Geoscience Reference
In-Depth Information
Precipitation climatology
Annual variations
The amount and variability in annual total precipitation is arguably the most
important aspect of the precipitation at a location because it determines the nature
of the region and its viability for human habitation. The basic information required
includes mean and standard deviation of annual totals and any discernable trends
in these.
Long-term trends in annual total precipitation are of considerable interest to
hydroclimatologists concerned with global science because trends may specula-
tively result from human intervention in the Earth system globally, regionally, or
locally. At the global scale, possible intervention mechanisms include climate
change due to increasing concentrations of radiatively active atmospheric gases of
human origin; at the regional scale, large-scale change in vegetation cover caused
by deforestation or overgrazing; and at the local scale, changes in precipitation due
to modified aerosol loading associated with upwind burning of agricultural land.
Similarly, fluctuations in annual total precipitation may speculatively be linked to
observable and perhaps predictable variations in aspects of the Earth system, such
as the regional climate impacts that result from oceanic phenomena such as
El
Niño-Southern Oscillation (ENSO)
, see Chapter 9.
Intra-annual variations
The dominant cause of intra-annual variations within the precipitation climatology
at a location is the seasonal change in the regional pattern of atmospheric
circulation discussed in Chapter 9. These changes determine the atmospheric
processes operating to generate precipitation and to a significant extent also the
short-term character of contributing precipitation events. The general nature of
seasonal changes in major precipitation patterns is illustrated in Fig. 13.1. Key
features include the season to season changes in the strength and location of the
westerly wind belts north and south of the equator, the north-south movement
and changing pattern of the precipitation band associated with atmospheric ascent
in the intertropical convergence zone, and the strong seasonal change in
precipitation associated with the northerly to southerly reversal in wind direction
in monsoon systems, especially the southeast Asian monsoon system.
At mid-latitudes the general features of the intra-annual variations in precipitation
that result from these large-scale movements in precipitation climate are as follows:
The western margins of continents are dominated by precipitation associated
with oceanic depressions, which give copious rain in all seasons but a marked
maximum in the relevant autumn and winter months in each hemisphere
when the westerly winds strengthen.
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