JC31: 'Ingredients' of the Southern Ocean


Cruise diary

Saturday 7 February 2009
Location: Drake Passage (57.13°S/68.25ºW)

Is the Antarctic Circumpolar Current getting stronger?

The Antarctic Circumpolar Current flows eastward all the way around the world at about 50°S to 60° latitude.  It is the largest current in the world ocean and it connects the Pacific, Atlantic and Indian Oceans.  Drake Passage between the southern tip of South America and Antarctica is the narrowest constriction through which the Antarctic Circumpolar Current must flow.  Our mission is to measure the flow of the Current through the Drake Passage chokepoint by taking a section from South America to Antarctica.

In the past the transport of the Antarctic Circumpolar Current through Drake Passage has been estimated to be 130 Sverdrups.  A Sverdrup is defined to be a million cubic meters per second.  The Amazon being the largest river has a transport of about 0.3 Sv, so we are measuring a Current with a flow 400 times larger than the world’s largest river.  The Antarctic Circumpolar Current is notable in that it is very wide, about 600 km in north-south extent, and it penetrates all the way to the bottom of the ocean at 4000 m depth.

Many scientists think that the transport of the Antarctic Circumpolar Current should be getting stronger with time.  The westerly winds (winds blowing toward the east) have been getting stronger due to a strengthening of the Antarctic polar low (increasingly low atmospheric pressure over the Antarctic continent may be related to the ozone hole).  These scientists believe that the wind forcing determines how strong the Antarctic Circumpolar Current is and they believe the 10% increase in winds should result in a measurably stronger transport.  Recent analyses published in Nature last November, however, suggest that the Antarctic Circumpolar Current transport has not increased and may in fact have decreased.  We are here to see for ourselves, concentrating on Drake Passage where the transport can best be observed.

To measure the flow of the Antarctic Circumpolar Current, we plan to take a series of stations oriented north to south across Drake Passage.  We lower an instrument package from sea surface to sea bottom measuring continuous vertical profiles of temperature, salinity, oxygen, east and north components of ocean current.  On the return upward, the package is stopped periodically to take 24 water samples distributed throughout the water column and these water samples are analysed in chemistry laboratories on board ship to measure the concentrations of salinity and oxygen (to calibrate the continuous profiles), nitrate, phosphate, silicate, total carbon, alkalinity, and chlorofluorocarbons.  The profiles of east and north currents represent a direct measure of the flow of the Antarctic Circumpolar Current.  We also have a traditional method to measure the current flow by using the temperature and salinity profiles to estimate geostrophic currents.  This traditional method has been the way in which historical observations of the transport have been made.

So our initial goal is to make about 45 stations across Drake Passage over the next 2 weeks, to process and analyse the temperature, salinity and current profiles to international standards and to have a preliminary idea whether the flow of the ACC is increasing, decreasing (or about the same as before) at the end of the expedition.

Left: Deployment of the CTD

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