Abstract
The impact of the Atlantic multidecadal variability (AMV) on the wintertime atmosphere circulation is investigated using three different configurations of the Community Atmospheric Model version 5 (CAM5). Realistic SST and sea ice anomalies associated with the AMV in observations are prescribed in CAM5 (low-top model) and WACCM5 (high-top model) to assess the dependence of the results on the representation of the stratosphere. In a third experiment, the role of ocean–atmosphere feedback is investigated by coupling CAM5 to a slab-ocean model in which the AMV forcing is prescribed through oceanic heat flux anomalies. The three experiments give consistent results concerning the response of the NAO in winter, with a negative NAO signal in response to a warming of the North Atlantic ocean. This response is found in early winter when the high-top model is used, and in late winter with the low-top model. With the slab-ocean, the negative NAO response is more persistent in winter and shifted eastward over the continent due to the damping of the atmospheric response over the North Atlantic ocean. Additional experiments suggest that both tropical and extratropical SST anomalies are needed to obtain a significant modulation of the NAO, with small influence of sea ice anomalies. Warm tropical SST anomalies induce a northward shift of the ITCZ and a Rossby-wave response that is reinforced in the mid-latitudes by the extratropical SST anomalies through eddy–mean flow interactions. This modeling study supports that the positive phase of the AMV promotes the negative NAO in winter, while illustrating the impacts of the stratosphere and of the ocean–atmosphere feedbacks in the spatial pattern and timing of this response.
Highlights
The relationship between sea surface temperature (SST) anomalies and the large-scale atmospheric circulation has been thoroughly investigated in the literature, from observational analysis and modeling experiments
Wind stress anomaly vectors are superimposed onto the SST anomalies in Fig. 1a–c to illustrate the adjustment of the near-surface atmosphere to the forcing, that directly impacts the turbulent heat fluxes
We have examined the relative importance of tropical SST, extratropical SST and sea ice anomalies in the response
Summary
The relationship between sea surface temperature (SST) anomalies and the large-scale atmospheric circulation has been thoroughly investigated in the literature, from observational analysis and modeling experiments. Observational measurements of the AMOC are too recent to investigate its impacts on long time scales, but coupled climate models suggest that a strong AMOC induces an increase of warm water transport from the tropics to the extratropics, resulting in the positive polarity of the AMV (e.g., Wang and Zhang 2013) Such an AMV-AMOC relationship is consistent with the pioneering work of Bjerknes (1964) who hypothesized that in the North Atlantic the atmosphere drives the SST variability on short time scales (up to interannual), while ocean dynamics is responsible for SST and potentially atmospheric variability at decadal/multidecadal time scales. This hypothesis is verified by recent observational work that has investigated the relationship between surface heat fluxes and the North Atlantic SST (Gulev et al 2013)
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