Responses to comments on esurf-2022-25

Abstract

Arctic river deltas define the interface between the terrestrial Arctic and the Arctic Ocean. They discharge sediments, nutrients, and soil organic carbon to the Arctic Ocean and provide key stratigraphic records of permafrost landscape evolution. As the climate warms, the future evolution of Arctic deltas will likely take a different course, with implications both local in scale and on the wider Arctic Ocean. One important way to understand and predict the evolution of Arctic deltas is through numerical models. Here we present ArcDelRCM.jl, an improved reduced complexity model (RCM) of arctic delta evolution based on the DeltaRCM-Arctic model (Lauzon et al., 2019). We have rewritten the DeltaRCM-Arctic model entirely in the Julia language and the final ArcDelRCM.jl model retains the option to execute as the former. Unlike previous models, ArcDelRCM.jl is able to replicate an important and ubiquitous feature observed in Arctic deltas — the underwater ramps extending from the shoreline of deltas tens of kilometres towards the ocean at a depth of roughly 2 m. This feature may form a buffer between ocean processes and the land portions of the deltas. We have found that the delayed breakup of bed-fast ice on and around the deltas is ultimately responsible for the development of the ramp feature. However, changes made to the modelling of permafrost erosion and protective effects of bed-fast ice are also important contributors. Through a simple graph analysis performed on ensemble runs, including the non-Arctic DeltaRCM (Liang et al., 2015a), we found that the Arctic processes considered in all the models and modifications did not lead to significant differences in the channel structures. Moreover, we found that the summer months contribute significantly to the growth and evolution of Arctic deltas, thus should not be neglected in simulations. Finally, we tested a strong climate-warming scenario on the simulated deltas of ArcDelRCM.jl. We found that the ramp features degrade on the time scale of centuries and effectively disappear in under a millennium. Ocean processes, which are not included in these models, may further shorten the time scale. With the degradations of the ramps, any dissipative effects on wave energy they offered would also decrease. This could expose the sub-aerial parts of the deltas to increased coastal erosion, thus impacting permafrost degradation, nutrients and carbon releases.