Acoustic Thermometry for the Indian Ocean
Andrew M. Forbes, Marine Technology & Information Program Leader, CSIRO
Marine Research, Australia
Introduction
For decades, ships have been employed as observing platforms, occupying discrete "stations" in some systematic spatial pattern. With the development of self-contained, internally recording instruments, it became possible to build time series of data at a number of points. Each of these methods still has merit, evidenced by the maintenance, worldwide, of a large fleet of oceanographic research vessels. Moored instrument arrays such as the equatorial TOGA-TAO array continue to demonstrate the value of maintaining large numbers of moorings on regional scales. Despite the billions of dollars that have been invested in ships and moorings, the oceans remain greatly under-sampled compared with the atmosphere. Our understanding of ocean processes consequently suffers from lack of data. Numerical ocean modeling has stepped in to partially fill the data gap, by providing local to global coverage where, at each grid point, physical processes are parameterised. From boundary and initial conditions, the fundamental equations of motion and the exchange of properties are allowed to operate in time and space, thus mimicking the behaviour of the real ocean, on some scales.
Developments
Two major developments have taken place in the last decade that reveal the relatively crude nature of most numerical ocean models.
First, drifting instruments were developed that could report their position, mapping out Lagrangian trajectories. These evolved into instruments capable of also transmitting observations of ocean properties to receivers in the laboratory. What had been thought of as simple currents turned out to be highly complex, with turbulence extending across all scales. The discovery that energetic mesoscale eddies populate the margins of major circulation features, and carry heat, salt and other properties across frontal regions stimulated the development of higher resolution numerical models, but without a corresponding improvement in the underlying model physics.
Second, satellites carrying infrared and microwave sensors have revolutionised our ability to systematically and repeatedly map the temperature, colour, roughness and height of ocean surface. Further complexities emerged that we currently have no way of analytically describing, and which have led to a burgeoning field, collectively called remote sensing.
