Looping surface drifters help to close the Arctic oceanic loopBy Cecilie Mauritzen and Inga Koszalka
|
Objective
Ocean currents transport heat towards the Arctic in the upper ocean and cold waters towards the equator in the deep ocean. Though the process is not well understood, heat is lost to the atmosphere and to the surrounding water masses. POLEWARD aims to quantify this process in the Norwegian Atlantic Current, just as the current is about to dive and become a deep current, losing contact with the atmosphere for hundreds of years. The project is affiliated with the Integrated Arctic Ocean Observation System (iAOOS) - Norway: Closing the loop.
POLEWARD represents the largest surface drifter experiment in the Nordic Seas to date. During the two-year period of International Polar Year (IPY), a set of 118 surface drifters , specially designed surface floats (fig.1) , have been released in the Norwegian Atlantic current at various locations along its path towards the Arctic. The instruments are tracked by the Argos satellite system. The resulting trajectories will be used to describe the structure of the current, and this in turn will help quantify the amount of heat lost by the current as it moves northward. The drifters in particular will help us quantify lateral mixing into the interior of the Nordic seas by the mesoscale eddy field. The data contributes to the Global Drifter Program (http://www.aoml.noaa.gov/phod/dac/gdp_drifter.html).
Fig. 1. Left: A Surface Velocity Program (SVP) drifter before the deployment. The instrument consists of a spherical surface float with a transmitter and a temperature sensor, and a drogue securing the desired buoyancy properties. Middle: A drifter after deployment. Right: A drifter on its ocean voyage. Foto: The courtesy of Global Drifter Program (http://www.aoml.noaa.gov/phod/dac/gdp_drifter.html).
Drifter deployments
Clusters of drifters are released at four sites along the pathway of the Norwegian Atlantic Current: at Svinøy, off Gimsøy and Bear Islands, at the opening of the Barents Sea. The most recent deployments in the Lofoten Basin aims to study the importance of this region for mixing processes (fig.2) . Thus far 148 drifters have been deployed, in June/July 2007, October 2007, March 2008, October 2008 and June and July 2009.
|
|
Fig.2. Left: POLEWARD deployment sites: Svinøy (red), Gimsøy (dark blue), Barents Sea (cyan), Bear Island (magenta), and the Lofoten Basin (black). Superimposed is a schematic of the pathway of the two branches of the Norwegian Atlantic Current. Right: Tracks of selected poleward drifters in the first 20 days after release, color coded after deployment site (see left), except of the deployment at Svinøy from October 2008, which plotted in green. These drifters got trapped in a big anticyclonic eddy before setting on their northward journey.
Results
While most of the drifters follow the eastern branch of Norwegian Atlantic Current, there are some that are carried along the coast by the Norwegian Coastal Current, and some that disperse from the Svinoy section directly into the Norwegian basin (fig.2).
Initial analysis of the trajectories reveals a general northward drift which is strongly influenced by topography. In regions with strong topographic slopes, the trajectories closely follow the isobaths. Over weaker slopes, the motion is more complex and chaotic (fig.3). The tendency of drifters to follow the topographically-steered main flow is less conspicuous at later times after deployment as more and more of them are swaped off by the smaller-scale flows. Gradually the trajectories spread all around the Nordic Seas (fig.3), which reflects their complex histories determined by the main current, recirculations and turbulent flows.
|
|
Fig.3. Left: Tracks of poleward drifters in the first 15 days after release, superimposed on the bathymetry. Right: Tracks of poleward drifters during 60-80 days after release.
Because the motion of drifters is chaotic, we focus on statistical descriptions, and we are currently calculating both single and multiple particle measures of dispersion. This dispersion is essential to the cooling of the current, because the chaotic motion prolongs the exposure of the warm surface waters to the atmosphere. This in turn permits the water to become dense enough to sink, a key dynamic in the thermohaline overturning circulation. The drifter trajectories from the deployments in the Barents Sea and Lofoten Basin are shown in (fig.4). There were 15 drifters deployed in the Barents Sea in 27-29 June 2009. All deployments were succesfull; However on the 21 September 2009 we note already 5 drifters that quit transmitting (erliest after 6 days, latest after 60 days). Trajectories of these drifters are plotted in (fig. 5). There were also 15 drifters deployed in the Lofoten Basin in 3-5 July 2009. Of these, one failed at the deployment while tracks of the remaining succesful drifters are plotted in (fig. 6).
Fig.4. The trajectories of drifters deployed in the Barents Sea (27-29 June 2009, 15, yellow) and the Lofoten Basin (3-5 July 2009, 14, pink) as in February 2011.
For an animation of POLEWARD trajectories, see HERE
