Mean annual cycles for the major terrestrial drainages 631 PP ET and ET

Some of the results just presented are summarized in Figure 6.10 as mean annual cycles (1960-89) of P, P — ET and ET for the four major drainage basins of the Arctic (the Ob, Yenisey, Lena and Mackenzie). The monthly values are simple averages of the data at all grid points located within each basin.

Following earlier results, in all four basins, precipitation is at a minimum in February and March and is at a maximum during July. P — ET tends to peak during autumn. These autumn maxima arise from the combined effects of a stronger vapor flux convergence and a strong seasonal decrease in precipitable water (i.e., a decrease in water vapor storage). The general inverse relationship between the annual cycles in P and P — ET over land noted earlier is best expressed in the Ob (the westernmost basin). Here, mean P — ET for June and July is about —10 mm. Mean July P — ET is close to zero for the Mackenzie. Walsh et al. (1994) obtained qualitatively similar results for the Mackenzie using fluxes calculated from interpolated rawinsonde data. The July maxima in ET estimated as the residual lie between 60 and 75 mm, highest for the Ob. The Ob also has the highest winter ET rates.

Plate 2 Mean evapo-transpiration (ET) for the Arctic terrestrial drainage (mm) for alternate months, estimated as a residual from bias-adjusted precipitation and aerological estimates of P — ET from NCEP/NCAR. Results are based on data from 1960 through 1999 (from Serreze et al., 2003a, by permission of AGU). See color plates section.

Figure 6.10 Mean monthly precipitation (P), precipitation minus evapo-transpiration (P — ET) and evapo-transpiration (ET) for the four major Arctic-draining watersheds, based on data from 1960 through 1989 (mm). ET is calculated as a residual from P and P — ET (adapted from Serreze et al., 2003a, by permission of AGU).

Figure 6.10 Mean monthly precipitation (P), precipitation minus evapo-transpiration (P — ET) and evapo-transpiration (ET) for the four major Arctic-draining watersheds, based on data from 1960 through 1989 (mm). ET is calculated as a residual from P and P — ET (adapted from Serreze et al., 2003a, by permission of AGU).

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