ECMWF季节预报模式对CO₂和人为气溶胶强迫的敏感性：实验设计及其对气候趋势的影响
Sensitivity of the ECMWF seasonal forecast model to CO2 and anthropogenic aerosol forcings: Experimental design and impact on climate trends
AI总结：
该研究利用ECMWF耦合季节预报系统开展反事实后报实验，探究模式对CO₂和人为气溶胶强迫的敏感性，发现反事实后报可改变长期温度趋势，为动态归因和诊断模式缺陷提供了有力工具。
AI中文摘要：
检测与归因研究通常依赖于修改了人为强迫的自由运行气候模式，但这些模式无法再现关键的观测年代际趋势。尽管频繁重新初始化可减少偏差、提高预测技巧、生成大集合并改进趋势，但季节预报模式尚未用于此类研究。我们引入了一组新的反事实季节后报，使用ECMWF耦合季节预报系统，通过修改大气强迫以及海洋和海冰初始条件，基于替代强迫场景开展研究。我们推导了基于观测的强迫海洋温度信号估计值，以从海洋初始条件中放大或去除该信号。回顾性预报包括控制配置，以及增强和减弱强迫实验：其中CO₂随人为海洋变暖加倍而增加，或CO₂固定且人为海洋变暖信号被去除；另有额外实验用于分离气溶胶强迫。这些反事实后报大幅改变了长期温度趋势，同时基本保留了季节预测技巧、年际变率和模式漂移。增强强迫强化了控制实验低估的若干观测气候趋势，包括顶大气辐射通量和热带太平洋大气环流的某些方面。然而，耦合海气系统无法通过Bjerknes反馈维持增强的热带太平洋温度梯度，这表明存在根本性的模式局限。气溶胶强迫影响很小，可能是因为未包含气溶胶间接效应。这些结果确立了反事实季节后报作为动态归因和诊断模式对人为气候强迫响应缺陷的强大工具。
英文摘要：
Detection and attribution studies typically rely on free running climate models with modified anthropogenic forcings, yet they fail to reproduce key observed decadal trends. Despite reduced biases, higher predictive skill, large ensembles, and improved trends through frequent reinitialization, seasonal forecast models have not been used for this purpose. We introduce a new set of counterfactual seasonal hindcasts using the ECMWF coupled seasonal forecasting system based on alternative forcing scenarios by modifying atmospheric forcing and ocean and sea ice initial conditions. An observation based estimate of the forced ocean temperature signal is derived to amplify or remove this signal from the ocean initial conditions. Retrospective forecasts include a control configuration together with enhanced and reduced forcing experiments in which either CO2 increases together with doubled anthropogenic ocean warming or CO2 is fixed and the anthropogenic ocean-warming signal is removed. Additional experiments isolate aerosol forcing. The counterfactual hindcasts substantially alter long term temperature trends while largely preserving seasonal prediction skill, interannual variability, and model drift. Enhanced forcing strengthens several observed climate trends underestimated by the control, including top of atmosphere radiative fluxes and aspects of tropical Pacific atmospheric circulation. However, the coupled atmosphere ocean system fails to sustain the strengthened tropical Pacific temperature gradient through Bjerknes feedbacks, suggesting a fundamental model limitation. Aerosol forcing has little impact, likely because indirect aerosol effects are omitted. These results establish counterfactual seasonal hindcasts as a powerful tool for dynamic attribution and diagnosing model deficiencies in responses to anthropogenic climate forcing.
