Geoscience Reference
In-Depth Information
85-110°E (central Eurasia), 110-135°E (east central Eurasia) and 110-135°W
(western North America). The area of each region is 3.08x10
6
km
2
. These regions
were chosen to roughly represent the four major Arctic watersheds. Equal areas
were chosen (rather than areas defined by the true basin boundaries) to allow for
direct comparisons. The western North American domain (Mackenzie) does include
some ocean areas as well as coastal regions where precipitation is very high. Results
are based on the 1960-1999 period for the Eurasian domains and 1960-1989 for the
western North American domain.
A number of different formulations of the recycling ratio can be found in the lit-
erature. Following the recommendation of Trenberth (
1998
), Serreze et al. (
2003a
)
employed the formulation of Brubaker et al. (
1993
). This is given as:
P
P
1
m
=
(6.5)
+
2*F
ET *A
1
+
where A is the area of the region and F+ is the advective moisture term. The advec-
tive moisture term is calculated as the line integral of the component of the verti-
cally integrated moisture flux directed into the region. It should not be confused
with the vapor flux convergence, which is the difference between F+ and the com-
ponent of the moisture flux directed out of the domain (F-). Calculation of F+ uses
the monthly mean vertically integrated moisture fluxes at the 2.5 x 2.5 degree grid
available from the NCEP/NCAR reanalysis. ET is averaged over the region and
is based on the difference between P and P-ET as obtained from the aerological
approach. The formulation assumes equilibrium conditions and a well-mixed atmo-
sphere. This means no changes in atmospheric moisture content, and, that the ratio
of advected to locally derived precipitation is equal to the ratio of average advected
to evaporated moisture in the air. Inspection of Equation
6.5
shows that a large pre-
cipitation recycling ratio will result when the moisture advection term F+ is small
in comparison with ET.
For the Eurasian domains (
Figure 6.13
), the ratio is largest during July. This is
primarily attributed to the peak in ET as the term F+ still tends to be fairly large in
summer. By contrast, the peak for the western North American (Mackenzie) domain
is one month earlier in June. Peak values range from 0.22 (Central and Eastern
Eurasia, or Lena) to 0.28 (central Eurasia, or Yenisey). This points to a significant
effect of the land surface on the summer hydrologic regime. Winter values range
from 0.0 to 0.11, largest for western North America.
6.5
River Discharge and Runoff
6.5.1
River Discharge Data
River discharge is measured as water volume per unit time. The unit time is often
implied (e.g., we might say that we have a certain mean discharge for June). Runoff
