Amelie Meyer

On the role of diapycnal mixing in the Antarctic Circumpolar Current on the Meridional Overturning Circulation

Supervisors: Prof Nathan Bindoff, Dr Helen Phillips, Dr Bernadette Sloyan

Background:

To date, theories and models of the Southern Ocean circulation have been built on the assumption that the flow is primarily along rather than across isopycnals (surfaces of constant density) in the ocean interior, with diapycnal processes (flow across density surfaces) largely confined to the upper ocean mixed layer. Much of our understanding of what sets the structure of the Southern Ocean Meridional Overturning Circulation (MOC), the Antarctic Circumpolar Current (ACC) transport, their dynamical coupling to the global ocean and a range of important biogeochemical and palaeoceanographic issues has been developed under this assumption. Recently, however, observational studies have found evidence for diapycnal mixing at great depth in the deep Southern Ocean, thought to be due to internal waves and turbulent mixing where the ACC impinges on rough topography.

A joint UK-USA-Australia observational effort has been conceived to directly measure all terms in the ACCs momentum balance, including diapycnal mixing rates. The crux of the observational effort is a research cruise in late 2008 to the northern flank of the Kerguelen Plateau where the two main jets of the ACC form a large standing meander. We anticipate that strong geostrophic flow over roughness on the northern Kerguelen Plateau slope will be an efficient generator of turbulence, either via internal wave breaking or another instability process. To measure this turbulence, and other quantities relevant to the momentum balance of the ACC, we will use a combination of traditional sampling techniques (such as shipboard CTD and ADCP) and newly-developed instruments (in particular, small, low-cost deep-ocean microstructure profilers, moored profiling CTDs/current meters, and velocity-sensing Argo floats). This study will be the first dedicated observational study of turbulence in the Southern Ocean.

The PhD project will focus on observations of diapycnal mixing collected by the velocity-sensing Argo floats, known as EM-APEX (ElectroMagnetic - Autonomous Profiling eXplorer), which will be deployed on the western, upstream, side of the Kerguelen Plateau during the research cruise. The EM-APEX will collect profiles of temperature, salinity, pressure and horizontal velocity over the upper 2000m of the ocean as they flow with the ACC across the Kerguelen Plateau and continue downstream. These measurements will be used to calculate diapycnal mixing from the vertical gradients of density and horizontal velocity. The EM-APEX will provide a broader spatial and temporal context for the shipboard hydrographic and mooring observations. Then, as the floats are advected through the Southern Ocean, the geographic spread of the observations will provide information on the variability of the diapycnal mixing rates in relation to bottom topography, cross-stream position, geographical regime (i.e. where the current is constricted or free), and atmospheric variability.

Project Outline and objectives

The project will comprise of 4 stages.

1. Defining an optimal deployment and sampling strategy for the EM-APEX. This will involve examining existing Argo float trajectories and eddy-resolving model simulations to determine typical pathways floats take along the ACC, and how rapidly they progress downstream.

2. The research voyage in October/November 2008, in which the student will participate.

3. Comparison of the diapycnal mixing observations from the EM-APEX with the shipboard microstructure observations across the Kerguelen Plateau. This work can begin straight after the voyage because the EM-APEX transmit data in real time and we expect they will have completed their crossing of the plateau soon after the voyage ends.

4. Analysis of diapycnal mixing rates from the EM-APEX downstream of the Kerguelen Plateau in relation to bottom topography, cross-stream position in the ACC, and other influences. Once the EM-APEX are pushed downstream of the plateau by the ACC, their profiling rate will slow to 1 profile every 10 days to extend their lifetime and give profiles over as wide a segment of the ACC as possible.

Objectives

 I. Using observations of diapycnal mixing from EM-APEX floats, test the hypothesis that the interaction of the geostrophic ACC flow with rough topography is the primary cause of widespread, strong diapycnal mixing in the Southern Ocean, and that the along-stream variability of diapycnal mixing is directly correlated with topographic roughness;

II. Contribute to the efforts of the international team to understand the role of the ACC momentum balance in the Meridional Overturning Circulation (MOC).