Contrasting responses of vegetation to intraseasonal rainfall in Earth System Models

The response of vegetation productivity to water availability provides a key link between the carbon and water cycles. Correctly representing this response in Earth System Models (ESMs) is essential for accurate modelling of the terrestrial carbon cycle and the evolution of the climate system. To investigate how well models capture this relationship at intraseasonal timescales, we use global datasets based on satellite observations to assess the land surface response to intraseasonal precipitation events, and evaluate the performance of CMIP6 ESMs in representing this response in the recent historical period. Whereas models are able to capture the observed surface soil moisture (SSM) response with reasonable agreement, there are large inter-model discrepancies in the response of Gross Primary Productivity (GPP), both in magnitude and timing, even in regions where land cover is similar between models. In particular, ACCESS-ESM and NorESM produce much lower-amplitude GPP responses to rainfall than UKESM and CNRM-ESM. All the models studied are able to represent that the regional amplitude of the GPP response is positively correlated with the amplitude of the SSM response, and negatively correlated with the amplitude of the vapour pressure deficit (VPD) response. All models except NorESM also capture that stronger SSM responses are associated with faster GPP responses. However, the models differ in their sensitivity to these drivers, and can produce very different GPP responses from similar variations in SSM and VPD, particularly in climatologically dry regions. This highlights the need for a better understanding of the uncertainties in the representation of water-vegetation relationships in ESMs, such as the effect of atmospheric vapour pressure deficit on stomatal conductance and the control of soil moisture stress on GPP.