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Enhanced seasonal forecast skill following stratospheric sudden warmings


Advances in seasonal forecasting have brought widespread socio-economic benefits. However, seasonal forecast skill in the extratropics is relatively modest1, prompting the seasonal forecasting community to search for additional sources of predictability2,3. For over a decade it has been suggested that knowledge of the state of the stratosphere can act as a source of enhanced seasonal predictability; long-lived circulation anomalies in the lower stratosphere that follow stratospheric sudden warmings are associated with circulation anomalies in the troposphere that can last up to two months4,5. Here, we show by performing retrospective ensemble model forecasts that such enhanced predictability can be realized in a dynamical seasonal forecast system with a good representation of the stratosphere. When initialized at the onset date of stratospheric sudden warmings, the model forecasts faithfully reproduce the observed mean tropospheric conditions in the months following the stratospheric sudden warmings. Compared with an equivalent set of forecasts that are not initialized during stratospheric sudden warmings, we document enhanced forecast skill for atmospheric circulation patterns, surface temperatures over northern Russia and eastern Canada and North Atlantic precipitation. We suggest that seasonal forecast systems initialized during stratospheric sudden warmings are likely to yield significantly greater forecast skill in some regions.

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Figure 1: Surface climate response to SSWs.
Figure 2: Observed versus forecast 1,000 hPa NAM index.
Figure 3: Vertical profile of the mean NAM and of the CSS of the NAM.
Figure 4: The CSS of various variables.


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M.S. gratefully acknowledges funding by Environment Canada through a Grants and Contributions Agreement with the University of Toronto. We thank B. Merryfield, J. Fyfe and N. Gillett for their helpful comments.

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M.S., J.F.S. and V.V.K. designed the experiments. All authors interpreted the results and contributed to writing the paper.

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Correspondence to M. Sigmond.

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The authors declare no competing financial interests.

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Sigmond, M., Scinocca, J., Kharin, V. et al. Enhanced seasonal forecast skill following stratospheric sudden warmings. Nature Geosci 6, 98–102 (2013).

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