For the first time it is possible to say that the surface of oceans are being adequately sampled for a number of properties, although those properties must be inferred from algorithms that relate the sensors' spectral response to the real ocean, and to properties of the intervening atmosphere.
Improving those algorithms and constraining numerical models depends heavily on our ability to directly measure those properties with "ocean truth" instruments.
Periodically, say every 30 years or so, internationally coordinated ocean sampling programs such as WOCE attract enough resources to repeat oceanographic sections (vertical slices of the ocean constructed from closely spaced stations where many properties are measured with vertically profiling instruments) across major ocean basins. This is valuable, but clearly inadequate compared with satellite remote sensing's ability to repeat surface observations along tracks every ten days or do. The rate at which the interior of the ocean varies is not as high as at the surface, but annual repetition of a global pattern of WOCE-like sections would approach the sampling rate that oceanographers demand. Clearly, even by pooling resources, we do not have that ship-based sampling capacity.
A development that approaches the desired sampling rate on regional to basin scales is the global XBT network of Volunteer Observing Ships (VOS), part of the Ship Of Opportunity Program (SOOP). XBT's provide temperature profiles of the upper 500 metres of the ocean along a ship's track. By using ships that repeat their track every few weeks, time series of temperature variability have been acquired in many areas of the ocean that would otherwise only be visited by a research vessel every two or three decades.
Motivation
Heat content is one of the critical determinants of ocean circulation, and its influence on weather and climate is profound. There is very strong motivation, not only from the oceanographic research community, but also from atmospheric scientists, agricultural industries and insurance underwriters to provide more realistic, higher density, more frequent temperature information from the ocean's interior. Satellites come close to giving us coverage of sea surface temperature, but we need an equivalent method of directly or remotely sensing the temperature of the ocean's interior. Acoustic thermometry may prove to be that method.
Acoustic Thermometry
Acoustic thermometry relies on the travel time of sound waves through the electromagnetically opaque ocean interior. Once an acoustic thermometry network is in place (typically one source and
