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Predictability of European winter 2015/2016

We present a case study of long range forecasts for Northern Hemisphere winter 2015/2016. This winter produced the strongest El Niño event since 1997/1998 and equatorial Pacific sea surface temperature anomalies exceeded 3 °C. Other factors relevant to the Northern Hemisphere extratropical atmosphere circulation included a strong westerly phase of the Quasi‐Biennial Oscillation (QBO) and very strong winds in the stratospheric polar night jet in early winter. At the surface, intense cyclonic extratropical circulation anomalies occurred in early winter in both the North Pacific and North Atlantic, consistent with known teleconnections to these phases of El Niño–Southern Oscillation, the QBO and the polar night jet. The midlatitude flow was very westerly in early winter and less westerly and sometimes northerly in late winter, when sudden stratospheric warming events also occurred. We show that initialised climate predictions were able to capture the winter mean flow pattern at seasonal lead times from well before the start of winter. In this special case, not only the winter mean flow pattern, but also some aspects of the sub‐seasonal evolution were skilfully predicted. We show that the winter of 1982/1983 was closely analogous to winter 2015/2016 in both the predictable driving factors and the forecast winter circulation. This case study adds to the evidence that the north Atlantic circulation can be predictable on seasonal timescales and advance warning of the increased risk of intense rainfall and storminess which caused extreme flooding in the UK in December was possible in this case.


Introduction
Recent evidence has suggested that not only the Tropics but also the extratropical winter flow is often predictable on seasonal timescales . This case study examines the latest real time seasonal forecasts for the winter 2015/2016. This winter was marked by a very strong El Niño, comparable in strength to the largest events in the observational record. We analyse the forecasts in the context of the known teleconnections to El Niño-Southern Oscillation (ENSO) and comment on other potential sources of predictability. Section 2 on Predictable Factors discusses potential sources of predictable signals, Section 3 on Extratropical Circulation discusses real time forecasts and Section 4 on The Analogous Winter of 1982/1983 shows a striking analogue in forecasts for the winter of 1982/1983, which was also a strong El Niño case.
The seasonal forecast data used in this study were generated by the Met Office operational Global Seasonal Prediction System GloSea . We use the fifth generation of this coupled ocean-land-atmosphere forecast system GloSea5 (MacLachlan et al., 2015). Forecasts were made in real time in late 2015, initialised approximately 1 month ahead of winter (MacLachlan et al., 2015). Forecasts for the winter of 1982/1983 were made using the same HadGEM3 model applied to a longer historical period for decadal prediction assessment . The atmospheric resolution of the model is 0.83 ∘ longitude by 0.55 ∘ latitude with 85 quasi-horizontal atmospheric levels and an upper boundary at 85 km. Ocean resolution is 0.25 ∘ globally with 75 quasi-horizontal levels. A full description of the system is given in MacLachlan et al. (2015). Sea level pressure observations are taken from the HadSLP2 dataset (Allan and Ansell, 2006) and sea surface temperature observations are from the HadISST2 dataset (Titchner and Rayner, 2014). Observational stratospheric data are from the same operational analysis used to initialise GloSea5 (MacLachlan et al., 2015). intense westerly wind bursts began in spring 2015 and continued into the late autumn (Levine and McPhaden, 2016). Tropical Pacific sea surface temperature anomalies grew rapidly through 2015 and peaked in winter, exceeding 3 ∘ C across parts of the central and eastern equatorial Pacific ( Figure 1). The event was similar in strength to the strongest events in the recent record which occurred in the winters of 1997/1998 and 1982/1983 ( Figure 1). Interestingly, the spatial tropical Pacific temperature structure more closely resembled that of the 1982/1983 event (pattern correlation of 0.95 compared to 0.91 with 1997/1998), with the strongest sea surface temperature anomalies well separated from the South American coast. We return to this similarity later. Real time seasonal forecasts very skilfully predicted the El Niño well in advance of the start of winter, including the position of the peak sea surface temperature anomaly (Figure 1). The observed amplitude was lower than the ensemble mean but within the ensemble spread of the forecast values (not shown). The stratospheric circulation also exhibited notable features in winter 2015/2016. The equatorial lower stratospheric winds reversed from easterly to westerly in mid-2015 as the Quasi-Biennial Oscillation (QBO) entered a strong westerly phase (Figure 2(b)). This provides a second similarity with the winter of 1982/1983, which also coincided with a strong westerly phase of the oscillation. Many observational and climate modelling studies have noted the relationship between the phase of the tropical QBO and the strength of the extratropical stratospheric winds, particularly in early winter (Ebdon, 1975;Holton and Tan, 1980;Pascoe et al., 2006;Garfinkel et al., 2012). The observed stratospheric wind anomalies in winter 2015/2016 matched the sign and strength of this teleconnection well. In particular, the polar night jet winds were extremely strong in early winter, exceeding 60 m s −1 in the upper stratosphere for many days (Figure 2(a)) and with a reduced chance of a sudden stratospheric warming (SSW) in early winter and increased chance in late winter ( Figure 2(d)).
