Dependence of Southern Ocean Overturning on Wind Stress

story by Helen Hill

Image credit: R. Abernathey

Ryan Abernathey is a fifth year Ph.D. student in MIT’s Program in Atmospheres, Oceans and Climate. His particular interest is in the geophysical fluid dynamics responsible for determining Earth’s climate. To this end, much of his work over the past 3+ years has involved using MITgcm to better understand, in particular, the role of eddies and turbulent processes in the large-scale circulation of the ocean and atmosphere. Most recently, in collaboration with MIT co-authors David Ferreira and advisor John Marshall, Abernathey has been using idealised MITgcm simulations to explore the dependence of Southern Ocean meridional overturning on wind stress.

In a very nice, process-oriented study (in the tradition of the group’s ocean convection work (!) from the early to mid-90’s) Abernathey et al. use an eddy-resolving numerical model of a zonal flow as a simple analogue to the Antarctic Circumpolar Current. In addition to wind and buoyancy forcing at the surface, the model contains a sponge layer at the northern boundary to permit a residual meridional overturning circulation at depth.

A novel feature of particular interest to other MITgcm users will likely be the use of the second-order-moment advection scheme of Prather (1986) (see also Hill et al. 2011) to maintain a “realistically effective diapycnal diffusivity (κv = 0.5× 10−5 m s−2) “.  Full particulars of the model’s configuation can be found in Abernathey et al.

The above movie depicts the 3D temperature field evolving over a period of one year in a zonally-periodic channel domain meant to represent the ACC, forced with wind stress and heating at the surface. The forcing leads to a baroclinically-unstable mean state, which produces the vigorous mesoscale eddies seen in the movie (animation credit – Ryan Abernathey, Marshall Lab.)

Having spun up the model from rest for approximately 200 years to establish a statistically steady state,  averages performed over 20-year intervals, were then analysed to diagnose the strength of the residual MOC for different strengths of surface wind stress.

Returning to the framework laid out in Marshall and Radko (2003) to describe and analyze their results the team make a number of interesting observations. They find that the eddy circulation, in large part, compensates for the changes in Ekman circulation and  that the extent of that compensation, and hence the sensitivity of the MOC to the winds, thus depends on the surface boundary condition. They also report finding that a fixed-heat-flux surface boundary condition severely limits the ability of the MOC to change while an interactive heat flux allows for greater sensitivity. Find out more in their paper The Dependence of Southern Ocean Meridional Overturning on Wind Stress, recently accepted to J. Phys. Oceanogr. To find out more about this work? Contact Ryan.

References:

Abernathey, R.P., J. Marshall and D. Ferreira (2011)
The Dependence of Southern Ocean Meridional Overturning on Wind Stress
accepted to J. Phys. Oceanogr.

Hill, C., D. Ferreira, J.-M. Campin, J. Marshall, R. Abernathey, and N. Barrier (2011)
Controlling spurios diapycnal mixing in eddy-resolving height-coordinate ocean models: Insights from virtual deliberate tracer release experiments.
Ocean Modelling (submitted)

Marshall, J. and Timour Radko (2003)
Residual-Mean Solutions for the Antarctic Circumpolar Current and Its Associated Overturning Circulation.
J. Phys. Oceanogr., 33, 2341–2354, doi: 10.1175/1520-0485(2003)033<2341:RSFTAC>2.0.CO;2

Prather, M., 1986: Numerical advection by conservation of second-order moments. J. Geophys. Res., 91 (D6), 6671–6681.

People

Ryan Abernathey has been using MITgcm since 2006.

John Marshall

David Ferreira has been using MITgcm since 2002.