The ACOUS source currently operating in the Arctic sends a 20 min signal every four days. Thus the notional grid shown could provide a snapshot of the entire Arctic Ocean in less than one hour every four days, and could operate unattended for years with all of the data being provided to researchers in real-time. This provides data that is very high resolution in time. The data would give information as a function of depth, through inversion of the mode arrivals, yielding the temperature changes in the upper layers, the Atlantic layer and the deep Arctic Water. Clearly, a tomographic inversion of the notional grid would yield a horizontal resolution that would vary from 100 kms to many 100's kms depending upon the density of the crossing paths at any given location. Even on this scale acoustic thermometry can provide real-time year-round information and identify areas where important changes are taking place. These areas can then be the focus of more detailed study with submarine, ice-camp, or ice breaker based research. Thus the human assets are used more efficiently. However, spatial resolution can be improved through assimilating the acoustic data and the point measurements continuously into a coupled atmosphere-ice-ocean model as described above, which is a key element of the on-going research. Ultimately then, resolutions to model scales could be achievable. The current 3D model mentioned above is being run on a 55.56 km resolution and a 17 km resolutlon model is under development.
The notional network shown in Fig. 4 represents approximately 24,000 kms of acoustic path length. In order to obtain an average temperature on these 18 acoustic paths that would be equivalent to the acoustic thermometry, by direct measurement of temperature, it would require sampling along each path at the mesoscale correlation length of approximately 25-50 kms (for the SCICEX cruises the CTD sample spacing has been an average of 40 kms). This would therefore require installing and maintaining a continuous presence of 600-1000 bouys equipped with at least a 1 km long thermister chains that must be mounted through the ice in locations that span the entire Arctic, which given the realities of Arctic logistics is operationally as well as economically infeasible. The International Arctic Buoy Program (IABP) maintains 20-30 buoys at any given time on the ice that measure atmospheric variables and are tracked for ice drift. Their goal is to maintain a spacing not less than 500 km. This is two orders of magnitude less than required and the buoys that would measure ocean temperature or acoustic data would be much more difficult to transport and deploy with their long arrays. However, the Arctic buoys are appropriate technology for sampling the atmospheric properties and ice drift and will be a critical part of an integrated Arctic observing system when coupled with the Arctic Ocean observing system. Smaller numbers of buoys with acoustic and oceanographic arrays would also be useful to augment the acoustlc grid for propagation loss measurements and measurements of the upper ocean stratification since these methods to not require the precise localization that the travel time measurements need.
CONCLUSIONS
Under the ACOUS project acoustic thermometry measurements are currently being made in the Arctic Ocean. These installations provide the first part of a larger more comprehensive monitoring system in the Arctic utilizing acoustic remote sensing and direct measurements with a real-time, deep-sea cable and mooring based network. The Arctic Ocean is a harsh environment in which to conduct scientific studies. The need for new technologies for synoptic, real-time, autonomous and unmanned operation is
