The achievements of space science and technology have made it relatively easy to place satellites in earth orbit from which to observe our own planet. This vantage point in Space is now widely used by ocean scientists, making a significant difference to marine science and its application in operational oceanography. Following a brief review of the unique capabilities of ocean viewing sensors in Space, this lecture presents two examples of ocean phenomena whose complex behaviour has been revealed and understood only because of the availability of satellite observations. Both phenomena are wavelike motions within the layered structure of the ocean and both have their maximum amplitude tens to hundreds of metres below the surface.
At the ocean basin scale, baroclinic planetary waves, or Rossby waves, propagate westwards at constant latitude, taking several years to cross large oceans. Although their behaviour was predicted theoretically their existence had not been confirmed by conventional ocean observations. Measurements from Space of sea surface height and sea surface temperature have not only revealed their ubiquity but also enabled their speed and its variation with latitude to be determined, prompting a refinement of the theoretical models. Moreover they are found, in some cases, to be associated with fluctuations of biological primary production in the ocean. We are now able to explore the implications of these phenomena for inter-annual to decadal scale climate change.
The second phenomenon to be discussed is the propagation of non-linear gravity waves at the ocean thermocline, at length scales of a few tens of kilometres. Driven by tidal flow interacting with ocean bathymetry, these waves are important for their contribution to the mixing of nutrients between the deep ocean and the upper, illuminated, part of the water column. They also present a potential hazard for the operation of offshore oil and gas installations. Only from Space, using imaging radar, is it possible to observe their two-dimensional spatial phase structure and their wave kinematic properties. Explanation will be given of the chain of processes by which motions many tens of metres below the surface can be detected by electromagnetic waves which do not penetrate below the surface. Some radar images reveal in detail the complex behaviour of trains of internal solitons as their profile is transformed on encountering shallower water.
The lecture concludes by looking forward to the growing possibility that satellite data will enable us to model and forecast the ocean in an operational sense, with consequent benefits for all users of the seas.