The general pattern of net evaporation in middle latitudes and net
precipitation in the tropics and higher latitudes, leads to the necessity
for compensating flows in the ocean. That is, the ocean must transport
water into the evaporation zones and away from the precipitation regions,
in order to avoid regional trends in sea level. An appreciation for the
magnitude of such flows can be obtained by integrating the surface flux
estimates shown in Figure 2. As the major gradients are meridional, a
zonal integration in 5 latitude bands is presented, that shows the
pattern of meridional transport (Figure 3, 
from
Wijffels et al., 1992). This can be compared with
estimates of the water vapor transport in the atmosphere [
Peixoto and Oort, 1983]. As noted by
Wijffels, et al., [1992] and Schmitt and
Wijffels [1993], the ocean and atmosphere fluxes are about equal and
opposite; meridional river transports are one or two orders of magnitude
smaller. The ocean/atmosphere hydrologic cycle accounts for a significant
portion of the poleward heat transport on the planet. For instance, a
water transport of 0.6 Sv corresponds to 1.5 petawatts of latent heat flux
(1 Sv = 1 Sverdrup = 10
m
/s=10
kg/s). At 25 N, the ocean
sensible heat flux is about 2 petawatts (= 10
watts = PW) [ Bryden, 1993], so the
(separate) heat transport associated with the ocean/atmosphere hydrologic
cycle (0.8 PW) is quite substantial. As discussed later, ocean data alone
can be used to estimate the latent heat component of meridional heat
transport, thus permitting the specification of a large part of the total
planetary energy budget [ Schmitt and Wijffels, 1993].