ocean CO 2 Atlas and ORCHESTRA-generated observations will be used to constrain short-term variability in surface disequilibrium.

Determination of the time-variability of transports and heat content associated with the lower limb of the Southern Ocean overturning using deep-ocean moorings data (T6) and annual repeats of short hydrographic sections (T1), the locations of which are optimally chosen to capture the core of the AABW as it flows north to become the bottom waters of the AMOC, will be made. Hypotheses that relate AABW export from the Weddell Gyre to changes in wind-forced and buoyancy-forced circulation by relating the observed variability in the transport to changes in surface circulation derived from satellite altimetry (T5) will be tested. The dynamics behind such linkages will be explored fully using high-resolution ocean modelling (T8).

The size and variability of the modern (T7, T8) and projected (T7) Southern Ocean heat and carbon budget will be estimated in a range of ocean and coupled climate models. The sensitivity of the component parts of the budgets (uptake, storage, export) to model scenario, resolution and diversity will be assessed across timescales from seasonal to centennial, and impacts of ORCHESTRA-derived model improvements will be quantified.