The spatial distribution of mean annual precipitation illustrates the summation of the various mechanisms producing moisture at the Earth's surface. While the presence of atmospheric moisture and the cooling of ascending air parcels by frontal, convectional, and orographic lifting mechanisms are requisites for producing precipitation, the spatial variation of annual precipitation is so complex that it appears to defy explanation. In reality, the spatial variation of annual precipitation is the combined expression of the global hydrologic cycle, global atmospheric circulation, and the influence of continental landforms.
The subtropical oceans are major source regions for atmospheric moisture that is supplied by uniformly high evaporation in the regions between 15° and
40° latitude (Hartmann, 1994). Global atmospheric circulation is responsible for exporting water vapor out of these regions to support precipitation maxima at other latitudes. Regions of meager rainfall are related to atmospheric subsidence, topographic features, and distance from major atmospheric moisture sources. The global mean precipitation rate is 2.6mmday~1 with 2.8mmday~1 occurring over oceans and 2.1mmday~1 over land (Adler et al., 2003).
Global precipitation based on NCEP Reanalysis data (Kanamitsu et al., 2002) is shown in Figure 7.13. Fekete et al. (2004) compared NCEP and five other precipitation datasets and concluded the overall pattern was fairly similar in each dataset. Figure 7.13 shows the greatest mean annual precipitation occurs in near-equatorial areas where atmospheric water vapor is abundant and the ITCZ provides a nearly continuous mechanism for promoting ascending air. Precipitation in this region exceeds 2000 mm and may reach 3000-5000 mm (Lydolph, 1985). Even larger values of mean annual precipitation occur in some tropical coastal settings where prevailing winds consistently converge and are forced to ascend mountain slopes. Also, heavy precipitation is common in mid-latitude coastal locations where synoptic weather systems embedded in the prevailing westerlies encounter mountain barriers and forced ascent. In south
Asia and India, the summer monsoon delivers abundant precipitation that is locally enhanced by mountain barriers.
The ITCZ influence as a precipitation delivery mechanism on the continents declines at latitudes approaching the Tropic of Capricorn and the Tropic of Cancer. This declining pattern of rainfall is particularly evident in North Africa. Meager precipitation is the rule in the subtropics where atmospheric subsidence in the general circulation is dominant. Land areas in these latitudes are particularly influenced by the absence of available water vapor.
Precipitation increases to modest levels at higher latitudes because of the increased frequency of synoptic-scale storm systems. Zonal precipitation differences result from the presence of mountain ranges that trigger heavy precipitation on windward slopes and suppress precipitation on leeward slopes. In addition, the expanse of continents in these latitudes promotes atmospheric drying as distance from marine moisture sources increases.
In polar regions, mean annual precipitation is small because the entire hydrologic cycle is slowed by the limited availability of energy. Evaporation is slowed and the water-carrying capacity of the atmosphere is low (Hartmann, 1994). Precipitation, commonly in the form of snow, is conserved at these latitudes and even small annual precipitation amounts may appear to be greater because of the accumulated snow (see Fig. 4.3).
The seasonal occurrence of precipitation is a second major consideration influencing its hydroclimatic significance. At most locations, precipitation is unevenly distributed throughout the year. In general, seasonal changes in precipitation are traced to shifts in atmospheric circulation driven by latitudinal variations in solar radiation. Latitudes near the center of a circulation feature remain under the control of a single system throughout the year and display relatively little precipitation variability. This influence is evident in the spatial characteristics of January (Fig. 7.14) and July (Fig. 7.15) precipitation. Locations near the equator dominated by the ITCZ receive abundant precipitation in both months. Areas dominated by subtropical highs (e.g. North Africa, central Australia) receive little rainfall in either month. Latitudes near the margin of major circulation features display the greatest seasonal precipitation changes as they are influenced by contrasting wind systems during the course of the year (Shelton, 1988). The west coast of North America poleward of 40° N and northern Australia equatorward of 20° S display these seasonal contrasts.
The convergence of the trade winds in the equatorial region provides a relatively consistent supply of warm, moist air and convective forces persist all year. Precipitation is relatively abundant in all months at locations in the latitudinal zone traversed by the ITCZ, but seasonal differences are evident. The
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