Closing the water budgets of the three great Siberian Rivers

Abstract:

The basins of the three great Siberian rivers – Ob, Yenisei, Lena – have come under scrutiny as symptomatic of the effects of Arctic amplification on regional water cycles. As the air moistens, Zhang et al. (2013) report more vapour convergence and greater discharges into the Arctic Ocean. The thawing permafrost is also extremely relevant. However, the warming signal can display counterintuitive patterns (Wegmann et al., 2018) and the consequences on moisture fluxes are not straightforward (Dufour et al., 2016).

This study draws on several reanalyses including ERA-5 (Hersbach et al., 2019) and discharge data (GRDC, 2014 ; Shiklomanov et al., 2018).

We replicate the significant moisture convergence trend found in Zhang et al., 2013 for the extended 1948-2018 period. However, the trend breaks down for the shorter 1979-2018 time windows and in other reanalyses except MERRA-2 over 1980-2018. The discharge observations summed over the mouths of all three rivers are significantly increasing in the long run but not during the satellite era. The initial findings appear sensitive to the starting and ending points of the study period. We believe part of the explanation is to be found in warm Arctic - cold Siberia pattern (Wegmann et al., 2018). The weak increasing trend in convergence and discharge is not driven by a straightforward global warming trend. Rather, it is the consequence of atmospheric variability which is inherently more noisy than thermodynamical signals.

Nearly all reanalyses exhibit significant negative net precipitation trend over the satellite era with the exception of NCEP NCAR R1 and MERRA-2. Further analysis indicates that these trends are due to a decrease in precipitation rather than an increase in evaporation. The decrease in precipitation occurred mainly in summer and is absent in the Global Precipitation Climatology Project. The conflicting trends between convergence and net precipitation indicate a severe unbalance of moisture budgets over Siberia.

Over the Siberian river basins, there are weak long-term increasing trends of moisture convergence and river discharge. The reanalyses show perplexing negative precipitation trends in contradiction with the increasing/constant moisture transport. The causes of either tendencies are likely atmospheric variability and strong analysis increments.