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Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports (ORCHESTRA)- Work Package 2

Work Package 2 addresses the Exchange between the upper-ocean mixed layer and the interior and the research will be conducted by the National Oceanography Centre (NOC), the British Antarctic Survey (BAS), and the Scottish Marine Mammal Unit (SMRU).

To address Q2, WP2 will combine observationally-derived data and model simulations to determine and understand the exchanges between the ocean mixed layer and its interior in unprecedented detail. Ocean profile data (T2, T3) will be used to produce 10-day maps of temperature and salinity at 1° resolution for the period since 2004. These will be analysed alongside improved estimates of surface fluxes (see WP1) and EO data (T5) to derive the first seasonally-resolved, decade-long subduction budgets for mode and intermediate waters in the Southern Ocean. Carbon subduction will be estimated by combining these with publicly-available carbon measurement compilations, and with ship section observations (T1).

An investigation on the role of atmospheric forcing (including large-scale modes of climate variability, e.g. the Southern Annular Mode, SAM, and the El Niño/Southern Oscillation phenomenon, ENSO) in influencing heat and carbon subduction on both seasonal and interannual timescales, and the role of individual mixed layer/subduction processes in setting the net budget will be done. Multi-month glider missions (T3) will provide detailed information on key processes, with autonomous measurements charting the erosion of water masses created in winter, and allowing us to determine the effect of individual weather events on the flux of heat into the ocean interior.

The processes underpinning the observed relationships, as well as larger-scale and long-term trends, will be investigated in model-based analyses (T7 and T8) using the same budget approaches applied to the observations. Subduction-related metrics will permit quantitative comparisons across diverse model ensembles including climate/Earth System models (T7) and forced-ocean simulations (T8). Perturbation experiments aimed at understanding and quantifying the dynamic relationship between mixed layer processes and seasonal/interannual subduction will be conducted (e.g. shortwave/turbulent heat flux perturbation, modified mixing schemes etc.), with the perturbations being informed by adjoint model runs (T9) and the relationships derived in the observational and low-resolution model subduction analyses. These analyses will inform improved model representation of mixed layer processes (e.g. Langmuir turbulence) in setting subduction rates and sensitivities. The downslope injection of dense waters into the deep Weddell Sea (a key process in AABW formation, which present models simulate poorly) will be made more realistic using improved vertical coordinates to reduce spurious mixing, and by including the effect of tides. Intercomparisons of coupled runs with ocean-only climate scale models (T7) and high-resolution models (T8) will be conducted to examine and improve existing biases and to enable quantification of structural uncertainty, the impact of mesoscale eddies, and the role of coupled feedbacks in setting subduction and overturning circulation.