ORCHESTRA WP2 meeting report

On the 4th of March the ORCHESTRA WP2 meeting was held at BAS. WP2 focuses on the processes that exchange heat between the ocean mixed layer and interior. Despite travel restrictions on many participants there was an active group of online listeners and speakers from ORCHESTRA and beyond, and a very productive meeting was held, hosted by WP2 lead Dave Munday aided by Andrew Meijers and Alex Brearley. The meeting focused on updates from the various observations, modelling and model analysis groups, as well as planning on how best to synthesize results together into cross disciplinary papers. Additionally ideas for the one year ORCHESTRA extension were scoped out.

Highlights included the showcasing of the regional z and sigma-z models developed at BAS and NOC, demonstrating that the control runs of the z-level model are completed and several perturbation runs are either completed or being run. Encouraging new results from the sigma-z model indicate that a version with fully realised ice shelves and cavities is now working, and producing dense water more ‘realistically’ than the equivalent z level model.

The coming ARCHER downtime will represent a challenge, but also an opportunity to consolidate work to date and get results published.

A slew of adjoint modelling papers looking at Southern Ocean heat uptake sensitivies have been published or in review from Dan Jones and Emma Boland. These models also are providing the basis for perturbation experiments in both ECCO and high resolution model setups to explore heat uptake dynamics.

Pat Hyder of the Met Office showed recent work demonstrating that care needs to be applied when interpreting the results of forced ocean experiments, notably in cases looking at surface heat transports under buoyancy forcing perturbation experiments.

Yavor Kostov gave a guest talk from TICTOC, demonstrating how historical SO SST trends are influenced by both the SAM and GHG forcing variability, but that variability in these processes still struggles to explain the modelled SST warming vs observed cooling.

Andrew Meijers showcased initial CMIP6 water mass heat uptake results, showing that while they largely agree with CMIP5, there is reduced sensitivities of AABW in the newer models.

Alex Brearley, Ryan Scott and Louise Biddle (Gothenburg) updated the group on the initial results from glider and EM-APEX based process studies. These are showing promising results, particularly mixed layer microstructure features and clear resolution of frontal/regional MLD deepening events at high temporal and spatial resolution. Discussion revolved around how to disentangle temporal/spatial features and how to compare these results with more traditional climatology scale analysis of MLD evolution.

Rachael Sanders, despite fighting off a bad illness gave a (remote!) presentation on her mixed layer budget work, showing that surface forcing, lateral advection and mixed layer entrainment dominates the mixed layer heat and freshwater budgets in the SAMW formation regions. Her ongoing work will be to investigate in ECCO how regional wind and buoyancy forcing impact water mass formation rates.

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