Geoscience Reference
In-Depth Information
basin-averaged P-ET correlates with the NAO (AO) index at 0.49(0.56), 0.42(0.57),
0.29(0.56), and 0.49(0.36), respectively. The strongest relationship is for annual
P-ET, which correlates with the NAO (AO) at 0.69(0.49). In accord with the analy-
ses just described, the strongest P-ET signals associated with the NAO are found
in the Atlantic sector and extending north into the Arctic Ocean during winter (see
also Hurrell et al.,
2003
).
Recall also from
Chapter 4
that the structure of the AO depends on how the AO is
calculated. When the AO is calculated separately for each calendar month, as done
by Ogi et al. (
2004
), the summer pattern has a distinct center of action lying over
the central Arctic Ocean; the positive mode is associated with low sea level pressure
over the central Arctic Ocean, linked in turn to a high frequency of cyclone activity.
It hence follows that the summer AO mode is associated with positive precipitation
anomalies over the central Arctic Ocean.
6.4
Annual Cycles for the Major Terrestrial Drainages
6.4.1
P, P-ET and ET
Figure 6.12
shows mean annual cycles (1960-1989) of estimated P, P-ET, and ET
for the four major drainage basins of the Arctic (the Ob, the Yenisey, the Lena, and
the Mackenzie). The monthly values of P and P-ET are simple averages of the data
at all grid points located within each basin. ET is computed as a residual from the
basin-mean values of P and P-ET.
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 dur-
ing 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 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 60-75 mm, highest for the Ob.
The Ob also has the highest winter ET rates.
6.4.2
Precipitation Recycling
The amount of precipitation falling over a region can be divided into: (1) precipita-
tion associated with water vapor transported into the region (advected precipitation);
and (2) precipitation associated with water that evaporates from the surface of the
region and falls within the same region (locally derived precipitation). The precipi-
tation recycling ratio is defined as P
m
/P, where P
m
is the precipitation of local origin
and P is the total precipitation. The recycling ratio can be thought of as providing
