Orchestra-1

The development and destruction of ocean stratification is a key control on the flux of heat and momentum into the ocean interior, yet the processes by which mixed layers are created, deepened and ultimately removed remain poorly understood both in both a qualitatively and quantitatively. This deficiency in knowledge is particularly acute in the Southern Ocean, which is known to be a region of strong heat and carbon uptake and which plays a pivotal role in setting the magnitude and configuration of the meridional overturning circulation. For example, it is known that surface-sourced internal waves driven by near-inertial wind motions effect mixing between the upper and lower overturning cells, yet the routes through which momentum is moved from the surface ocean to mid-depths are only partly understood. In addition, it is known that surface fluxes strongly regulate the formation of both mode and deep waters in the Southern Ocean, which in turn ventilate much of the global abysss.

Progress to date in this field has been strongly hindered by a lack of high-resolution upper ocean observations over the appropriate spatial scales to capture the mixed layer processes of interest. Many of these processes (e.g. Langmuir cells) occur at a range of sizes known as the submesoscale, typically 1-10km. This scale is strongly undersampled, meaning that parameterizations of mixed layer physics remain relatively crude. The sampling strategy we propose, which combines observations of surface fluxes from a surface vehicle (subject to a separate SME) with high-resolution ocean observations from underwater gliders, will target these scales directly. These observations will achieve several objectives:

1. Quantify the development of summertime stratification to the south of the Polar Front.
2. Understand how surface fluxes affect the breakdown of the wintertime mixed layer, and the role of submesoscale processes in these changes.
3. Quantify the changes in turbulent mixing that occur during the summer season and how these affect near-surface stratification.
4. Understand the magnitude and timescales of mixing of heat from the surface layer into the ocean interior.