Both Arctic sea ice loss and La Nina events can result in cold conditions in midlatitude Eurasia in winter. Since the two forcings sometimes occur simultaneously, determining whether they are independent of each other is undertaken first. The result suggests an overall independence. Considering possible interactions between them, their coordinated impacts on the Northern Hemisphere winter climate are then investigated based on observational data analyses, historical simulation analyses from one coupled model (MPI-ESM-LR) contributing to CMIP5, and atmospheric general circulation model sensitive experiments in ECHAM5. The results show that the impacts of the two forcings are overall linearly accumulated. In comparison with one single forcing, there is intensified cooling response in midlatitude Eurasia along with northern warmer-southern cooler dipolar temperature responses over North America. Despite the additive linearity, additive nonlinearity between the two forcings is identifiable. The nonlinearity causes midlatitude Eurasian cooling weakened by one-tenth to one-fifth as much as their individual impacts in combination. The underlying mechanisms for the weak additive nonlinearity are finally explored by transient adjustment AGCM runs with one single forcing or both the forcings switched on suddenly. The day-to-day evolution of responses suggests that the additive nonlinearity may arise initially from the forced wave dynamics and then be amplified because of the involvement of transient eddy feedbacks.
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