As the ocean hydrologic cycle remains one of the most poorly understood components of the climate system, all of the above approaches must be pursued. Large programs such as the Global Energy and Water Cycle Experiment (GEWEX), though focussed on the ``fast'' atmospheric and terrestrial processes, will help refine our picture of the water cycle. Intercomparison of the various techniques will help to identify the best methods for monitoring. However only long term records, such as those planned as part of the Global Ocean Observing System (GOOS), will allow us to describe and understand the functioning of the hydrologic cycle in decadal and longer time-scale climate processes. The next decade offers ideal opportunities to begin to build a much more complete picture of the ocean hydrologic cycle.
Acknowledgments. Support of the NOAA Atlantic Climate Change Program (ACCP), the National Science Foundation and the Office of Naval Research is gratefully acknowledged. This review was prepared under ONR grant N00014-92-J-1323. Woods Hole Oceanographic Institution Contribution Number 8808.