Large‐scale hydro‐climatology of the terrestrial Arctic drainage system

Abstract

[1] The large-scale hydro-climatology of the terrestrial Arctic drainage system is examined, focusing on the period 1960 onward. Special attention is paid to the Ob, Yenisey, Lena, and Mackenzie watersheds, which provide the bulk of freshwater discharge to the Arctic Ocean. Station data are used to compile monthly gridded time series of gaugecorrected precipitation (P). Gridded time series of precipitation minus evapotranspiration (PÀET) are calculated from the moisture flux convergence using NCEP reanalysis data. Estimates of ET are obtained as a residual. Runoff (R) is obtained from available discharge records. For long-term water-year means, PÀET for the Yenisey, Lena, and Mackenzie watersheds is 16-20% lower than the observed runoff. In the Ob watershed, the two values agree within 9%. Given the uncertainties in PÀET, we consider the atmospheric and surface water budgets to be reasonably closed. Compared to the other three basins, the mean runoff ratio (R/P) is lower in the Ob watershed, consistent with the high fraction of annual precipitation lost through ET. All basins exhibit summer maxima in P and minima in PÀET. Summer PÀET in the Ob watershed is negative due to high ET rates. For large domains in northern Eurasia, about 25% of July precipitation is associated with the recycling of water vapor evapotranspirated within each domain. This points to a significant effect of the land surface on the hydrologic regime. Variability in P and PÀET has generally clear associations with the regional atmospheric circulation. A strong link with the Urals trough is documented for the Ob. Relationships with indices of the Arctic Oscillation and other teleconnections are generally weak. Water-year time series of runoff and PÀET are strongly correlated in the Lena watershed only, reflecting extensive permafrost. Cold-season runoff has increased in the Yenisey and Lena watersheds. This is most pronounced in the Yenisey watershed, where runoff has also increased sharply in spring, decreased in summer, but has increased for the year as a whole. The mechanisms for these changes are not entirely clear. While they fundamentally relate to higher air temperatures, increased winter precipitation, and strong summer drying, we speculate links with changes in active layer thickness and thawing permafrost.

Contemporary estimates of Pan‐Arctic freshwater discharge from GRACE and reanalysis

“…Large overestimates of total basin discharge are noted in the Ob basin. Here, in contrast to the other three river basins, significantly less permafrost extent ($4% -10% of the drainage area), an appreciably greater number of snow-free days and the highest percentage of wetland cover result in relatively higher ET rates ($74% of annual precipitation) [Serreze et al, 2003] which are poorly captured by ERA and NRA.…”

“…This unconformity is best explained by the fact that estimated total basin discharge, as previously discussed, encapsulates all forms of water mass losses, including all of gauged and ungauged discharge. Additionally, observed low flows during winter are far more prone to measurement errors relative to peak flows, due to ice jams and river freeze-up, with potential errors ranging between $15% and $30% [Grabs et al, 2000;Serreze et al, 2003].…”

“…The limitations of reanalyses [Cullather et al, 2000] notwithstanding, the (À @W @t À div Q) term is an extensively used alternative to P-E in regional and global water balance studies [e.g., Dai and Trenberth, 2002;Serreze et al, 2003] owing to the assimilation of observed wind, temperature and humidity profiles. The availability of additional meteorological data from the dense rawinsonde network makes the assimilation even more effective in the Pan-Arctic region [Serreze et al, 2003]. In contrast, observed precipitation in the Arctic often contains large errors [Yang et al, 2001], and observations of evapotranspiration are significantly lacking.…”

Precipitation characteristics of the Eurasian Arctic drainage system

“…Peterson et al (2002) document a general increase in annual Siberian river discharge since the advent of routine monitoring in the 1930s. Serreze et al (2003) recently addressed terrestrial aspects of the Arctic hydrologic system, including variability in monthly precipitation P , evaporation E and P − E, links with atmospheric variability, and relationships with river discharge. The effort used gridded fields of monthly P compiled from station records, P − E calculated via aerological methods (Cullather et al, 2000), E obtained as a residual and discharge records at gauging stations closest to the river mouths.…”

Synoptically forced hydroclimatology of major Arctic watersheds in general circulation models; Part 2: Eurasian watersheds

“…Differences in net precipitation (precipitation minus evaporation; P -E) between observations and the GCM ensemble support this hypothesis (Figure 10). Both observations (Serreze et al, 2002) and GCMs give negative values of P -E in the Ob basin throughout summer, indicating the region becomes a net source of atmospheric moisture. This source is overemphasized in models, which produce a negative P -E bias relative to ERA-40 between April and August.…”