Stratospheric initial conditions provide seasonal predictability of the North Atlantic and Arctic Oscillations

To understand the role of initial atmospheric conditions in predicting the winter NAO, figure 1(a) shows the correlation between the initial zonally-averaged zonal wind anomaly on 1 November from the observational reanalysis and the 45-member ensemble mean predicted NAO index for the winter mean (DJF). A significant positive correlation is found in the polar upper stratosphere with further positive correlation at lower latitudes, implying that the initial stratospheric conditions may play a role in subsequent winter NAO predictions. For comparison, the correlation between the initial wind anomaly and the ensemble mean predicted DJF AO index is displayed in figure 1(b). The correlation shows a similar spatial structure to figure 1(a), suggesting that the stratospheric initial conditions also play a role in AO predictions. Note, there is a similar but weaker pattern in observational reanalysis (not shown). This is to be expected given that the NAO/AO in observational reanalysis is in but one realization and contains unpredictable NAO/AO variability which is removed from our ensemble mean. Figure 1 suggests that the initial upper stratospheric zonal wind anomalies may exert a significant influence on the model predicted winter NAO/AO.

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To represent the variation of initial stratospheric zonal wind anomalies, a stratospheric zonal wind index U_i is defined by averaging the zonal-mean zonal wind anomaly on 1 November over ($60^\circ {\rm{N}}\mbox{--}90^\circ {\rm{N}},1\mbox{--}5\,{\rm{hPa}}$) where the positive correlation with the predicted winter NAO/AO index is strongest in figure 1. Figure 2(a) shows the time series of standardized U_i as well as the standardized DJF NAO indices in ensemble mean forecasts and observational reanalysis. The correlation between the initial zonal wind index and the predicted NAO index is 0.47, which is significant at the α = 0.05 level by Student's t-test. This implies that around 22% of the predicted NAO variability could come from the initial stratospheric zonal wind anomaly, and suggests that initial atmospheric conditions are important for seasonal prediction as in Stockdale et al (2015). The correlation between U_i and the single DJF NAO in observational reanalysis is 0.19, which is well within the range of ensemble member correlations. Similar correlation analysis is also applied to the AO. As shown in figure 2(b), the correlation coefficient between U_i and the predicted ensemble-mean AO index is around 0.34, implying that approximately 12% of the variance in the seasonal forecast of the AO can be explained by the stratospheric initial conditions.

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It is interesting to note the 2004/2005 winter, for which the forecast NAO anomaly was previously noted for its difference from the observed NAO anomaly and that implied by predicted tropical rainfall (Scaife et al 2017). For this unusual winter, the initial zonal wind index U_i reached its lowest value during 1994–2012. Given the relationship seen in the other years, this negative value of U_i is linked to the unusually negative phase of the NAO predicted by the model ensemble mean, implying that the initial strong easterly wind anomaly gave rise to the negative phase of the NAO predicted for this winter. Figure 2(c) further shows the spatial pattern of the zonal wind anomaly on 1 November 2004. The strong easterly anomaly in the upper-stratospheric polar region suggests a weakening of the polar night jet at the start of the forecast. This anomalous signal persists in the November-mean (not shown), explaining the predicted negative phase of the NAO in 2004/2005 winter by the exceptional easterly anomaly of the upper stratospheric wind on the initial date. Although the observed 2004/2005 winter NAO is opposite to the ensemble mean predicted NAO, it is still within the range of the ensemble spread. There may not be an error in the model forecast for this winter, it is just that the ensemble mean was shifted down by the stratospheric initial conditions whereas the observations happened to be higher, presumably due to internal unpredictable variability. It is also noteworthy to point out that the 2006/2007 winter, which is the year exhibiting second lowest value of U_i during 1994–2012, shows a positive phase of NAO prediction. The predicted NAO index is opposite to the NAO suggested by the stratospheric initial condition but is consistent with the observed value. As suggested by Scaife et al (2017), the positive phase of NAO this year can be well explained by the tropical rainfall (their figure 12). The above case studies further suggest that the initial stratospheric zonal wind anomaly may provide an additional seasonal predictability source to the NAO besides tropical rainfall.

To explore the connection between the initial zonal wind anomaly in the stratosphere and the predicted winter NAO/AO, figure 3 shows the correlation between the initial wind index U_i and the model predicted zonal-mean zonal wind anomaly from November to February relative to the climatological mean states. The zonal wind climatology in all months shows a strong polar night jet in the stratosphere and two tropospheric jets: subtropical jet and extra-tropical jet. As shown in figure 3(a), the largest positive correlation between U_i and November zonal wind anomaly is found in the polar lower stratosphere, suggesting a strengthening of the polar night jet in November. This stratospheric anomaly then extends down to the troposphere in December. As shown in figure 3(b), the strongest positive correlation is located around 60°N, suggesting a poleward shift of the tropospheric extra-tropical jet. This anomaly in the extra-tropical jet persists well into late winter as illustrated in figures 3(c)–(d), consistent with the positive phase of the predicted NAO/AO. This downward propagation of upper stratospheric zonal wind perturbation to the troposphere is consistent with similar signals in many other observation and modeling studies (Kodera et al 1990, Kodera 1995, Baldwin and Dunkerton 1999).

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To further quantify the relative contribution of the initial stratospheric zonal wind anomaly to winter ensemble mean NAO prediction, figure 4 shows the correlation between the predicted DJF-mean NAO index and the predicted zonal-mean zonal wind anomaly from November to February compared to the climatological mean states. By doing so, the evolution of the signals in winds accompanying the DJF-mean NAO can be clearly shown and compared with the wind evolution induced by the stratospheric initial conditions. In November, as shown in figure 4(a), a strong positive correlation is found in the stratosphere, suggesting a strengthening of the polar night jet before positive NAO. In December (figure 4(b)), the correlation exhibits a dipolar structure in the troposphere, with positive correlation at high latitude and negative correlation at low latitude, thus indicating a poleward displacement of the midlatitude jet with positive phase of the NAO. This dipolar pattern of correlation persists into January and February and becomes stronger, as shown in figures 4(c)–(d). Comparison between figures 3 and 4 illustrates that the spatial and time evolution of the zonal wind anomaly induced by the initial upper stratospheric conditions is similar to the zonal wind evolution accompanying the predicted NAO. This further demonstrates that the downward propagation of initial stratospheric wind perturbations plays a role in the prediction of the winter NAO and demonstrates that the influence on the NAO is likely mediated by the zonal wind anomalies (figure 4). The conclusions for the NAO also hold for the AO (results not shown).

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The impact of the initial upper stratospheric zonal wind anomaly on the surface climate is examined through composite analysis. Figures 5(a) and (b) present the composite DJF-mean SLP anomalies with initial easterly and westerly anomalies in the stratosphere. By definition, the easterly (westerly) phase of U_i refers to years when U_i is below (above) −0.5 (+0.5) standard deviations. As shown in figure 5(a), for the initial easterly case, the winter SLP increases in the Arctic region and decreases at mid-latitudes in both the Atlantic and Pacific basins corresponding to a negative NAO and AO. For the initial westerly case, as shown in figure 5(b), the composite DJF SLP anomalies display opposite spatial pattern which represents positive phase of NAO/AO. The composite of SLP difference between initial easterly phase and westerly phase displays a stronger negative phase of NAO/AO as shown in figure 5(c). The above results illustrate that the initial upper-stratospheric easterly/westerly anomaly induces a negative/positive phase of the NAO and AO. Very similar patterns are seen in observed surface climate response to stratospheric variability (Gerber et al 2012, Kidston et al 2015).

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