MERGE SP: Using the regional Earth system model RCA-GUESS to quantify vegetation-atmosphere feedback on a hotspot of CO2 flux variability - Australia

Terrestrial ecosystems drive the seasonal and inter-annual variability of the land CO2 sink, and semi-arid ecosystems such as drylands charateristic of the Australian continent making a disproportionate contribution (Poulter et al., 2014; Ahlström et al., 2015). This is partly due to high sensitivity of photosynthetic carbon uptake and soil decomposition to the fluctuations of water availability, and to the influence of ocean circulation modes, notably the El Niño Southern Oscillation which dominates wet/dry cycles for Australia (Teckentrup et al., 2021). Boom versus bust behaviour of drought-adapted
vegetation of Australia’s dry inland accentuates rainfall effects on ecosystem carbon cycling, a single wet episode in 2011 contributing ~25% to a record global land sink anomaly of 1.50.9 PgC relative to that decade’s mean (Haverd et al., 2016). Alteration of land surface biophysical properties due to vegetation dynamics undoubtedly influence such variability, as demonstrated in our novel work applying RCA-GUESS, the first published regional Earth system model, to Africa and South America (Wu et al., 2016, 2017). Changes in canopy cover, stomatal conductance and surface roughness alter dissipation of energy flux, which will affect water flux returned to the atmosphere and the depth of atmospheric boundary layer. Using the Earth system model that coupled vegetation dynamics and their responses to climate and CO2, we can explore if the land-atmosphere interaction will intensify current disproportionate patterns of water distribution and how it will influence interannual variability of CO2 flux. By focusing on Australia as a global hotspot of hydrological and carbon cycle variability, there is an opportunity to advance understanding of the implications of vegetation-atmosphere biophysical coupling for the integrity of the land carbon sink, climate extremes and resultant impacts on nature and humans via intensified droughts, bushfires and floods, already being experienced in Australia and other world regions.

PURPOSE AND AIM
1. Applying the regional Earth system model (RCA-GUESS) to the Australasia CORDEX domain (https://cordex.org/domains/region-9-australasia/) to understand the influence of vegetation-atmosphere feedbacks on carbon cycle variability, for which Australia has been identified as a global hotspot (Harvet, 2016; Poulter et al., 2014).
2. Comparing simulations with and without vegetation feedbacks to determine if vegetation-precipitation (VP) feedback in a CO2-enriched climate contributes to the intensification of temperature and rainfall extremes, exemplified by droughts and heatwaves that preceded the Black Summer megafires of 2019-2020, and repeated and extensive flooding in Eastern Australia during the triple La Niña episode ending in 2023.
3. Configuring and testing the performance of the model for Australasia in preparation for detailed studies of biosphere-atmosphere influences on climate extremes, carbon cycle variability, forest dieback and bushfires, capitalising on new relevant functionality in LPJ-GUESS.