Introduction

The US (NASA) and German (DLR) space agencies are committed to the launch in mid-2001 of a spacecraft called the Gravity Recovery and Climate Experiment (GRACE). A summary of the goals of this experiment would be that it is intended to measure over a period of about five years the earth's global gravity field--both the time-mean and its temporal fluctuations. A general discussion of the applications of such missions may be found in National Research Council (1997).

The structure of Earth's gravity has implications for much of earth science ranging from geophysics to hydrology to oceanography. Wahr et al. [1998] have described the basic mission configuration, and many of its potential applications to geophysics and hydrology. From the point of view of physical oceanography, GRACE will produce two complementary types of results. The first result should be a significant improvement in existing estimates [e.g., Lemoine et al., 1997] of the earth's geoid height. The need for an improved static geoid has come, in physical oceanography, to be conspicuous in discussions of the use of satellite altimetry for determining the absolute ocean circulation.

GRACE will however, also be able to measure the time varying gravity field of the earth, and with accuracies and precisions that are somewhat startling. Figure 1 shows the expected precision of the change in the gravity field detectable when averaged over circular areas of various diameters, over time spans of order one month. Changes in the gravity field below a spacecraft are caused by redistributions in the mass on the earth. Thus Figure 1 has been converted to the equivalent mass distribution of a layer of water. Over the oceans, the net change in the total mass would correspond to the change in bottom pressure which is just the integral,

$$p_{b}\left( \theta,\lambda,t\right) =\int_{z=-h}^{z=a}\rho gdz$$ (1)

where a denotes the top of the atmosphere (or the spacecraft altitude) z=-h is the local ocean depth at position $\theta,\lambda$ and the density $\rho$ varies from nearly zero at z=a to the density of seawater at the seafloor. GRACE thus has the startling promise of making visible to oceanographers seafloor fluctuations in pressure as observed from space, with precisions at the sub-millimeter level. Experience with satellite missions suggests that engineering specifications tend to be somewhat conservative.

 

Figure: Figure 1. The expected precision of GRACE estimates of monthly mass variations at the earth's surface, in units of cm of equivalent water thickness. The mass is, in effect, averagedover circular regions, and the precision is plottedas a function of each region's radius.

This idea, that elements of the ocean circulation manifesting themselves at the seafloor can be measured from space, has led a growing group of oceanographers and geophysicists to consider how such measurements can best be interpreted, how the data might best be analyzed, and how to optimize the results. The issues raised by these questions are varied and complicated, and as a result of the discussions of the US-GRACE Science Team, it was considered useful to convene an international meeting of some of the scientists known to be interested in GRACE and related measurements, to plan how to deal with the relevant ocean science coming from GRACE (that is, deliberately omitting most considerations of geophysics and hydrology, but with some significant exceptions). A meeting was held on the premises of the Royal Society of London on 13-15 April 1999 with partial financial support from NASA, and the UK Proudman Oceanographic Laboratory, Bidston Observatory.