Contrasting responses of vegetation productivity to intraseasonal rainfall in Earth system models

Correctly representing the response of vegetation productivity to water availability in Earth System Models (ESMs) is essential for accurately modelling the terrestrial carbon cycle and the evolution of the climate system. We evaluate this response at the intraseasonal timescale in five CMIP6 ESMs by analysing changes in Gross Primary Productivity (GPP) after intraseasonal rainfall events and comparing to the responses found in a range of observation-based products. When composited around all intraseasonal rainfall events globally, both the amplitude and the timing of the GPP response show large inter-model differences, demonstrating discrepancies between models in their representation of water-carbon coupling processes. However, the responses calculated from the observational datasets also vary considerably, making it challenging to assess the realism of the modelled GPP responses. The models correctly capture that larger increases in GPP at the regional scale are associated with larger increases in surface soil moisture and larger decreases in atmospheric vapour pressure deficit. However, the sensitivity of the GPP response to these drivers varies between models. The GPP in NorESM is insufficiently sensitive to surface soil moisture perturbations when compared to any observational GPP product tested. Most models produce a faster GPP response where the surface soil moisture perturbation is larger, but the observational evidence for this relationship is weak. This work demonstrates the need for a better understanding of the uncertainties in the representation of water-vegetation relationships in ESMs, and highlights a requirement for future daily-resolution observations of GPP to provide a tighter constraint on global water-carbon coupling processes.