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Abstract
AbstractThe winter North Atlantic Oscillation (NAO) has varied on interannual and decadal timescales over the last century, associated with variations in the speed and latitude of the eddy‐driven jet stream. This paper uses hindcasts from two operational seasonal forecast systems (the European Centre for Medium‐range Weather Forecasts's seasonal forecast system, and the U.K. Met Office global seasonal forecast system) and a century‐long atmosphere‐only experiment (using the European Centre for Medium‐range Weather Forecasts's Integrated Forecasting System model) to relate seasonal prediction skill in the NAO to these aspects of jet variability. This shows that the NAO skill realized so far arises from interannual variations in the jet, largely associated with its latitude rather than speed. There likely remains further potential for predictability on longer, decadal timescales. In the small sample of models analyzed here, improved representation of the structure of jet variability does not translate to enhanced seasonal forecast skill.
Highlights
The leading mode of variability in the large-scale circulation over the North Atlantic in winter is the North Atlantic Oscillation (NAO), which strongly impacts the weather and climate in the Euro-Atlantic sector
The ASF-20C hindcasts exhibit a positive bias in jet strength, with the median and mean jet speed greater than the reanalysis seasonal mean for the majority of the forecast period
This paper has examined the contributions from interannual and decadal variations in jet latitude and jet speed to the winter NAO skill in model hindcasts
Summary
The leading mode of variability in the large-scale circulation over the North Atlantic in winter is the North Atlantic Oscillation (NAO), which strongly impacts the weather and climate in the Euro-Atlantic sector. The level of skill itself appears to vary on decadal timescales (see, e.g., Müller et al, 2005; Shi et al, 2015) To investigate this variation, an ensemble of atmospheric seasonal hindcast experiments using the European Centre for Medium-range Weather Forecasts (ECMWF)'s Integrated Forecasting System model for the period 1900–2009 was created to explore the skill in predicting the winter NAO over a long period (Weisheimer et al, 2017). The NAO reflects changes in both the speed and latitude of the jet stream These two features are linearly independent in their interannual variability (Woollings et al, 2014) and likely sensitive to different dynamical drivers (Baker et al, 2018). An additional complexity is that the decadal variations in jet speed can modulate the amount of variability in latitude on shorter timescales, with decades where the jet was strong and steady being interspersed with decades where the jet was weak and variable (Woollings et al, 2017)
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