Southern Ocean overturning circulation

The zonal transport of water masses in the Southern Ocean is brought about by the Antarctic Circumpolar Current (ACC) which, driven by strong westerly winds and the temperature differences between the subtropics and the icy Antarctic continent, represents the mightiest ocean current on Earth. The eastward zonal transport with the ACC is superimposed by a meridional overturning circulation, which is caused by a divergence in the large-scale mean wind field.

The deep water masses upwelling in the Antarctic Divergence, sucked in from their North Atlantic source regions, are partly driven northward near the surface towards the northern flank of the ACC, where in a zone of confluence they subduct to intermediate layers around 1000 m depth. Termed Antarctic Intermediate Water, it spreads northward until the northern temperate latitudes, where it influences through changes in temperature and salinity the large-scale weather patterns and though carrying nutrients the fishery yields.

Another part of the upwelled deep water gets to the Antarctic continental margins by advection with the clockwise rotating gyres of the Ross and Weddell Seas. Heat losses to the atmosphere, interactions with the adjacent ice shelves, and salt enrichment by brine ejected from freezing sea ice result in very dense water masses which run down the continental slope towards the deep sea floor. The newly formed and well ventilated bottom water underrides, steered by bottom topography, the ACC to fill the deep basins of the world ocean.

The upwelling deep water in the Antarctic Divergence is rich in nutrients and in natural CO2; without sea ice coverage hindering the gas exchange, it would release CO2 to the atmosphere. Due to primary production of organic carbon compounds through photosynthesis by phytoplankton however CO2 is taken up. Part of the organic material sinks as biogenic particles and so carries carbon and nutrients downward to the deep ocean layers or to the sediment. This process is called the biological pump. Whether the Southern Ocean at large acts as a source or a sink of CO2 for the atmosphere depends on the balance between the physical carbon pump coupled to the overturning circulation and the biological pump. The wind-driven upwelling of deep water is counteracted by mixing caused by mesoscale eddies. These eddies of typically 10 – 100 kilometers in diameter, which form continuously in the ACC and then decay over weeks to months, also influence the depth of the surface mixed layer hence the availability of sun light for the phytoplankton and the supply of nutrients, including the trace nutrient iron.
Controversially discussed among the international science community is whether the increase in winds, observed over the last decades, results in a stronger overturning circulation or in enhanced eddy activity. To this end it is presently also uncertain in which direction the delicate balance between the physical and biological pumps of carbon will shift in future, hence whether changes in climate due to anthropogenic greenhouse gas emissions will be amplified or dampened.

As physical oceanographers we are investigating primarily physical changes and mechanisms, but also chemical variations; and we work closely together with biologists to better understand the interaction between physical, biological and biogeochemical processes. Our studies consider the whole water column, extending from the open or ice-covered surface into the bottom water layer, and cover time and space scales which reach from microscale turbulence over the mesoscale dynamics up to decadal variations in the large-scale circulation.

Our long-term observations with focus on the Weddell Sea started in 1984 and represent the longest time series that exists in the open polar ocean. They are based on repeated measurements along hydrographic sections, on moored instruments, and increasingly during the last years on autonomous recording floats, the position of which is acoustically tracked under the ice and which transmit their data via satellite links. Our long-term time series document that the Weddell Sea bottom water became warmer during the last decades, and that its content of anthropogenic CO2 increased continuously; by detailed investigations it could be shown that the densest part of the newly formed bottom water is in equilibrium with the atmosphere.

Apart of long-term observations we conduct as well in the Weddell Sea as in the ACC interdisciplinary process studies dedicated to create better or new knowledge about the fluxes of energy and substances between the atmosphere and the ocean, about the interaction between mixed-layer turbulence and mesoscale dynamics, and on the control exerted by the physical environment on the plankton and the biogeochemical fluxes. Our data analysis frequently involves numerical modelling.