A rough global picture of the net water flux between ocean and atmosphere
is given in Figure 2 
. This is a composite
picture, assembled from modern climatologies for evaporation minus
precipitation (E-P) for the Atlantic [ Schmitt et al.,
1989] and over the rest of the ocean estimates due to [
Baumgartner and Reichel [1975]. These are based on older coast
and island data for the most part and thus lack resolution over the open
ocean. The Schmitt et al., [1989] estimates are derived
from the latent heat flux data of
Bunker [1976] and the rainfall data of
Dorman and Bourke [1981]. Their estimates have more detail than the seminal study of
[ Baumgartner and Reichel, 1975], since the data are based
on millions of ship observations from the 1940s to the 1970s, and display
much more structure. New features are seen, such as a minimum in E-P
trending from northeast to southwest across the North Atlantic subtropical
gyre. Also, a prominent region of net water loss near the Gulf Stream has
no corresponding pattern in the North Pacific near the Kuroshio. Such a
pattern would be present, if modern climatologies were to be assembled for
the Pacific Ocean as well.
Despite the limitations of such a map, the major features of the water cycle are clearly seen. In the tropics, rainfall dominates over evaporation within the Intertropical Convergence Zone (ITCZ). The subtropics are characterized by an excess of evaporation, except for the South Pacific Convergence Zone, a band of net precipitation trending to the southeast away from the western equatorial Pacific. Horizontal gradients in net water flux can be large. For instance in the Atlantic at 30 W, well over 1 m/yr of net precipitation is found at 5 N, and more than 1.4 m/yr of net evaporation is estimated just 10 degrees poleward.
In subpolar latitudes precipitation dominates,
though more so in the
North Pacific than the North Atlantic. There is little data near the
poles themselves. The amplitude of the hydrologic cycle is reduced at
high latitudes by the low water vapor capacity of a cold atmosphere.
There the processes of freezing, melting and transport of sea ice play an
important role in the hydrologic cycle. In
general, the patterns of E-P are strongly zonal, except for the North
Indian Ocean, where evaporation dominates in the Arabian Sea,
precipitation in the Bay of Bengal.