What are you aiming to achieve?

Of the earth’s surface, 70% is ocean, of which almost 90% is deeper than 3,000 m. Today’s relatively mild global environment is maintained by global ocean circulation, the slow circulation of the ocean with its enormous thermal capacity. The role of “engine” of this ocean circulation is played by the polar oceans, which are cooled by the atmosphere, and cold seawater, with a higher density, which has been formed in association with sea ice formation, sinks down into the deep ocean and spreads towards lower latitudes (Fig. 1). This sinking motion into deep water also causes oxygen, carbon dioxide (carbon) and other substances to be transported from the surface layers. Furthermore, the surface layers compensate for this flow of deep water by transporting warm seawater from low to high latitudes. This circulation transports heat and materials throughout the global ocean, and forms and maintains a mild global environment. However, ocean circulation and sea-ice formation are difficult to observe directly, and as such many questions remain to be answered. We are working in cooperation with domestic and overseas research organizations to answer these questions, and our research mainly consists of observation experiments in the Arctic and Southern Ocean.

Fig. 1 Water temperature distribution at ocean bottom deeper than 4,000 m. The purple and green arrows indicate movements of cold seawater flowing out of the polar oceans, originating near the Antarctica and the North Atlantic (●symbols are main formation areas of cold seawater near the Antarctica). The brown area is the Kerguelen Plateau, and ● symbol is the region of observation. (Diagram: After Gordon (2001) with modifications)

 

What kind of experiment do you carry out?

Oceanographers normally carry out shipboard observations, but they are not always possible in polar oceans far from civilization and with sea-ice cover in winter. For this reason, we use so-called “moorings”, which consist of floats, ropes and weights to create fixed points and oceanographic instruments attached to ropes. Moorings are deployed for a year or longer to collect continuous time-series data (Fig. 2). Using this method allows us to collect data even during winter, when sea ice is present and cold water is mainly formed. This type of observation is also extremely important for the monitoring of global warming.

Recently, we have collaborated with an Australian research organization in the area around the Kerguelen Plateau, which is one of the main outflow routes for cold seawater formed near the Antarctica (Fig. 1), and have carried out extensive moored observations over two years. As a result, we have established that an extremely strong flow of cold seawater, averaging 20 cm/sec or faster, flows towards the lower latitudes in this area (Fig. 3). This is the fastest speed for seawater so far measured at a depth of 3,500 m or deeper, and its volume transport is roughly the 40 times the Amazon river transport, indicating the importance of northward ocean circulation from the south.

What are you aiming next?

In order to clarify the state of sea ice formation, which is an important process affecting global ocean circulation, it is important to measure sea-ice thickness in polar oceans. The only way this can possibly be done is satellite observation, but we do not have enough in situ data to validate its estimate based on satellite observation. We are therefore currently using moorings to measure sea-ice thickness in the Arctic and Southern Ocean. With these data, we will improve the accuracy of satellite sea-ice thickness estimate and hence enhance our understanding of global ocean circulation.

Fig. 2 Mooring deployment (floats and a current meter). Fig. 3 Two-year averaged north-south flow speed (contours in cm/sec, positive flow northward) and water temperature (indicated in color) distribution. Symbols ○, △ and □ represent the locations of oceanographic instruments.