Geoscience Reference
In-Depth Information
1500
(A)
1000
500
0
-500
-1000
-1500
30
(B)
20
10
0
10
20
30
90°N
60°N
30°N
0°
30°S
60°S
90°S
Figure 4.4
Meridional aspects of global moisture. A: Estimates of annual evaporation minus precipitation
(in cm) as a function of latitude; B: Annual meridional transfer of water vapor (in 10
15
kg).
Source: A: After J. Dodd. From Browning 1993. By permission NERC. B: From Sellers 1965. Courtesy of University of Chicago
Press.
precipitation. For example, 32 percent of the
summer season precipitation over the Mississippi
River basin and between 25 and 35 percent of that
over the Amazon basin is of 'local' origin, the
remainder being transported into these basins
by moisture advection. Even when moisture is
available in the atmosphere over a region, only a
small portion of it is usually precipitated. This
depends on the efficiency of the condensation and
precipitation mechanisms, both microphysical
and large scale.
Using atmospheric sounding data on winds
and moisture content, global patterns of average
water vapor flux divergence (i.e.,
E
-
P
>
0
) or
convergence (i.e.,
E
-
P
<
0
) can be determined.
The distribution of atmospheric moisture
'sources' (i.e.,
P
<
E
) and 'sinks' (i.e.,
P
>
E
) form
an important basis for understanding global
climates. Strong divergence (outflow) of moisture
occurs over the northern Indian Ocean in
summer, providing moisture for the monsoon.
Subtropical divergence zones are associated with
