marked, especially in the spring of 1978, when the limit of shelf water was twice as far offshore as had been recorded before or has been since. Rapid advection of shelf water into the region off Nova Scotia. was the evident cause of the phenomena.

Integration of water column data from ships of opportunity with the remote sensing data has provided information on the influence of the rings on water mass exchange across the shelf-slope front. Offshore entrainment of shelf water into the warmer, more saline slope water has been characteristically visible at the sea surface in satellite imagery; whereas intrusions of slope water and ring water onto the shelf have been largely restricted to the subsurface. Furthermore, each of these exchange processes is sometimes amplified by interaction between rings and Gulf Stream meanders. In oceanographic terms, the warm-core rings are a mechanism which transfers Gulf Stream water and energy shoreward into the proximity of the continental slope and outer shelf. Exchanges of water across the shelf-slope front are, in part, a function of this transfer.

Research is now in progress on changes in the position and flow pattern of the Gulf Stream and on cross correlation of the movements of the different fronts. Several considerations bear on plans for continuation and expansion of biological oceanographic research with remote sensing data:

In recent years, satellites have provided the first source of fairly regular synoptic data on what may be called the "oceanic influence" on hydrodynamics in the shelf and slope regions. Routine data have been available for many years on other forcing functions such as surface wind stress, air temperature, coastal runoff, and long-term periodicities in the strength of tidal currents.

Because of the short period of the satellite data base, the significance of annual and seasonal differences in the "oceanic influence," determined from these data, cannot be satisfactorily evaluated yet. Other data bases, such as hydrological data, are also very short of meeting requirements.

Despite the demonstrated annual and seasonal differences in the prevalence of warm-core rings, the importance of their effect on crossfrontal exchange will not be known until completion of ship-board investigations of volume transports. Such investigations will be initiated in the fall of 1981 by oceanographers from several institutions (with NSF funding).

New technology in satellite remote sensing may improve the quality of the ongoing research. The color scanner, for example, reveals spatial and temporal patterns even when surface thermal contrasts

are lacking. Also, radar altimeters and other microwave instrumentation on satellites permit "all-weather" detection of water mass boundaries at the surface.

New satellite technology can also permit new lines of fishery oceanographic research. For example, the color scanner data, if available routinely, will reveal the spatial and temporal patterns of primary biological productivity on the outer continental shelf, and indicate. the importance of nutrient enrichment resulting from upwelling of deep slope water.

The region off the northeast coast is one of the most favorable in the world ocean for satellite oceanography because of the energetic circulation and consequent strong differences in properties between water masses. For this reason, water mass boundaries tend to be visible from satellites even when atmospheric transparency is reduced. 2.3.4 World Ocean Exploration with Drifters (WOED)

The practicability of large-scale deployment and the scientific utility of drifting buoys was demonstrated in the Global Weather Experiment (GWE), 1979. The buoy program for the GWE was invented and implemented for meteorological purposes. The data fields, however, as shown in Figure 7, are also useful per se to define some of the oceanic circulation. The success of the program has stimulated new technical efforts to develop drifters of several types into instruments of broader oceanographic use-better sensors, reliable thermister chains to obtain temperature profiles, subsurface flotation with tracking and data relay via the sound channel, or via a pop-up technique and satellite data relay, etc.

An exciting research prospect, feasible in the second half of the 1980s, is exploration of ocean circulation on a global basis using drifters both as tracers of horizontal advection and as platforms from which scalar properties are measured. The objective of this exploration would be development of worldwide maps of statistical indicators of the general circulation, such as mean flow, eddy energy, and Reynolds stress, and of lateral mixing as indicated by drifter dispersion. Eventually, it will be necessary to map variability in various frequency bands at various depths on a global basis. Nearly continuous satellite positioning and data telemetry permit intensive measurement of the upper ocean on a global basis at a reasonable level of effort. Present methods of communicating with drifters at depth are more costly than is ultimately desirable. This will probably limit the use of very frequently positioned subsurface drifters to regional studies in the near future. However, for describing.

Figure 7. Composite of drifting buoy tracks during the Global Weather Experiment, 1979. Some 300 buoys were deployed in 1978 and 1979 to maintain a network of stations in the Southern Ocean for observing surface pressure and temperature. In addition, the drift patterns provide valuable insight into oceanic circulation characteristics.

the mean general circulation, including lateral eddy dispersion, the use of satellite-positioned pop-up drifters may permit global coverage at a reasonable level of effort.

