Direct estimates of oceanic freshwater fluxes are
determined in the same way as those of heat (Bryden
and Imawaki, Chapter 6.1), using long hydrographic lines that enclose volumes of ocean, for
which budget equations can be written. The technique relies on the assumptions that we can measure the steady-state portion of the velocity and
the salinity field, and that geostrophic dynamics
apply everywhere, except for an Ekman balance in
the mixed layer. Freshwater fluxes achieved by the
temporal rectification of the salt and velocity fields
at either the seasonal or eddy time scales are
ignored in these estimates, although, as seen below,
these fluxes have been found to be small over most
of the ocean.
For a volume of ocean enclosed by a hydrographic line, salt conservation applies in the steady
state as the flux of salt through the atmosphere is
6.2 Ocean Transport of Fresh Water
479
Wijffels
1 0 0 0
1 0 0 0
1 0 0 0
1 0 0 0
1000
1 0 0 0
10 00
1 0 0 0
1 0 0 0
1 0 0 0
1000
1 0 0 0
1 0 0 0
1 0 0 0
20 00
2000
2 5 0
2 5 0
25 0
2 5 0
2 5 0
250
Mean Annual Evaporation Rate
(mm yr
–1 )
Standard Deviation of Annual
Evaporation Rate (mm yr
–1 )
50°E
100°
150°
200°
250°
300°
350°
80°S
60°
40°
20°
0°
20°
40°
60°
80°N
80°S
60°
40°
20°
0°
20°
40°
60°
80°N
(a)
(b)
Fig. 6.2.2 Average (a) and standard deviation (b) of an ensemble of six estimates of evaporation over the oceans
based on the products in Table 6.2.1. Units are mm yr
91 and the contour interval is 250 mm yr
91
.