In the latter part of winter, the polar night jet weakened dramatically and a minor SSW occurred in mid-January, followed by a major sudden warming at the end of winter (Figure 2(a)). This late winter weakening of the polar night jet often occurs during El Niño events (Brönnimann et al., 2007;Bell et al., 2009;Butler and Polvani, 2011;Scaife et al., 2016). It is associated with a shift in surface conditions over the north Atlantic towards a more blocked flow (Cagnazzo and Manzini, 2009;Ineson and Scaife, 2009). Seasonal forecasts from ahead of the winter predicted a strong increase in the risk of a major SSW, with around 80% probability of a late winter event (Figures 2(c) and (d)). This sharp rise in the probability of a late winter SSW is also typically seen in seasonal forecasts during El Niño (Domeisen et al., 2015;Butler et al., 2016;Scaife et al., 2016). A major SSW eventually occurred in the first few days of March (Figure 2(a)).
Other factors have been associated with predictability of the north Atlantic region such as the area of Arctic sea ice (Mori et al., 2015;Furtado et al., 2016), Atlantic Ocean temperatures (Folland et al., 2012) and solar variability (Ineson et al., 2011). We briefly consider each of these in turn. The area of Arctic sea ice was close to the long term trend and therefore below average in winter 2015/2016. Although it is controversial, this has been suggested to drive a weakening of the Atlantic jet (Kim et al., 2014), so it is unlikely to be responsible for the strong jet seen in this particular winter ( Figure 3). With respect to solar variability, winter 2015/2016 was around 2 years after a maximum in the 11 year solar cycle, and the occurrence of a strong Atlantic jet and strong westerly flow in the stratosphere (Figure 2) at this time lag agrees with the usual observed response  via integrated effects in the Atlantic ocean heat content Andrews et al., 2015). The Atlantic Ocean did also show an enhanced pole to equator temperature gradient off Newfoundland in autumn 2015, consistent with the pattern associated with solar variability in these earlier studies. However, we note that the recent solar maximum was weak in amplitude (Svalgaard et al., 2005) and so although there could still be a degree of phase locking between atmospheric circulation and solar variability (Thieblemont et al., 2015), we focus on shorter interannual factors such as ENSO (Figure 1) and the QBO (Figure 2) here as these showed large deviations from their mean state.

Extratropical circulation
The Aleutian low in the north Pacific typically deepens in response to El Niño as part of the Pacific North American teleconnection pattern which is highly predictable in the forecasting system used here (Athanasiadis et al., 2014). As expected, a very deep Aleutian low also occurred during winter 2015/2016 (Figure 3), with anomalies exceeding −10 hPa in the north Pacific corresponding to the very strong El Niño. This deepening of the climatological Aleutian Low leads to increased meandering of the North Pacific jet stream, which in turn increases the amplitude of stationary waves (Manzini et al., 2006;Ineson and Scaife, 2009;Smith and Kushner, 2012). Stationary waves can propagate readily into the stratosphere in winter and their increased amplitude during El Niño increases the risk of an SSW in late winter (Bell et al., 2009;Scaife et al., 2016). This occurred in forecasts of the late winter (Figure 2(c)) and eventually in observations (Figure 2(a)).