The efficiency of pop-up drifters arises from low unit cost and the fact that the initial and final drifter positions for a deployment of months yield an integral of velocity more representative of the mean general circulation than individual velocity measurements are. For example, accurate determination of the mean velocity requires removing sampling errors associated with eddies and this requires knowledge of the average of observed velocity over long times. The measurement error is roughly

proportional to record length to the minus one-half power and, for MidOcean Dynamics Experiment (MODE) eddy scales, about five years of data are required to achieve an oceanographically significant accuracy of 3 mm/s. Since system accuracy is dependent mainly on the total measurement time, it is clearly efficient to take advantage of the natural integration associated with pop-up drifters. Similar criteria apply for measuring lateral dispersion (or eddy diffusivity, in cases where the notion applies), and end-point drifters are again efficient tools.

Essential to the pop-up drifter concept is availability of inexpensive floats which, in turn, depends on availability of satellite tracking which permits inexpensive equipment in the drifter. At the same time, accuracy requirements are an order of magnitude less stringent than that available from the Argos system. Cost is a critical factor because large numbers of floats will be required to map several levels of the global ocean. For example, using the MODE-derived estimates of dispersion, two-year deployments produce statistical characteristics of regions approximately 200 km on a side, and three floats in each region are required to establish the mean velocity to within 3 mm/s. If it is assumed that regions requiring greater resolution and accuracy are balanced by regions where lower resolution and/or accuracy will suffice, it seems that 103 floats might produce a quite useful global map at one depth and something like 104 might be used over a decade in mapping out various levels, exploring year-to-year variability, and filling in holes found in the deployment array.

Assuming that buoy development will proceed as planned (a substantial project is now under way that is supported by NASA and NOAA and that involves collaboration by a group of researchers as well as sensor and buoy engineers) and assuming that a suitable DCLS is available, a substantial program would be feasible to produce worldwide maps of statistics of ocean circulation for four frequency bands: band (i), one cycle per two to 40 days, which is a spectral band containing the results of direct atmospheric forcing; band (ii), one cycle per 40-150 days, the temporal mesoscale; band (iii), one cycle per 150 days to the length of a feasible program, say three to five years, which contains the secular climatic variability scale; and band (iv), the long-term mean, representative of the general circulation. All buoys would include sensors for temperature and pressure, and surface drifters would profile down to 100-200 m. Drifters would be distributed at the surface, in the thermocline, and at an abyssal level, say 3,000-4,000 m. Satellite DCLS or acoustic relay, or a combination, would be used.

• Superdrifter: If a drifter, equipped with a 100-200 m thermister chain, could also be equipped with a Doppler logging device to obtain current profiles down to 100-200 m, then substantial improvement

could be made on observations necessary to develop further models of ocean response to the atmosphere and to validate their performance. Associated field programs could be envisioned also. The superdrifter would provide valuable information in frequency band (i), and there may also be some linkage in the air-sea interaction aspects in bands (ii) and (iii).

Pop-up drifters: Pop-up drifters would be crucial where advection is slow and at depths and locations where acoustic tracking is not practical. The drifters must be inexpensive and reliable; the associated satellite DCLS must have a wide frequency acceptance band, since there will be a signficant temperature change as the pop-ups surface, and it would not be practical to include a highly regulated or stable oscillator on them.

The WOED effort should probably be built up through regional studies, and close coordination should be included with altimeter flights (TOPEX) and with scatterometer observations. It might also be useful, in light of the discussion earlier, to try to coordinate extensive drifter experiments with the operation of a color scanner, since the scanner images may also contain valuable information on movement of surface structures.

2.3.5 Remote Sensing of the Heat Budget of the Ocean

This discussion is closely related to several other sections, particularly the following one.

Ocean circulation is strongly affected by atmospheric winds and the spatially varying net radiant heating and sensible and latent cooling of the ocean surface, while the ocean influences atmospheric circulation through the transfer of heat and moisture to the lower layers of the atmosphere. Because the ocean flow and ocean diffusion transport heat from one place to another, the thermal energy which the ocean might absorb in one place might be given back to the atmosphere at a very different place and at a different time. The ocean's capacity to store thermal energy for long periods and move it around over large areas can result in slow changes of atmospheric circulation patterns or in climate changes. The determination of the oceanic heat budget and the dynamical processes which change it is paramount to understanding and modeling climate changes over large areas of the globe.

Remote ocean sensing can play an important part in measuring the air-sea exchange process over large areas of the open ocean. For example, climatologists have shown that slowly varying ocean surface temperature patterns, and the implied heat exchange patterns, in the equatorial and southern Pacific are directly related to winter climate patterns over North America. But this ocean area is not crossed by ships on a regular basis