Abstract:
In this talk, we demonstrate an ability to forecast carbon fluxes accurately at multi-month leads in the Northern Hemisphere boreal region, an ability that appears to be linked in part to snowpack initialization in the forecast model. Using 20 years of forecasted meteorology from NASA GMAO’s S2S ensemble forecast system (from forecasts initialized in December each year) to drive offline runs of the Catchment-CN model, we evaluate the degree to which we can forecast greening onset date and Gross Primary Production (GPP) relative to a fully independent observation dataset. We find that skillful forecasts of the greening onset date largely occur where we have skillful forecasts of snow cover removal date in western North America and Europe, and this snow cover removal date is naturally tied to the amount of snow at the start of the forecast. The ability to predict greening onset date in turn leads to an ability to predict annual (or more specifically, January-September) GPP, presumably because an earlier greening start date implies a longer time period for carbon uptake. We also, however, identify some regions for which GPP is accurately predicted without the snow mechanism; in these areas, it appears to be the initialization of the carbon/vegetation reservoirs in December that leads to the skill. Overall, this study demonstrates the significance of accurate land initialization for S2S carbon forecasts.