FIRST ASSESSMENT

Caught in the ice of the Arctic transpolar drift, Tara has covered more than 1000 km in a straight line within nine months since September 2006 when she started off from the Laptev sea.

This drift in the ice is twice as fast than what was imagined and Tara has practically covered half of the way that leads her to the Fram Strait, that is her exit from the Arctic Ocean. Positioned towards the geographic North Pole, this drift enables Tara to be the vessel imprisoned by the ice having reached the most northern latitude. A historical record.
All this information can be explained for several reasons. The present Arctic climate system is characterized by a weak extension of the anticyclonic circulation in the Beaufort Sea (Canadian basin), a mean atmospheric pressure field weakened in the centre of the Arctic, air temperatures above the seasonal average as well as a quantity of sea ice, exported from the Arctic Ocean towards the Atlantic Ocean greater than the norm.
During the first eight months of the drift, several important mechanisms that characterise the ocean circulation and the formation of sea ice were identified. These concern mainly the inertial oscillations that were observed since the beginning of the drift in September and in October 2006. They are linked on the one hand to the wind stress acting on the ocean mixed layer 20 to 30 meters thick under the surface and on the Coriolis force linked to the Earth’s rotation on the other hand. When the wind ceases, the water mass moved previously by the wind, adjusts itself to the Coriolis effect by inertial (centrifugal force). These inertial oscillations as quasi circular movements of the Tara drift are easily identifiable thanks to her frequent and precise positioning by GPS. The radius of curvature of the inertial circles is directly a function of the speed induced by the wind on the water mass divided by the inertial frequency. The inertial period being of about 12 hours at this latitude, each circle is described in 12 hours. The inertial circles radius being about 2 km, this corresponds to a drift speed of 30cm/s. These inertial oscillations trigger a very efficient mixing process within 20 to 30 meters of the Ocean under the surface called the Ocean mixed layer.
A second identified mechanism concerns the sea ice formation at about twenty to thirty meters underneath the surface, which is located at the base of the ocean mixed layer. At that depth, the mixed water mass and super cooled by the atmosphere is maintained in a liquid state by the effect of pressure. If the pressure diminishes under the effect of internal waves for instance ice crystals that are called frazil, can appear at depth. They surface rapidly thanks to the buoyancy they have acquired when changing from a liquid state to a solid state (ice).
The making of frazil ice phenomenon arose with strength to the point of scrambling the bathymetry readings of the ocean bottom by acoustic survey that were accomplished from Tara, as well as the salinity and water masses temperature measurements. This phenomenon was considered of marginal importance to explain sea ice formation in the Arctic. Observations accomplished on Tara during the course of the winter 2006-2007 oblige us to change our point of view on this subject. This might explain why we have discovered in April 2007, one hundred miles south of Tara very thick new winter ice (over two meters) and not deformed.
Generally speaking, it seems as though the Arctic sea ice has a tendency to behave more and more like the Antarctic sea ice. In the coming decades, it would mean a complete disappearance of the Arctic sea ice in summer and of the formation of new ice in winter.
It would change drastically the radiation planet assessment with 80% of the incident solar energy that would be absorbed by the Arctic Ocean, free of ice in the summer instead of being reflected back into space from an ice surface of 7 million km2 (equivalent to Europe’s surface area). Paradoxically more ice would form in the winter but this ice would disappear during the summer thawing. Is the perennial Arctic ice in the process of disappearing and why? The European integrated Damocles project is precisely addressing these two questions and trying to answer them. This project concerns 48 institutions from more than ten European countries. Fifteen scientists of seven different nationalities came to participate in the crew change of eight persons who have spent winter onboard of Tara since the month of September 2006. Ten persons in charge of routine measurements have taken their turn.
The onboard scientific programme has now fully started with observations that are simultaneously carried out in the atmosphere up to 1500 meters high, in the ocean down to 4000 meters depth and through the few meters of ice that separate them. It is essentially a matter of measuring the air, water temperature and the pressure according to altitude and/or depth. The humidity of the air and the salinity of the sea water are observed as well as the intensity and the wind direction and currents. Ultra violet, infra red and visible radiations are analyzed permanently thanks to radiometers oriented toward the zenith and the nadir. A web of 16 weather beacons has been set up within a square with sides measuring 500 km of which Tara is at the centre. By the end of the summer, a web of 20 oceanographic beacons will be installed in the heart of the Arctic Ocean. At that time, Tara will already be located at the front of this web and in the process of exiting the Arctic Sea Ocean planned before spring 2008.
The data collected by these automatic networks of platforms as well as observations collected at Tara and from the drifting Russian station NP35 that will be deployed in September 2007, should enable us to establish more reliable predictions concerning the current evolution of the Arctic in the context of global warming.

By Jean Claude Gascard, CNRS Research Director
Coordinator of the European project Damocles at the University Pierre et Marie