Control of the ACC system by eddies, topography and localised forcing

Lead Research Organisation: University of Southampton
Department Name: School of Ocean and Earth Science


Strong winds blow from west to east in the Southern Hemisphere. These winds drive a major ocean current around Antarctica, known as the Antarctic Circumpolar Current. The Antarctic Circumpolar Current is the largest and strongest current on the surface of the Earth, squeezing more than 130 billion kilograms of water between the southern tip of South America and Antarctica every second. This current links the Pacific, Indian and Atlantic Ocean basins, and permits a truly global ocean circulation. Perpendicular to the Antarctic Circumpolar Current's intense, West-East flow is a more subtle yet critical North-South circulation, known as the Meridional Overturning, or 'Global Conveyor'. Although weaker, constituting an exchange of 10-30 billion kilograms distributed across the entire Southern Ocean, the Meridional Overturning is critical to the Earth's climate. The Meridional Overturning transports heat from the equator towards the poles. The Overturning upwells water from the deep ocean around Antarctica, exposing carbon and nutrient rich water to the surface, feeding biological production, and influencing the amount of greenhouse gases absorbed by the Ocean. The intense East-West Antarctic Circumpolar Current is intimately linked to the North-South Meridional Overturning. As the current is forced by the winds it can accelerate. This accelerated current often becomes turbulent and collides with islands, seamounts and continents. As it does so, it forms swirling vortices of water, like cyclones or storms in the atmosphere. It is these swirling vortices that move heat, salt and CO2 from North to South. The winds, the current and turbulent processes all contribute to the fate of the Meridional Overturning. Understanding how this interplay works is the subject of our research. To do this we will make use of state of the art computer models, observations of the ocean from satellites, and go to sea, to measure the dynamics of the Ocean Circulation in-situ. Understanding the Antarctic Circumpolar Current and the Meridional Overturning, how they interact and may change, is critical. Decade to decade, the evolution of the Meridional Overturning will strongly affect the evolution and impact of climate change, including greenhouse gas concentrations (hence global temperatures), Antarctic Glacial Melt (hence global sea levels), and marine ecosystems (hence fisheries and biodiversity).


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Description The strength of the Antarctic Circumpolar Current (ACC), the largest and most energetic current in the climate system, has been found to be dependent on the local strength of the wind along the coast of Antarctica. Although it is far windier along the main path of the ACC - approximately mid-way between Antarctica and the southern continents (Africa, Australian and America) at latitudes known as the Roaring 40s and Furious 50s - the unique physics of the region result in winds along the coast of Antarctic being far more efficient at changing the ACC transport strength (doi: 10.1175/JPO-D-13-091.1).

New methods have been developed for diagnosing the Southern Ocean Overturning. The Southern Ocean Overturning runs perpendicular to the Antarctic Circumpolar Current and the two are intimately linked. The overturning is highly energetic - involves upwelling and downwelling of waters of different densities - and diabatic - involves the transformation of waters into different temperature and salinity classes. To this end a new framework has been developed which explicitly measures the vertical exchanges of water masses and their transformation. This has led to revised estimates (revised up) of the role of mesoscale eddy processes in the overturning and the response of that overturning to changes in wind forcing (DOI:10.1175/JPO-D-12-0178.1 and 10.1175/JCLI-D-11-00309.1). A new theory about the role of southern hemisphere winds in setting the ocean's vertical heat balance has also been presented (10.1175/JPO-D-12-0179.1).

The pursuit of water mass based approaches has led to new observationally based methods for diagnosing the influence and magnitude of surface heat fluxes, fresh water fluxes and mixing in the Southern Ocean and globally (doi: 10.1175/JPO-D-13-0213.1,10.1175/JPO-D-14-0039.1 and 10.1002/2014JC010097). These ideas have also led to progress in understanding the thermodynamic circulation of the Atmosphere (10.1175/JAS-D-13-0173.1).
Exploitation Route Findings relating to the ACC are of key interest to observational oceanographers and climate modellers. Observationalists are attempting to measure the ACC and relate its variability to climatic conditions. Climate modellers use the ACC to validate their ocean models as it is one of the only observable parameters in the climatically critical Southern Ocean region which is representative of the circulation. These results will help modellers to determine how winds influence the ACC transport in their models as opposed to, for example, surface warming.
Sectors Environment

Description So far this research has had academic impact. This impact can be measure in term of citations. There is unlikely to have been measurable societal impact as yet.