Description of task
This task aims at understanding the mechanisms that regulate the inflow of Atlantic water from the Atlantic through the Nordic Seas into the Arctic Ocean through the Barents Sea and Fram Strait, and its variation due to a mix of both local and remote forcing.
To estimate the transport of mass, heat and salt to the Arctic Ocean, we will continue moored arrays that have been running successfully for about a decade (partly in EU-funded projects VEINS and ASOF-EC-N) and that have provided crucial insight in the propagation of signals from the North-Atlantic into the Arctic. The observed time lags between sites carry the potential of prediction: eg long-term observations have recently demonstrated a link between the volume transport of the Norwegian Atlantic Current passing 62°N and the wind stress curl at 55°N 15 months earlier.
These input mooring lines are located off Norway (62°N Svinøy; 2 moorings, UoB), in Fram Strait (16 moorings; AWI, IOPAN, NPI) and across the Barents Sea Opening (5 moorings; IMR). In addition, in close co-operation with WP8, an optimally cost-effective observatory will be developed and first implemented in Fram Strait where intense mesoscale activity and the strongly variable recirculation of Atlantic water demand an innovative improvement of the existing system. Acoustic thermometry and gliders are the selected means of providing both higher resolution and integrative properties of the inflowing and outflowing watermasses.
Acoustic tomography will provide synoptic measures of ocean temperature integrated over different depth layers and over a 200 km distance with an accuracy of about 0.5°C close to the surface and 0.01°C for the deeper water layers. Such integrated ocean parameters can and will be used for assimilation in ocean circulation models. Acoustical observations can also be used to monitor climate change. Climate model predictions and acoustic propagation simulations for the Fram Strait indicate that temperature changes in the West Spitzbergen Current corresponding to a doubling of CO2 over 80 years (1970-2050) can be acoustically observed. Acoustically navigated sea gliders will provide year-round, high-resolution hydrographic measurements along desired sections or at fixed positions. Being buoyancy-driven they are extremely energy-efficient, running at about half a knot on half a Watt. The resulting high-resolution sections will be combined with data from the moored array to estimate volume, heat and freshwater fluxes more precisely. Data transmission (3 times per day) and course control is done via satellite.
In DAMOCLES for the first time, data from conventional moorings, gliders, acoustic tomography, and profiling sensors will be combined with coupled ice-ocean models through advanced data assimilation (core theme 4) to obtain an enhanced ocean observing system. The assimilation schemes for a Fram Strait model will be refined to incorporate acoustic data (core theme 4, task 2). The resulting estimates feed back on the observational programme by allowing recommendations to be made for an optimal future Fram-Strait integrated observing system, and it is envisaged that DAMOCLES will be developed into such an operational monitoring and forecasting system at the close of the Project.
The optimization of the observatory array in Fram Strait will thus include 4 components:
a) The Fram Strait acoustic tomography system to monitor the integrated temperature of the western and eastern half (NERSC, FORTH). The acoustic system will consist of two acoustic sources located at either side of the strait and one receiver array in the middle. It will transmit processed data in near real time; the components will be built and tested in WP8. For the western part of the strait propagation models taking into account the effects of the ice and matched-field inversion methods will be used (NERSC), whereas for the eastern part of the strait open-water acoustic models and matched-peak inversion methods will be applied (FORTH).
b) The system of gliders to provide quasi-continuous CTD cross-sections through the Fram Strait (AWI, IOPAN in cooperation with APL). The gliders will be equipped with acoustic modems (AWI, NERSC) to serve for near real time data acquisition from the moorings. In the open water gliders will be controlled by Iridium communication while in the ice-covered part a triangle of RAFOS beacons will be used for navigation and data recovery (the necessary acoustic system will be developed in WP8).
c) Research vessels (RV 'Polarstern', RV 'Oceania') to service the gliders and provide supplementary CTD casts for system calibration
d) Moored profiling CTD systems (NPI) for calibration. A deployment of the profiling CTD during IPY will be coupled with the first acoustic monitoring experiment.
