Seasonal winter forecasts and the stratosphere

A A Scaife,N Dunstone,M Gordon,C Maclachlan,M P Baldwin,A Brookshaw,A Yu Karpechko,N Martin,A H Butler,R Eade,D Smith

doi:10.1002/asl.598

A A Scaife, N Dunstone + Show 9 more

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https://doi.org/10.1002/asl.598

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Journal: Atmospheric Science Letters	Publication Date: Oct 25, 2015
Citations: 161	License type: CC BY 4.0

Affiliation: Met Office, University of Exeter

Abstract

AbstractWe investigate seasonal forecasts of the winter North Atlantic Oscillation (NAO) and their relationship with the stratosphere. Climatological frequencies of sudden stratospheric warming (SSW) and strong polar vortex (SPV) events are well represented and the predicted risk of events varies between 25 and 90% from winter to winter, indicating predictability beyond the deterministic range. The risk of SSW and SPV events relates to predicted NAO as expected, with NAO shifts of −6.5 and +4.8 hPa in forecast members containing SSW and SPV events. Most striking of all is that forecast skill of the surface winter NAO vanishes from these hindcasts if members containing SSW events are excluded.

Highlights

Previous studies have quantified the deterministic limit on the predictability of the timing of sudden stratospheric warming (SSW) events
Given the stratospheric influence on winter surface climate (e.g. Boville, 1984; Scaife and Knight, 2008; Kolstad et al, 2010; Mitchell et al, 2013; Sigmond et al, 2013), we investigate the relationship with winter seasonal forecasts of the surface North Atlantic Oscillation (NAO)
SSW events are defined to occur if this wind decreases below zero at some time in the winter, while strong polar vortex (SPV) events are defined to occur if this wind increases above 48 m s−1 on some day in the winter

Summary

Introduction

Previous studies have quantified the deterministic limit on the predictability of the timing of sudden stratospheric warming (SSW) events These studies typically indicate significant forecast skill out to around 2 weeks (Mukougawa et al, 2005; Stan and Straus, 2009; Marshall and Scaife, 2010; Sigmond et al, 2013) and very occasionally 1 month (Kuroda, 2008) ahead. We use ensembles of 24 forecasts starting in early November for each of the 20 winters from 1992/1993 to 2011/2012 from the fifth generation of GloSea (MacLachlan et al, 2014) These winter forecasts have statistically significant forecast skill for the surface NAO (Scaife et al, 2014). This SPV (upper) threshold is chosen as it is broken with the same frequency as the lower SSW threshold in our forecasts

Predictability of stratospheric events beyond the deterministic range

Relationship with the surface NAO

Impact on surface forecast skill

Findings

Conclusions