The distribution of evaporation and precipitation over the ocean (its
hydrologic cycle) is one of the least understood elements of the climate
system. However, it is now considered one of the most important,
especially for ocean circulation changes on decadal to millennial
time-scales. The ocean covers 70% of the Earth's surface and contains
nearly all (97%) of its free water, thus, it plays a dominant role in the
global water cycle. The atmosphere only holds a few centimeters of liquid
water, or 0.001% of the total. However, most discussions of the water
cycle focus on the rather small component associated with terrestrial
processes [ Chahine, 1992]. This is understandable, since
the water cycle is so vital to agriculture and all of man's activities.
Yet current estimates indicate that 86% of global evaporation and 78% of
global precipitation occurs over the oceans [ Baumgartner
and Reichel, 1975]; (Figure 1) 
. Since the
oceans are the source of most rainwater, it behooves us to work toward a
better understanding of the ocean hydrologic cycle; small changes in ocean
evaporation and precipitation patterns may have dramatic consequences for
the much smaller terrestrial water cycle. For example, if less than 1%
of the rain falling on the Atlantic Ocean were to be concentrated in the
central US, it would double the discharge of the Mississippi river!
Similarly, it is now apparent that the hydrologic cycle has a direct impact on the thermohaline circulation of the ocean (that part of the circulation driven by heat and salt differences), which is recognized as a key element of the climate system for variations on decadal to millennial time scales. The poleward transport of heat by the ocean and atmosphere serves to moderate high latitude temperatures. Meridional ocean heat transports are about equal to those in the atmosphere, and a large fraction of the ocean transport in the northern hemisphere is carried by the thermohaline overturning cell in the Atlantic [ Bryden, 1993]. If this circulation is disrupted, as it appears to have been in the past [ Broecker, 1987; [ Keigwin et al., 1991], the consequences for high and mid-latitude continental climate are severe [ Manabe and Stouffer, 1993]. The cause of collapse of the thermohaline circulation is thought to be the ``halocline catastrophe'' [ Bryan, 1986]; whereby deep convection and the formation of bottom waters can cease if surface salinity decreases sufficiently through enhanced freshwater input. Since the distribution of evaporation, precipitation, ice and continental runoff is the primary factor in the determination of surface salinity, it is essential that we adequately understand the ocean hydrologic cycle. In the following sections I review recent progress in understanding the patterns of water exchange between ocean and atmosphere, and the oceanic flows resulting from mass conservation and simple dynamics. Implications for climate models and ocean monitoring are also discussed.