Geoscience Reference
In-Depth Information
Figure 4.4
Meridional aspects of
global moisture. (A) Estimates of annual
evaporation minus precipitation (in
mm) as a function of latitude; (B)
Annual meridional transfer of water
vapour (in 10
15
kg).
Source
: (A) After J. Dodd, from Browning
(1993). By permission of NERC. (B)
From Sellers (1965).
for latitude zones shows that in low and middle latitudes
P
>
E
, whereas in the subtropics
P
<
E
(Figure 4.4A).
These regional imbalances are maintained by net
moisture transport into (convergence) and out of (diver-
gence) the respective zones (D
Q
, where divergence
is positive):
It is important to stress that local evaporation is, in
general, not the major source of local precipitation. For
example, 32 per cent of the summer season precipitation
over the Mississippi River basin and between 25 and
35 per cent 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
vapour 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 mois-
ture occurs over the northern Indian Ocean in summer,
providing moisture for the monsoon. Subtropical diver-
gence zones are associated with the high-pressure areas.
The oceanic subtropical highs are evaporation sources;
divergence over land may reflect underground water
supply or may be artefacts of sparse data.
E
-
P
= D
Q
A prominent feature is the equatorward transport into
low latitudes and the poleward transport in middle
latitudes (Figure 4B). Atmospheric moisture is trans-
ported by the global westerly wind systems of middle
latitudes towards higher latitudes and by the easterly
trade wind systems towards the equatorial region (see
Chapter 7). There is also significant exchange of
moisture between the hemispheres. During June to
August there is a moisture transport northward across
the equator of 18.8
10
8
kg s
-1
; during December to
February the southward transport is 13.6
10
8
kg s
-1
.
The net annual south to north transport is 3.2
10
8
kg s
-1
, giving an annual excess of net precipitation
in the northern hemisphere of 39 mm. This is returned
by terrestrial runoff into the oceans.