Conditions over the North Atlantic were also far from normal in winter 2015/2016 with December sea level pressure anomalies well in excess of 10 hPa and winter mean anomalies well in excess of 5 hPa (Figure 3), and there was a strong contrast between early and late winter. From as early as November, and for the whole of December, anomalously strong meridional sea level pressure gradients dominated the North Atlantic  ( Figure 3). Lower than normal pressure over Iceland and higher than normal pressure over the Azores correspond to a positive North Atlantic Oscillation index which was well above normal for December. The associated strong jet stream and a string of deep cyclones gave rise to frequent wind storms, intense rainfall and persistent flooding across north-west Europe, for example in the UK.
In contrast, the later winter period showed more frequent Atlantic ridging and north-westerly flow into Europe. This transition occurred briefly in January, and then again in late February, close to the time of the major SSW. Note that the stratospheric westerly jet never recovered after this event and so the SSW would technically be classified as a final stratospheric warming (Black and McDaniel, 2006). However, like mid-winter SSW events it did mark a shift to more blocked, Atlantic ridging conditions as has occurred in association with other sudden warming events (Baldwin and Dunkerton, 2001;Kuroda, 2008;Scaife and Knight, 2008;Kolstad et al., 2010;Marshall and Scaife, 2010;Fereday et al., 2012;Sigmond et al., 2013).
Real time seasonal forecasts using Met Office climate predictions initialised from well ahead of winter in early November 2015 (Maclachlan et al., 2015;Dunstone et al., 2016) predicted the enhanced early winter westerlies and the more blocked late winter flow (Figures 3 and 4). Northern Hemisphere pattern correlations are 0.52 for December and 0.72 for the winter as a whole. Note that while the patterns are very similar, the amplitude is low. One might at first attribute this to the ensemble averaging which naturally reduces the size of anomalies due to cancellation of noise between ensemble members. However, earlier work based on multiple winters shows that Atlantic seasonal forecast signals are often unusually weak in amplitude Scaife et al., 2014). Nevertheless, such clear forecast signals at such long lead times demonstrate that the extratropical flow in winter 2015/2016 contained highly predictable signals. This is consistent with the influence of ENSO and the QBO, as they are both predictable at seasonal or even interannual lead times (Luo et al., 2008;Scaife et al., 2014;Dunstone et al., 2016) and both El Nino (Moron and Gouirand, 2004;Brönnimann et al., 2007;Fereday et al., 2008) and the westerly phase of the QBO (Ebdon, 1975;Hamilton, 1998;Pascoe et al., 2006) tend to drive a stronger than normal Atlantic westerly jet during early winter.

The analogous winter of 1982/1983
Given the identification of a few driving factors for winter 2015/2016, we ask whether similar conditions have occurred in the past? We pointed out earlier that both the phase of ENSO and the QBO in winter 2015/2016 were similar in the winter of 1982/1983. Given that both factors are highly predictable on seasonal timescales and both have teleconnections to the North Atlantic, it is likely that they also influenced the forecasts in Figure 3. Here we ask whether this analogy between the 2015/2016 and 1982/1983 winters can be taken further: were forecast circulation patterns similar for these two winters? Figure 4 shows predicted sea level pressure anomalies for the winters of 2015/2016 and 1982/1983 using the same model as was used for the winter 2015/2016 forecasts . A striking similarity is seen between forecast anomalies for these two strong El Niño/westerly QBO winters. The November-December period shows strong westerly flow in both cases, while the January-February period undergoes a transition to more ridging/blocking in the North Atlantic. It seems that for forecast signals at least, the winter of 1982/1983 is closely analogous to the winter of 2015/2016 and that the predictable part of the flow represented by the ensemble mean is remarkably similar in the two cases, even for the intraseasonal evolution.
