reduce the record length required for detecting acceleration of the rise.
These analyses illustrate both the possibilities and the difficulties involved in dealing with data collected by operational measurements. These should be borne in mind in planning future observing systems, for it is clearly on such operational measurements that we will depend if we are to acquire sufficient data coverage to monitor climate variability.
The explosion of the electronics industry and the advent of the Space Age with its technological advances have had obvious implications for oceanographic observations. The development of electronic CTD (conductivity-temperature-depth) instruments now allows virtually continuous vertical sampling of the water column, whereas individual bottle samples are typically spaced 10-25 m apart in shallow waters and may be separated by several hundred meters at depth. Temperature is regularly measured to millidegree precision with an accuracy of ±0.002°C, and salinity is measured to a precision of 0.001 permil, with typical accuracies on the order of ± 0.002%. These accuracies are capable of revealing local and regional changes of water-mass properties over time even at depth, where the magnitude of the variability is usually < 0.01°C and 0.02 permil (see, for example, Levitus et al. (1995)). Besides obtaining the necessary accuracy of measurements, it is essential to establish time series of velocity, temperature, and salinity. These goals are being accomplished with the use of new technologies and improvements to existing instruments: continuation, expansion, and extension of XBT collection to high-resolution, deeper sampling; implementation of global arrays of surface drifters and subsurface floats; acoustic tomography; and satellite measurements. In addition, expendable CTDs (XCTDs) are becoming a viable though still expensive means of increasing coverage of salinity as well as temperature observations in thermocline waters.
Both surface drifters and subsurface floats have been in use for decades as a means of measuring absolute water velocities, which cannot be determined from hydrographic data alone. However, recent design improvements have increased their usefulness, as well as their lifetimes. Surface drifters, for example, are now drogued properly and designed for minimal windage to sample surface velocities accurately. Subsurface floats can be programmed to follow an isopycnal surface rather than a constant-pressure surface, and to change their depth periodically to provide vertical sampling; they can also be equipped with temperature and conductivity sensors for sampling hydrographic properties. Floats either transmit their data to acoustic transceivers moored on the ocean bottom, which must then be retrieved, or they surface periodically to telemeter their position and other stored data to a satellite, which transmits the information to a shore-based lab, allowing near-real-time data analysis. Floats can live up to four years, are easily deployed, and are generally considered "expendable."
Acoustic tomography takes advantage of the changing speed of sound in seawater, due to changes in density. At mid-depths a "channeling" effect allows transmitted acoustic signals to travel thousands of kilometers with little attenuation. (Note the summary of Dr. Munk's speech in this section.) Moreover, since density is a strong function of temperature, the measured travel time of an acoustic signal between two transceivers is related to the heat content of the water between them, suggesting the use of large arrays of acoustic transceivers as a potential tool for monitoring long-term changes of heat content at transoceanic scales.
Another technological advance of the past two decades is the development of remote sensing capabilities, that is, observations of the sea surface from instruments mounted on satellites in orbit around the earth. Different frequency bands are exploited to image different aspects of the ocean's surface. For example, infrared (IR) imagery can be used to deduce and map sea surface temperature, but because its ability is limited by the extent of cloud cover over the ocean, it is a more useful tool in subtropical and tropical latitudes than near the poles. On the other hand, microwaves penetrate the cloud cover, and several satellite-borne instruments are based on this frequency band. Radar altimeters can be used to determine the absolute distance between satellite and sea surface; scatterometers yield information on wind speed and direction over the sea surface; and synthetic aperture radar (SAR) can be used to map or image a wide variety of dynamical features at the sea surface and in the upper ocean. Clearly, satellites offer the potential of global coverage in space and more or less continuous temporal coverage of the ocean's surface. However, for future monitoring capability, it is essential that consistency be maintained: Sequential satellite missions must provide continuity in time, and they must sample in overlapping frequency bands—something past measurements have not.
The past several decades also have led to the almost routine sampling of a host of other physical and chemical properties, including concentrations of helium and tritium, halocarbons ("freons"), and radiocarbon, which exist in trace quantities in the ocean. Some occur naturally, and some have been anthropogenically produced; in some cases the anthropogenically induced signal overwhelms an existing natural signal. All of these quantities act as "tracers" of water masses in the Wüstian sense: Once a water parcel has acquired its characteristic value of a tracer from contact with the atmosphere, it retains that value as it sinks and participates in the ocean circulation. Wüst's core method for tracing deep-water flow is thus appropriate, with one fundamentally important difference: Unlike temperature and