WQfo P Ef0 rl 0 248

pA Pa

Adding (2.4.5) and (2.4.8) and considering the equality (/Q + /, + /L) = 1 results in the obvious relation expressing the law of conservation for the fresh water mass in the atmosphere-ocean-sea ice-land system.

Let us discuss in turn those elements of the hydrologic cycle on which we have more or less reliable information.

Moisture content in the atmosphere. For annual mean conditions this decreases systematically from the equator to the poles. Such characteristic changing of the moisture content in the atmosphere is explained by the strong dependence of specific air humidity on temperature. Let us note the presence of deviations from zonal symmetry related to the effect of the underlying surface; as a result, atmospheric moisture content is higher over the ocean than over the land. Significant deviations also take place in the vicinity of the east and west boundaries of continents, due to the effect of orography and of the warm and cold ocean currents. There is one more distinguishing feature: the distribution of the moisture content in the Southern Hemisphere is closer to the zonal distribution than in the Northern Hemisphere. This is caused by the different ratio of land to ocean areas in the two hemispheres. The total average moisture content of the atmosphere per year is 13.1 x 1015 kg; this is equivalent to a water layer of 0.025 m thickness.

An idea of seasonal variability can be obtained from Figure 2.9. It is obvious from the diagram that the moisture content of the atmosphere in all latitudes of the Northern Hemisphere is larger in summer than in winter. The opposite situation takes place in the Southern Hemisphere. The maximum seasonal variability of the moisture content in the atmosphere occurs in the zonal belt limited by parallels 20° and 30°N. This feature has its origin in the monsoon circulation over South-East Asia and Africa. In the equatorial region the seasonal variability conditioned by displacement of the moisture content maximum to the summer hemisphere is much less.

Meridional moisture transport in the atmosphere. Let us integrate Equation (2.4.1) over longitude to determine this. As a result we obtain the following expression:

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