Geoscience Reference
In-Depth Information
(a) DJF
60°N
5
5
5
30°N
5
5
5
5
Equator
5
5
5
5
30°S
5
60°S
0°
60°E
120°E
180°
120°W
60°W
0°
(b) JJA
60°N
5
30°N
5
5
5
5
5
5
Equator
5
5
30°S
5
60°S
0°
60°E
120°E
180°
120°W
60°W
0°
Figure 2.27 Precipitation climatology for (a) the December-January-February (DJF)
mean and (b) the June-July-August (JJA)mean. Contour interval is 2 mm/day.
Evaporation rates are very difficult to measure, and we currently do not
have a global climatology compiled from direct observations.
Figure 2.28
shows estimated zonal mean evaporation rates from a reanalysis. Compare
this figure with the zonal mean precipitation rates shown in
Figure 2.25
. Note
that the greatest values of evaporation rates do not occur at the same latitudes
as those of precipitation. Instead, they are largest in the subtropics where pre-
cipitation rates are relatively low and they decrease fairly uniformly toward the
poles. The local evaporation minimum near the equator is co-located with the
surface wind speed minimum (
Fig. 2.10)
.
The geographic distribution of annual mean evaporation is displayed in
Fig-
ure 2.29.
Note that, in general, the evaporation distribution reflects the struc-
ture of the surface temperature field (
Fig. 2.6)
much more closely than it reflects
the structure of the precipitation field (
Fig. 2.26)
. An exception is very close to
the equator, where the low surface wind speeds of the ITCZ (the “doldrums”;
see
Figs. 2.10
and
2.11)
inhibit evaporation despite warm temperatures.
Specific humidity
,
q
, is a variable commonly used to quantify the amount of
water vapor in the atmosphere. Specific humidity is defined as the ratio of the
mass of water vapor to the total mass (dry air plus water vapor) of a volume of
