Why the Southern Ocean?
The Southern Ocean, and in particular Drakes Passage which runs between the south coast of Argentina and the Antarctic Peninsula is well known, perhaps infamous, as host to some of the roughest seas on the planet. Despite this, we have chosen to conduct our field experiment to the south of the Falkland Islands, just to the east of Drakes Passage, at the onset of the Southern Ocean winter. With winds expected to be consistently over 30 knots and wave heights reaching more than 10 metres at times, you may ask why we have chosen this time and place?
Schematic of the Southern Ocean overturning circulation. Subantarctic Mode Water (SAMW) is modified and subducted to the north of the Subantarctic Front (SAF). Previously considered to be due to air-sea interaction alone, the aim of SMILES is to assess the extent to which the transformation of surface waters occurs due to submesoscales throughout the surface mixed layer, the generation of which may arise through the forcing by westerly, down-front winds.
The reason for compromising our comfort aboard the RRS James Clark Ross during this cruise is the particular combination of oceanic and atmospheric conditions at this location and time of year which are particularly favourable for the generation of submesoscales.
The reason for compromising our comfort aboard the RRS James Clark Ross during the cruise is that the particular combination of oceanic and atmospheric conditions at this location and time of year are particularly favourable for the generation of submesoscales. Whilst in some instances submesoscales appear spontaneously, a specific mechanism that we know is particularly effective in generating submesoscales are strong winds blowing in the same direction as the strong currents found at major oceanic fronts. When this happens, the combination of friction and the earth’s rotation drives a transport of water across the front, a process referred to as Ekman transport. In the case of the subantarctic front being blown by the usual westerly winds, this results in the transport of cold water from the south across the front to the north where it sits above warmer water. This situation is gravitationally unstable as cold water is denser than warm water. The overturning that results is a particularly effective mechanism for generating the small-scale instabilities that we know of as submesoscales. As the frontal jet at the subantarctic front is strong and persistent, and the winds are almost always directed from west to east in the same direction as the jet, this is an excellent place to study submesoscales!
Subantarctic Mode Water
Once submesoscales have been generated in this region, they have a crucial role to play in the climate system. North of the SAF, Subantarctic Mode Water is formed by atmospheric cooling. This water mass, which is formed throughout the Southern Ocean, is then subducted to the ocean interior where it finds its way into every one of the world’s oceans and is effectively locked away from further contact with the atmosphere. Crucially, as 40% of anthropogenic CO2 enters the ocean to the south of 40S (see the figure below from Sallee et al, 2012), the mechanisms that regulate the transfer of CO2 between the atmosphere and the ocean have widespread implications for the oceans ability to buffer the impact of CO2 emissions on climate. As SAMW is formed in a region that we believe to be host to pronounced submesoscale activity and that these submesoscales directly impact on the exchange of gases between the upper ocean and the atmosphere and therefore climate, we consider there is an imperative need to better understand the conditions under which submesoscales evolve and how their subsequent behaviour impacts on SAMW properties.
"If we want to understand climate change we need to appreciate how the oceans interact with the atmosphere at their boundary. And for the ocean, the top 100 metres is crucial – it’s what we term the surface mixed layer.
It’s where gases such as carbon dioxide are exchanged between the ocean and atmosphere; where nutrients are brought up from the deep ocean that in turn enable plankton to grow. But its effect upon the climate, while understood in a regional sense, is inadequately reflected in global climate models in areas where small scale processes have a disproportionate impact.”