Ocean Dynamics Research at RSES

The dynamics of the global ocean depends on the forcing from climate and the physical processes which occur in the ocean. Studies into Ocean Dynamics concentrate on determining the role of physical processes through both laboratory experiments and numerical modelling. These studies ultimately feed into models of the earth's climate and ecosystem.

Current Research projects

Ocean thermohaline circulation

The global meridional overturning circulation of the oceans (a major component of the thermohaline circulation) is forced by density differences owing to heat and water fluxes at the sea surface. Wind stress on the surface and injections of energy into turbulent mixing from the winds and tides modify this circulation. In our approach we examine the extent to which the density differences and interior turbulent mixing together, but in the absence of large-scale wind stresses, could force the overturning. The fundamental dynamics of convective overturning is being examined using experiments in the Geophysical Fluid Dynamics laboratory. The ultimate goal is to determine the role of such convection in the global circulation.

Relevant Papers:

Hughes, G.O. and Griffiths, R.W. (2005) A simple convective model of the global overturning circulation, including effects of entrainment into sinking regions. Ocean Modelling 12 , 46-79 (doi:10.1016/j.ocemod.2005.04.001).

Hughes, G.O. Griffiths, R.W., Mullarney, J.C. and Peterson, W.H. (2006) A theoretical model for horizontal convection at large Rayleigh number. J. Fluid Mech ., in press, to appear June 2007.

Mullarney, J.C., Griffiths, R.W., and Hughes, G.O. (2007) The role of freshwater fluxes in the thermohaline circulation: insights from a laboratory analog. Deep-Sea Res. I , doi:10.1016/j.dsr.2006.10.001.

Dynamics of the Southern Ocean

The Southern Ocean is characterised by turbulent flow associated with the world's strongest ocean current, the Antarctic Circumpolar Current (ACC). This turbulence is dominated by mesoscale eddies – vortices which are small (~50km) compared with the size of the Southern Ocean. Mesoscale eddies play a variety of roles in the dynamics of the Southern Ocean: they help to balance the forces contributing circulation in this region, thereby controlling the zonal momentum balance of the ACC, and are responsible for carrying heat across the Southern Ocean. We model the ACC using an eddy-resolving ocean model to determine the dynamics of this important ocean basin.

Relevant Papers :

Meredith M. P., and Hogg A. McC. (2006) Circumpolar response of Southern Ocean eddy activity to a change in the Southern Annular Mode. Geophys. Res. Lett. 33 , L16608, doi:10.1029/2006GL026499.

A. McC. HOGG & J. R. BLUNDELL, (2006). Interdecadal variability of the Southern Ocean. J. Phys. Ocean., 36, 1626-1645.

Mixing in ocean straits

We are measuring the amount of mixing that occurs when two fluids of different density exchange in opposite directions through a constriction or over a sill. Such exchange flows commonly occur through ocean straits and over bottom sills. The constrictions control the rate of transport of water into or out of estuaries and marginal seas and between abyssal ocean basins. This process may contribute to the overall amount of mixing required to maintain the stratification of the oceans and the present global rate of overturning.

Analytical methods are frequently used to estimate exchange flows through these constrictions; we have focused on extending the simple analytical solutions to more realistic cases. Exchange flows involve strong velocity and density gradients between the two layers flowing in different directions, and this flow can become ‘critical' (i.e. reach the speed of gravity waves on the density interface) so that there is a hydraulic control point in the strait or above the sill.

Relevant Papers:

Prastowo, T. J., Griffiths, R. W., Hughes, G. O. and Hogg, A. McC. (2006) Mixing due to exchange flows through a horizontal constriction. J. Fluid Mech

Ocean circulation through the Indonesian Seaway

The Indonesian throughflow (ITF) is thought to play an important role in global thermohaline circulation and influence global climate by funneling Pacific Warm Pool water into the Indian Ocean. Observations suggest the ITF is composed of North Pacific subtropical and thermocline waters that flow through Makassar Strait. At the southern end of Makassar St. most of the water flows eastward through the Flores Sea into the Banda Sea and then into the Indian Ocean.

Throughflow variability and flow volume is highly correlated with the state of ENSO. La Niña causes increased sea surface height in the Western Pacific Warm Pool region enhancing the dynamic height gradient with the Indian Ocean and therefore throughflow volume increases, the converse during El Niño is also true. The core of the ITF is also influenced by vertical mixing. Heat and freshwater are transported down toward the thermocline and cool water mixes upward resulting in potential influences on atmosphere-ocean heat flux . The ITF results in a significant export of heat and freshwater from the tropical Pacific into the Indian Ocean and may influence atmosphere-ocean coupling and tropical SST patterns and has the potential to impact the Asian Monsoon and ENSO. One way to examine the sources and variability of the throughflow is to use a water mass tracer. The radiocarbon ( D 14 C) content of waters can be used to track ocean currents, vertical mixing, and air-sea CO 2 exchange.

Measurements of sub-annual samples from coral skeletal material provide a proxy time-series record of the D 14 C of the dissolved inorganic carbon (DIC) of the surrounding seawater. This study will investigate interannual and decadal changes in tropical ocean circulation (including El Niño/Southern Oscillation) and advection of Pacific water masses through the Indonesian Seaway using a reconstruction of the spatial and temporal variability in 14 C distribution many decades into the past. We will focus on the acquisition of multi-decadal, high-resolution, high-precision ? 14 C time series from coral sites in the western Pacific and the Indonesian Seaway in order to study 1) variations in the proportion of waters mixing in the western tropical Pacific and 2) variability of the Indonesian throughflow itself and its relation to El Niño/Southern Oscillation.

Wake flows in coastal oceans

In a collaboration with the university of New South Wales we are exploring the interactions between topography and flow in shallow coastal regions, where headlands and islands are common features that disrupt mean currents, creating wakes and eddies that may trap or disperse nutrients, sediments and biota. In particular, we are using laboratory experiments to examine the effects of flow irregularities such as eddies or turbulence when these are carried into a wake region from upstream. To date we have demonstrated a strong coupling between incident eddies and eddies generated on the wake flow, creating dipolar vortices, and there are preliminary indications that incident disturbances lead to a greater release of kinetic energy from the mean shear into eddy kinetic energy.