SO-CHIC WorkPackage 2

Upper ocean ventilation pathways

Objectives

This WP will seek to determine the lead rate-controlling processes of ventilation in a region of upwelling of warm deep waters in the Southern Ocean. Specifically, we will:

1. Use an established process model to inform and test the design and implementation of the WP1/WP2 field campaign to document the evolution of the winter mixed layer.
2. Quantify a time series of mixed layer depths and properties during austral autumn, winter and spring seasons.
3. Quantify the dissipation of turbulent kinetic energy in both the mixed layer and thermocline and relate this to the entrainment of thermocline waters into the ocean mixed layer.
4. Quantify and understand the mechanisms (Ekman buoyancy forcing, convective buoyancy forcing and submesoscale instabilities) that control the evolution of the mixed layer and the subduction of heat and carbon into the Southern Ocean interior.
5. Use the outputs of the field experiment to test newly developed model parameterisations of Langmuir circulation and submesoscale processes.

This work package will bring together observationalists and modellers to address one of the key sources of uncertainty in current predictions of the future climate state, namely the rate of uptake of heat and carbon into the Southern Ocean interior and the mechanisms that drive this sequestration. New parameterisations of mixed layer processes have recently been developed but remain largely untested (e.g. Bachman et al., 2017). Ensuring the best available parameterisations are available to climate modellers is key to addressing the significant biases in mixed layer depths in current IPCC-class models.

Tasks

• Task 2.0: Scientific Coordination [Co-leads: A. Brearley (UKRI-BAS), D. Marshall (UOXF)]
• Task 2.1: Use submesoscale resolving model to test and refine the sampling strategy for the field experiment [lead D. Marshall (UOXF); participants: D. Ferreira (UREAD) and A. Brearley (UKRIBAS)]
• Task 2.2: Deploy, pilot and recover an array of autonomous surface vehicles and underwater gliders to document the evolution of the ocean mixed layer in the autumn, winter and spring when deepening and restratification processes reach their peak [lead A. Brearley (UKRI-BAS); participants: S. Swart (UGOT)]
• Task 2.3: Output time series of mixed layer depths (MLD) from the hydrographic data [lead A. Brearley (UKRI-BAS); participants: A. Naveira-Garabato (SOUTHAMPTON), S. Swart (UGOT)]
• Task 2.4: Quantify the turbulent kinetic energy dissipation rate, diapycnal diffusivity and heat and salt fluxes through the base of the mixed layer [lead A. Brearley (UKRI-BAS); participants: A. Naveira-Garabato (SOUTHAMPTON)]
• Task 2.5: Assess the importance of different processes in setting the evolution of ML properties [lead A. Brearley (UKRI-BAS); participants: S. Swart (UGOT) and A. Naveira-Garabato (SOUTHAMPTON)]
• Task 2.6: Investigate the impact of resolved and parameterised submesoscale processes and parameterised vertical mixing processes on the modelled ML annual cycle [lead D. Ferreira (UREAD); participants: D. Marshall (UOXF) and A. Brearley (UKRI-BAS)]

Partners

WP co-leaders: A. Brearley (UKRI-BAS) and D. Marshall (UOXF)

Other participants: D. Ferreira (UREAD), A. Naveira Garabato (SOUTHAMPTON), S. Swart (UGOT)