Trend in seasonal amplitude of northern net ecosystem production: Simulated results from IAP DGVM in CAS-ESM2

Net ecosystem production (NEP) over northern latitudes has an obvious seasonal variation, which dominantly drives the seasonal variation in atmospheric CO2. Investigating these carbon processes and understanding the underlying drivers is a key issue of climate research. This study focuses on the seasonal amplitude of northern NEP (40 degrees-90 degrees N) and investigates its trend in the period 1990-2014 by using the dynamic global vegetation model (IAP DGVM) in the second version of the Chinese Academy of Sciences Earth System Model (CAS-ESM2). Basing on a spin -up simulation, the authors conduct a control simulation to evaluate the modeled trend in the seasonal amplitude of northern NEP, and three sensitivity simulations to detect the contributions of the climate and atmospheric CO2. The results show that the modeled seasonal amplitude of northern NEP increases significantly from 1990 to 2014, with a trend of 9.69 TgC month-1 yr- 1 , which is mainly because of the increasing maximum NEP. The positive trend is largely reduced when CO2 fertilization effects and climatic effects are separately excluded. These significant reductions suggest important effects of atmospheric CO2 and climate change on the seasonal amplitude of northern NEP. Even though model uncertainties remain, the results favor further development of IAP DGVM in accurately simulating the terrestrial carbon cycle, and also provide an important reference for the application of CAS-ESM in further investigating carbon-climate interactions.