Diagnostic evaluation of runoff into the Arctic Ocean and its impact on freshwater transport through Arctic gateways.

Rapid surface warming in the Arctic region has strong impacts on the Arctic water balance and its individual hydrological components. With the Arctic Ocean being almost entirely surrounded by landmasses and some of the world’s largest rivers draining into it, the link between ocean and surrounding land is remarkably strong. Hence runoff forms one of the key variables in the Arctic freshwater budget and builds the main focus of this study.

Seasonal cycles, as well as annual and seasonal runoff trends are analyzed for the major Arctic watersheds. We first compare river discharge data taken from the reanalysis component from the Global Flood and Awareness System (GloFAS) to available observed river discharge records. GloFAS combines the land surface model from ECMWF’s most recent reanalysis effort ERA5 with a hydrological and channel routing model. Results show that seasonal river discharge peaks are underestimated by GloFAS as well as by direct ERA5 runoff.

Further analysis shows that this discrepancy can be tracked to non-stationary biases in the snow analysis of ERA5, which affect melt and subsequently runoff (Zsoter et al. (2020), https://doi.org/10.21957/p9jrh0xp). It is shown that this bias is substantially improved in ERA5’s downscaled counterpart ERA5-Land. An experimental version of GloFAS that uses ERA5-Land forcing, exhibits improved river discharge values.

Seasonal cycles of ERA5 snow melt show that there is a lag of 1-2 months between the peak in snow melt and observed river discharge, which can be explained by the time it takes for the water to reach the river mouth, but it may also be influenced by water resources management (e.g., Yang et al. (2004), https://doi.org/10.1016/j.jhydrol.2004.03.017 ; Ye et al. (2003), https://doi.org/10.1029/2003WR001991).

In addition, runoff is calculated over the whole pan-arctic region to account for the total freshwater entering the Arctic Ocean from land. Independent mooring-derived estimates of net freshwater flux through the Arctic oceanic gateways show a consistent and strong imprint of the runoff seasonal cycle.