Currently, less than 20 direct measurements of bottom pressure are available from in situ gauges. These are generally distributed at a few ''choke points," primarily in the Southern Ocean. Even  at these locations current technology limits the periods detectable by the measurements to 12 months or less because of compression effects on sensor accuracy performance that result from the high pressure of the benthic environment (Woodworth et al., 1996). Thus, the current suite of in situ bottom pressure measurements represent a complementary data set that could provide "ground truth" for the time-dependent gravity data. Its use in this capacity, however, may be limited to deployments with sufficient spatial coverage.

Gravity data provide a direct estimate of the change of total mass within an oceanic basin or large sea. These data could open up a completely new set of applications of which we can suggest only a few possibilities. The semi-enclosed Arctic Ocean, for example, is subject to an enormous increase in freshwater input during summer due to continental runoff. The residence time of the freshwater along the continental shelves at the southern margins of the Arctic Ocean is likely to be a month or longer. The increase in mass could potentially raise the geoid by 15 mm. Gravity data could provide an important constraint on this increase, and thus on the residence time of the freshwater (see, for example, Aagaard and Carmack, 1989).

Another example is determining changes in the oceanic conveyor belt that supplies deep and bottom water to the major ocean basins. Deep geostrophic currents are proportional to the gradient of bottom pressure. Thus, gravity  measurements  provide  the possibility of directly estimating changes in the poleward and equatorward transports in the lower limb of the conveyor. At this time, we can only put forward "back of the envelope" calculations in support of this statement. However, choosing scales from the North Atlantic, formation rates of North Atlantic Deep Water suggest basin-averaged meridional velocities of ~2 × 10^-3 m/s. Integrated zonally across 4000 km, this meridional transport implies a pressure difference across the basin of approximately 1 mbar, corresponding to a surface slope of 2 × 10^-9. The accuracy versus spatial resolution comparison in Figure 4.3 shows that the SST and SSI missions are sufficiently accurate to measure annual and secular variations in this transport if the variations are assumed to be 10% or greater of the mean transport.

Another way in which gravity measurements can contribute to improved understanding of ocean circulation is through their complementary relationship to altimetry and in situ measurements of temperature and salinity. The difference between the observed sea level

FRONT MATTER (R1-R14)

EXECUTIVE SUMMARY (1-6)

1 INTRODUCTION (7-14)

2 MISSION SCENARIOS AND MEASUREMENT TECHNIQUES (15-28)

3 THE GRAVITY FIELD AS A TOOL FOR SCIENCE (29-34)

4 OCEAN DYNAMICS AND HEAT FLUX (35-46)

5 SOLID EARTH PROCESSES (47-58)

6 WATER CYCLING (59-66)

7 SEA-LEVEL CHANGE (67-78)

8 THE DYNAMIC ATMOSPHERE: UNRAVELING THE CONTRIBUTIONS OF THE EARTH'S SUBSYSTEMS (79-86)

9 CONCLUSIONS (87-88)

AFTERWORD (89-90)

APPENDIX A (91-98)

APPENDIX B (99-104)

ACRONYMS (105-106)

REFERENCES (107-112)