High-latitude climate warming has caused widespread permafrost thaw, especially over discontinuous and sporadic permafrost regions. Measurements at the Alaska Peatland Experiment (APEX) site (i.e., a hillslope thermokarst bog interior Alaska) have shown rapid permafrost thaw and contrasting methane (CH4) emissions between the bog edges and the bog center. We hypothesize that the coupled water and heat transport, specifically the advective heat transport associated with water flow vertically and laterally, greatly impacts the spatial-temporal variability of bog inundation dynamics, permafrost thaw rates, and the CH4 emissions from the bog.
To examine the role of advective heat transport in affecting permafrost thaw and CH4 emissions, we conducted meter-scale simulations along a ~350 m transect extending from upland forest permafrost to the lowland thermokarst bog using the Energy Exascale Earth System Model (E3SM) land model (ELM-ECA). We first spun up the transect-scale model under accelerated decomposition mode for 200 years with heterogeneous vegetation (forest-shrub-arctic grass transition along the transect) and fully saturated homogenous soils without lateral connection. Then we continued 200 years of regular spin-up with horizontally connected heterogeneous soils by prescribing the transect-scale varying soil properties, followed by transient simulations from 1901 to 2019 with reanalysis forcing bias-corrected by the nearby weather station and tower data. We increased snowfall over the low-elevation bog areas to mimic the snow increment/redistribution from the valley. Results demonstrated that incorporating advective heat transport improved the transect-scale simulations of soil temperature and moisture profiles compared to models omitting this process, reasonably recapturing summer thaw depth along the transect from the forest to the bog area. Rainfall intensity and timing in the warming season and rain-on-snow events in the cold season strongly modulated soil freeze/thaw cycles, bog inundation dynamics, and bog plant phenology. Results also revealed an interplayed role of advective heat and the spatial heterogeneity in bog plants and soils in controlling the variations of bog-area CH4 emissions. We concluded that the coupled water and heat transport significantly impacts permafrost thaw and possibly triggers abrupt thaw and thermokarst development, especially over hillslope landscapes affected by ice-rich permafrost, causing large carbon emissions from permafrost regions.