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Tropical Pacific impacts on cooling North American winters


The North American continent generally experienced a cooling trend in winter over the early 2000s. This cooling trend represented a significant deviation from expected anthropogenic warming and so requires explanation. Previous studies indicate that climate variations in the tropical Pacific contributed to many mid-latitude climate variations over the early twenty-first century. Here we show using large ensembles of fully coupled, partially coupled and uncoupled model simulations that in northwest North America the winter cooling was primarily a remote response to climate fluctuations in the tropical Pacific. By contrast, in central North America the winter cooling appears to have resulted from a relatively rare fluctuation in mid-latitude circulation that was unrelated to the tropical Pacific. Our results highlight how decadal climate signals—both remote and local in origin—can together offset anthropogenic warming to produce continental-scale cooling.

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Figure 1: Observed winter trends from 2001–2002 to 2013–2014.
Figure 2: Anomalies associated with a weakened tropical Pacific temperature gradient on interannual and decadal timescales.
Figure 3: Simulated and observed winter trends from 2001–2002 to 2013–2014 in North American SAT and the tropical Pacific SAT gradient.
Figure 4: Climate trends in winter from 2001–2002 to 2013–2014 from fully coupled, partially coupled and uncoupled model simulations.
Figure 5: Probability of an unusually cold central North American winter.
Figure 6: Composite of climate trends in fully coupled simulations with the largest central North American winter cooling.


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We thank O. Saenko and W. Lee for helping design and perform the pacemaker simulations, J. Scinocca for helping extend the CanESM2 large ensemble, Y. Jiao for technical assistance, N. Gillett, B. Merryfield and N. Swart for their comments on an earlier draft, C. Fletcher for helpful discussions and the CESM group for providing the CESM large ensemble model output.

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M.S. designed and performed the uncoupled simulations, helped to carry out the analysis and wrote the initial draft. J.C.F. helped design the pacemaker experiments, helped with the analysis and edited the manuscript.

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Correspondence to Michael Sigmond.

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The authors declare no competing financial interests.

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Sigmond, M., Fyfe, J. Tropical Pacific impacts on cooling North American winters. Nature Clim Change 6, 970–974 (2016).

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