Geoscience Reference
In-Depth Information
In tropical regions, there is solar radiation driven seasonality in precipitation with
at least one wet season caused by:
the north-south movement of the central ascent in the intertropical conver-
gence zone, see Fig. 9.4;
●
the large-scale reversal in wind direction in regions affected by monsoons
and the southeast Asian monsoon in particular, see Fig. 9.10;
●
the seasonal cycle in the strength of oceanic influences, notably the rate of
occurrence of tropical storms, see Fig. 9.11.
●
The need to provide measures of the intra-seasonal variation in precipitation is
most critical in tropical and subtropical regions because in these regions the often
marked seasonality has significance for human welfare; the potential for
catastrophe is high because agricultural systems are more marginal and water
resource infrastructure tends to be less resilient. In such regions there may, for
example, be a need to understand the extent and timing of seasonality in
precipitation for planning and advisory purposes, but the available data to do this
are often sparse. Consequently, there is a need to interpolate from limited data
using contour graphing techniques or multiple regression techniques to describe
relationships with features that can influence the precipitation, such as distance
from the sea, altitude, and latitude.
Seasonality can be expressed in visual form in several ways including as:
maps of the percentage of precipitation falling in each month of the year,
called
isomers
;
●
maps of the ratio of precipitation falling in each month relative to one twelfth
of the annual average precipitation, called
pluviometric coefficients
;
●
polar diagrams
of the monthly rainfall, with the angular direction indicating
the month of the year, and distance from the origin proportional to the
monthly average rainfall; or
●
'pie' diagrams of the monthly rainfall, with angle subtended as a fraction of
360° for each month proportional to the monthly percentage contribution of
annual rainfall.
●
One numerate way to express seasonality in precipitation is by calculating a
Seasonality Index
, an example being:
X
1
12
∑
SI
=
X
−
12
a
(13.1)
n
1.83
X
=
n
1
a
where
X
a
is the total annual precipitation and
X
n
are the individual total monthly
precipitation values. Values of
SI
<0.2 indicate a 'very equitable' precipitation cli-
mate, values in the range 0.6-0.8 a 'seasonal climate, while those with
SI
>1.2 arise
if almost all the precipitation falls in one month.
