vations from ships or instrumented drifters are required to improve the mathematical relationships (algorithms) for calibrating satellite observations to in situ values.

A suite of satellites launched by the United States, the European Space Agency, and Japan and with many joint satellite missions should provide many data required by physical oceanographers during the next decade. Advances in physical oceanography are tied closely with satellite views of the ocean and telemetry of data from Earth's surface. Coordination of satellite projects with ocean science objectives and in situ ocean measurements is required. The satellite time series of sea surface temperature, winds, sea ice, ocean color, and ocean height must be continued without interruption to provide a complete record of their variability.

Tracers Distribution of ocean properties can be used to depict the pattern of ocean currents and the effects of mixing within the ocean because water masses from different source regions vary in their physical and chemical signatures. Traditionally, temperature, salinity, oxygen, and nutrient concentrations have been used to track the movement or spreading of ocean water masses. Other more exotic chemicals present in minute concentrations (tracers) are now commonly used to track water masses. For example, chlorofluorocarbons, carbon-14 and tritium from atmospheric nuclear bomb testing, and other natural and synthetic substances with known rates and times of input to the atmosphere have been used as tracers. Tracers average the spreading action of ocean circulation at a variety of scales and integrate the effects of many processes. The infiltration of naturally occurring and synthetic chemical tracers into the ocean provides insight about the time scales of ocean circulation and mixing. The development of baseline time series of tracer concentrations is important.

Acoustic Techniques Because the ocean is transparent to sound but opaque to light, acoustic techniques provide oceanographers with the opportunity to see the interior of the ocean. In a real sense, the hydrophone array serves as the underwater eyes and ears of the oceanographer. The enormous bandwidth of available underwater acoustic instrumentation (10-3 to 106 hertz) allows sound to be used as a probe of structures and processes whose scales range from millimeters to ocean basin scales. The ocean is especially transparent to low-frequency sound. Consequently, underwater sound is becoming an important means of studying the three-dimensional structure of the ocean below its surface. Continuous measurements of current velocity from shipboard acoustic



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