Geoscience Reference
In-Depth Information
Although precipitation over much of the land area peaks in summer,
evapotranspiration rates are fairly high, such that summer net precipitation
is small or even negative in these areas. Over land regions, a considerable
fraction of summer precipitation results from regional recycling of water
vapor, pointing to the strong effect of the land surface. From autumn
through spring, most of the precipitation is stored as snow. Consequently,
river discharge to the Arctic Ocean exhibits a pronounced early summer
peak as the snow melts. For much of the Arctic, river discharge is strongly
impacted by the presence of permafrost which limits infiltration. The
bulk of the river discharge to the Arctic Ocean is contributed by four
major river systems: the Ob, the Yenisey, the Lena, and the Mackenzie.
Discharge from Arctic draining rivers in Eurasia has exhibited a positive
trend over the period of record, consistent with increased net precipitation
as assessed from atmospheric reanalysis data.
6.1
Precipitation
6.1.1
Difficulties in Measurement
There is still uncertainty regarding even mean precipitation totals in the Arctic.
The basic problems were articulated in
Chapter 2
. To reiterate the major issues:
(1) there is significant gauge undercatch of solid precipitation for which there are
only imperfect adjustment procedures, the problem is compounded by the variety of
gauge types and reporting practices used by different countries and by a switch to
automated stations for some locations in Canada; (2) while the station network has
always been sparse, it has further declined since about 1990; and (3) precipitation
estimates based on satellite data retrievals or from atmospheric reanalyses can have
large biases.
Figure 6.1
highlights the large size of bias adjustments applied to gauge
data, using (as an example) data from the NP drifting stations. Note in particular,
that the application of corrections for wind loss and the neglect of trace events acts
to shift the annual peak in precipitation from July to September! To highlight the
network problems,
Figure 6.2
shows the distribution of land stations north of 40°N
with at least ten years of data over the 1960-1989 period. Note the sparse network
over northern Canada and Siberia. There is a network of observations over central
Greenland, but these are from automated stations for which data quality is open to
question. The situation is even worse over the central Arctic Ocean, where the NP
stations provided spotty coverage from 1950 through 1991, and, with renewal of the
program, from 2003 onward.
6.1.2
Fields of Estimated Monthly Precipitation
Annual mean precipitation is shown in
Figure 2.28
. Maps for the four mid-season
months based on the same data sources appear in
Figure 6.3
. A number of different