Interestingly, it has also been observed that the strongest of El Niño's, such as occurred in the winters of 1982/1983, 1997/1998 and now 2015/2016 show early winter (November-December) westerly anomalies (Moron and Gouirand, 2004;Fereday et al., 2008). Whereas, in late winter (January-February), the strongest El Niño events show an Atlantic high pressure pattern tends to dominate, such that late winter stratospheric influences (Domeisen et al., 2015;Richter et al., 2015;Scaife et al., 2016) are outweighed by tropospheric effects (Bell et al., 2009;Ineson and Scaife, 2009). This has been interpreted as a Rossby wave train emanating from the Caribbean region in response to strong ENSO forcing (Toniazzo and Scaife, 2006) and is in very good agreement with the January-February patterns in Figure 4.
The similarity between the predicted North Atlantic circulation in the winters of 2015/2016 and 1982/1983 suggests that the predictable component of the circulation in these two winters was similar and strong enough to dominate the forecast ensemble mean. It also confirms that there were clear predictable signals in the winter of 2015/2016 as opposed to the winter mean flow being simply a result of unpredictable chaotic variability. Similarity between these two winters is consistent with explanations of a reasonable proportion of winter interannual variability from a moderate number of predictable factors (Folland et al., 2012;Hall et al., 2014).

Conclusions
Long range forecasts from the Met Office GloSea5 system identified signals for strong westerly flow over the Atlantic in winter 2015/2016, particularly in early winter (December). These forecasts verify well against observed sea level pressure anomalies: the winter mean pressure pattern and intense anomalies in December were both in good agreement with observations. The signal for intensified cyclonic flow and positive North Atlantic Oscillation (NAO) in early winter is consistent with the extreme rainfall and UK flooding (Svensson et al., 2015) experienced in December 2015 and the reduced extent of winter Baltic Sea Ice (Karpechko et al., 2015;FMI, 2016, pers. comm.) (personal communication from Jouni Vainio of observed Baltic Sea Ice extent of 110 000 km 2 which is well below average). These impacts of the unusual atmospheric circulation in winter 2015/2016 were therefore also predictable at very long lead times.
The deepened Aleutian low in the North Pacific and declining strength of the stratospheric polar night jet from very strong westerlies in early winter to easterlies in late winter were also well predicted. These features are well-established aspects of the ENSO and QBO teleconnections to the extratropics and suggest that the winter of 2015/2016 was subject to strong and predictable forcing from a near record El Niño event and a persistent westerly phase of the QBO.
We also showed that very similar Atlantic pressure patterns and hence winds, including the intraseasonal transition, occurred in the winter of 1982/1983. The striking similarity between forecasts for these two winters is consistent with conditions of strong El Niño and westerly QBO providing highly predictable signals in the extratropics.
The last El Niño winter occurred in 2009/2010, and this turned out to be a very harsh and disruptive winter across northern Europe (Cattiaux et al., 2010;Jung et al., 2011;Fereday et al., 2012). However, the Atlantic response to El Niño depends on the El Niño strength and the solar cycle was in opposite phase in 2015/2016 so the risk of a cold winter was much lower than in the winter of 2009/2010. Instead, the winter of 2015/2016 followed an evolution similar to the analogous strong El Niño/westerly QBO/post solar maximum winter of 1982/1983. Not all winters are predictable, and chaotic, unpredictable variability can dominate in some cases. Nevertheless, winter 2015/2016 provides additional evidence for seasonal predictability of atmospheric circulation over the Atlantic (cf. Scaife et al., 2014). Whether this is due to larger predictable signals from the recent increasing variability of the NAO (Hanna et al., 2015) and/or a stronger connection with ENSO in recent years (Yu and Lin, 2016) remains to be seen, but operational seasonal forecasts for this winter gave a good indication of the subsequent conditions out to months ahead, including the period of intense cyclonic activity in December.