Earth system models are intended to make long-term projections, but they can be evaluated at interannual and seasonal time scales. Although the Community Earth System Model (CESM2) showed improvements in a number of terrestrial carbon cycle benchmarks, relative to its predecessor, our analysis suggests that the interannual variability (IAV) in net terrestrial carbon fluxes did not show similar improvements. The model simulated low IAV of net ecosystem production (NEP), resulting in a weaker than observed sensitivity of the carbon cycle to climate variability. Low IAV in net fluxes likely resulted from low variability in gross primary productivity (GPP)—especially in the tropics—and a high covariation between GPP and ecosystem respiration. Although lower than observed, the IAV of NEP had significant climate sensitivities, with positive NEP anomalies associated with warmer and drier conditions in high latitudes, and with wetter and cooler conditions in mid and low latitudes. We identified two dominant modes of seasonal variability in carbon cycle flux anomalies in our fully coupled CESM2 simulations that are characterized by seasonal amplification and redistribution of ecosystem fluxes. Seasonal amplification of net and gross carbon fluxes showed climate sensitivities mirroring those of annual fluxes. Seasonal redistribution of carbon fluxes is initiated by springtime temperature anomalies, but subsequently negative feedbacks in soil moisture during the summer and fall result in net annual carbon losses from land. These modes of variability are also seen in satellite proxies of GPP, suggesting that CESM2 appropriately represents regional sensitivities of photosynthesis to climate variability on seasonal time scales.

中文翻译：

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以社区地球系统模型 (CESM2) 为代表的碳循环变率的年际和季节驱动因素

地球系统模型旨在进行长期预测，但可以在年际和季节时间尺度上进行评估。尽管社区地球系统模型 (CESM2) 相对于其前身显示了许多陆地碳循环基准的改进，但我们的分析表明，净陆地碳通量的年际变率 (IAV) 并未显示出类似的改进。该模型模拟了净生态系统生产 (NEP) 的低 IAV，导致碳循环对气候变化的敏感性低于观察到的敏感性。净通量中的低 IAV 可能是由于总初级生产力 (GPP) 的低变异性——尤其是在热带地区——以及 GPP 和生态系统呼吸之间的高协变。虽然低于观测值，但 NEP 的 IAV 具有显着的气候敏感性，NEP 正异常与高纬度地区的温暖和干燥条件以及中低纬度地区的潮湿和凉爽条件有关。我们在完全耦合的 CESM2 模拟中确定了碳循环通量异常中季节性变化的两种主要模式，其特征是生态系统通量的季节性放大和重新分布。净碳通量和总碳通量的季节性放大表明气候敏感性反映了年度通量。碳通量的季节性重新分配是由春季温度异常引发的，但随后在夏季和秋季土壤水分的负反馈导致土地的年度净碳损失。这些变异模式也可以在 GPP 的卫星代理中看到，