Acoustic Monitoring of the Ocean Climate in the Arctic Ocean (AMOC). Final Report

The overall objective of AMOC was to develop and design an acoustic system for long-term monitoring of the ocean temperature and ice thickness in the Arctic Ocean, including the Fram Strait, for climate variability studies and global warming detection. AMOC is feasibility study based on climate and acoustic model experiments and is complementary to the real experiments carried out in TAP. The approach of AMOC was to study detection and quantification of warming in the Arctic Ocean, using gyre scale acoustic long range propagation for measuring basin wide ocean temperature and ice thickness changes. Predicted climate change scenarios until 2050 have been used as input to acoustical models in order to assess the capability of using acoustical travel time to measure small changes in average temperature expected in the next few decades in the Arctic Ocean. Acoustic propagation, which has been successfully tested for climate monitoring in other oceans (Munk et al., 1995), can be an important component in detection of global
warming because the method gives an average estimate of temperature change in the area between source and receiver. A future acoustical monitoring system should consider the following recommendations:

• Propagation paths between source and receiver must avoid shelf areas and other bathymetric effects at water depths shallower than 1500 m.
• Sufficient signal to noise ratio, which does not require post processing, can be achieved by using a transmission level of 160 db in Fram strait and 180 - 195 db in the interior of the Arctic, depending on frequency.
• Deployment depth of source is a crucial factor for successful transmission. In the Arctic Basin relevant source deployment depths should be at 500 m or deeper to capture mode 2 and 3 waves propagation through the Atlantic water masses. Shallow source deployment in the upper water masses will not capture the expected warming of the Atlantic water which is predicted by the climate model simulations.
• For the Fram Strait our result shows that both a deep and a shallow source will provide information about the mean temperature of the water masses. A deep source gives the best results for long term monitoring of the climate, while the shallow source will provide information about the seasonal changes in the upper water masses. Due to strong mesoscale eddy activity, it can be difficult to separate arrival times of the different rays for every transmission. However the simulations show that there are enough deep going rays which can be recognised to observe long-term changes in temperature.
• Choice of frequency must be adapted to the parameters to be observed. For ocean temperature observations 20 Hz can be used for propagation across the whole Arctic with minimum influence of sea ice. To observe ice thickness frequencies from 100 to 5000 Hz can used to obtain regional estimates. In the Fram strait it is possible to use higher frequencies, especially in the ice-free part (West-Spitzbergen Current). The project has demonstrated that acoustic transmission data can be used to observe long-term changes in temperatures in the Arctic Ocean. The acoustical system should be used in combination with remote sensing from satellites, in situ observations and ice-ocean models