JC31: The 'ingredients' of the Southern Ocean


JC31: The 'ingredients' of the Southern Ocean

About the cruise

On 3 February 2009 the RRS James Cook will sail from Punta Arenas, Chile to embark upon the first of two consecutive research cruises (JC31 and JC32).  During JC31 the ship will be home to a group of scientists who will measure different properties of the ocean in Drake Passage.  Drake Passage is the narrowest stretch of water in the Southern Ocean, spanning approximately 500 miles between the southern tip of South America and the West Antarctic Peninsula.  The scientific party will have a range of specialties such as measuring dissolved oxygen and nutrient concentrations, chlorofluorocarbons, carbon, atmospheric analyses and also physical measurements of the ocean such as temperature, salinity and water velocity.

You may ask yourself what is the point in looking at the oceans.  Well, the climate system is made up of a group of subsystems; the ocean, atmosphere, geosphere, cryosphere and the biosphere (all living things), that are all intimately linked by numerous chemical and physical pathways and feedback processes.  The oceans are particularly important because they cover such a large expanse of the Earth’s surface (approximately ¾), and due to the unique properties of water, they have an enormous capacity to store and transport heat and chemical constituents like carbon around the globe.  As a result, the ocean plays an important role in determining the timing and location of climatic change.  The capacity of the oceans to buffer the effects of human activities (i.e. absorption and transport of carbon dioxide (CO2) and heat) play a large part in determining the rate of global climatic change, and this is the issue with which we are concerned.

JC31 will conduct measurements along a line in the ocean (known as a section).  A section is comprised of a series of stations (sampling points) where instruments are lowered from the ocean’s surface to the bottom, making precise measurements throughout the water column of temperature, salinity, and dissolved oxygen as well as various other chemical analyses of water samples.  This is achieved by a combination of bottles fired at different depths to collect water samples and an instrument called a CTD (conductivity, temperature, depth), which makes continuous measurements of conductivity (from which we derive salinity), temperature and pressure (from which we derive depth) through the water column.  Also attached to the CTD frame is an LADCP (Lowered Acoustic Doppler Current Profiler).  This uses sound pulses (sonar) and the principle of the Doppler shift to determine the velocity of the currents in the water column.  The water samples collected from different depths by the bottles will be divided between the scientists on board so that they can conduct their individual analyses.  As technology of the instruments advances, the accuracy and reliability of the data improves which in turn improves the predictions of ocean models.

Figure 1: Drake Passage with the sections to be taken by JC31 

Figure 2: Map of the South Atlantic and Southern Ocean

(The colour scheme in figures 1 and 2 maps the bathymetry of the ocean floor. For example, bathymetric features such as the Mid Atlantic ridge and continental shelves can be clearly identified.  The dark blue areas represent the deepest regions of the oceans, 8000 metres deep).

The sections to be surveyed during JC31 are A21 in the Drake Passage and SR1B, located further to the east (Figure 1).  SR1B has been occupied annually since 1993 except for two years.  A21 has previously been covered twice, once by METEOR in 1990 and once by the RRS James Clark Ross in 1999.  Data from JC31 will allow us to examine changes in water mass properties via comparison with these previous sections.  Also the sections in the Drake Passage will be combined with other sections taken in the South Atlantic/Southern Ocean at 24°S (to be occupied during JC32) and 30°E in the Southern Ocean (occupied by the French in 1996 and on a US cruise in February 2008). Currently, exchanges of heat and chemical properties between the Atlantic and Southern Ocean are poorly understood.  The combination of data from all these cruises will form a box around the Atlantic sector of the Southern Ocean allowing scientists to work out the budgets (i.e. what is flowing in and out) of heat, freshwater, carbon and other biogeochemical tracers in this part of the southern ocean the Southern Ocean.  The data will also help to improve the accuracy and reliability of predictive ocean circulation models, which are important in formulating strategies geared to either mitigating or adapting to the effects of climate change.  For example, monitoring the circulation of ocean currents transporting heat to countries such as the UK allow more predictions to be made concerning the rate of change of this circulation, and therefore how it is likely to impact the climate of these regions.

The scientists onboard the RRS James Cook for JC31 come from the following institutions; National Oceanography Centre Southampton (NOCS), University of East Anglia (UEA), University of Bangor, University of Reading, University of Glasgow and University of Sao Paulo. Find out more by visiting our Team page.

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