Winter : Illustrating the Strength of WCS Methods

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1 Winter : Illustrating the Strength of WCS Methods Amplified and persistent climate patterns in the winter of brought unusual conditions to many parts of the Northern Hemisphere. Highly publicized forecasts based primarily on near-record autumn Siberian snow cover failed to foresee the dominant circulation pattern, but the World Climate Service forecasts successfully identified both key drivers of the pattern and the corresponding outcome. The World Climate Service approach relies on a multitude of seasonal climate predictors and provides a stable, credible, and skillful basis for anticipating seasonal anomalies. 1. Introduction The primary circulation features that influenced Northern Hemisphere climate in winter were a persistent ridge over western North America and a strong and persistent trough over the northern North Atlantic Ocean and Greenland. The northern Atlantic anomaly was highly characteristic of the positive phase of the North Atlantic Oscillation (NAO), and the mean NAO phase was the most positive on record. The prevailing circulation produced a warm winter in most of Europe, especially the north and east, but persistent northwest flow brought numerous Arctic outbreaks to the northeastern United States after the turn of the year. In late autumn 2014, many seasonal climate analysts and forecasters began to expect highlatitude blocking to bring cold air to Europe and eastern North America during the subsequent winter. Discussion was heavily focused on the very rapid advance of Eurasian snow cover in October and the extreme value of a much-publicized October Pattern Index. Both of these predictors are said to be highly skillful in forecasting the winter mean Arctic Oscillation (AO) index. As a result of the unusual October conditions, some forecasters were highly confident that a negative AO index would prevail in winter and that the winter climate anomalies would reflect the negative AO pattern. However, the AO index persisted in the positive phase for most of the winter and became increasingly positive as the winter advanced, and forecasts based solely or primarily on the snow cover anomaly were badly incorrect over much of the Northern Hemisphere. In areas where these forecasts did correspond well to the observed outcome, such as the northeastern U.S., the success was for the wrong reason, because the AO phase was the opposite of what was expected.

2 2 The October snow cover anomaly was addressed in the November 2014 World Climate Service (WCS) forecast discussion and partly influenced the winter WCS forecast. However, the WCS approach is to construct a forecast based on a wide array of seasonal forecast guidance, and therefore the winter forecast was influenced by many additional predictors besides the snow cover anomaly. The following sections describe the forecasts, the observed outcome, and the primary guidance that was used, for North America and Europe. 2. Forecast for North America The forecast for winter temperature anomalies in North America included broad areas of cold conditions from Texas to the Southeast and in the northeastern United States (Figure 1). Above-normal temperatures were indicated for the West Coast, northern Rockies, and southwestern Canada. The forecast was quite successful (Figure 2). Figure 1. World Climate Service temperature anomaly forecast for North America for December 2014 February 2015.

3 3 Figure 2. Observed temperature anomaly for December 2014 February The World Climate Service typically makes use of seasonal dynamical model forecasts, but the skill of the models is variable and sometimes limited. Expert human interpretation is often able to discern which forecasts are less likely to be correct, and winter provides an example of this procedure. The November 2014 dynamical model forecasts for North America did not indicate any significant areas of below-normal temperatures, but this was regarded as extremely unlikely in the WCS forecast discussion. The forecast also noted an increased level of uncertainty in the dynamical model forecasts this month and that confidence [in the dynamical models] is low this month. In contrast to the dynamical models, statistical predictors based on historical analogs provided useful information for the WCS winter forecast. Several independent analogs pointed to colder than normal conditions in the South, Southeast, and/or Northeast, and warm conditions in the western U.S. In particular, a robust signal was obtained by examining past years in which the Pacific Decadal Oscillation (PDO) was significantly positive in December (Figure 3), which was deemed highly likely to occur when the forecast was made in early November. The October Eurasian snow anomaly also contributed to one of the analogs and produced a similar forecast signal. The WCS discussion noted that the analog temperature forecasts this month are in remarkable agreement and are unusually robust.

4 4 It should be noted that the winter forecast for North America was not based principally on an expectation for a negative AO phase, although the AO was expected to become negative in mid- to late winter. In contrast, the WCS summarized the expected pattern as follows: These influences will anchor a ridge over the Great Basin and a trough over the eastern Great Lakes. This pattern creates the possibility of occasional intrusive flows of high latitude cold air into the central and eastern U.S. Figure 3. Percentage of years with above normal temperature during December through February from analog years with a December PDO index greater than Forecast for Europe The WCS winter forecast for European temperature anomalies indicated a large area of abovenormal temperatures centered over west-central Europe, and colder than normal conditions were predicted over northern Russia and eastern Turkey and northern parts of the Middle East (Figure 4). The verification showed warmer than normal conditions north and east of the Low Countries, and thus the forecast was correct for the major populated areas of central and northern Europe (Figure 5). However, southern France was slightly cooler than normal, and the cold anomalies expected in northern Russia and in the far southeast did not occur.

5 5 Figure 4. World Climate Service temperature anomaly forecast for Europe for December 2014 February Figure 5. Observed temperature anomaly for December 2014 February 2015.

6 6 The forecast for Europe was based on the consensus of dynamical model and analog guidance as well as guidance concerning the expected NAO phase. A WCS analysis of the Eurasian snow cover analog revealed a surprising absence of a cold signal across Europe despite the fact that the winter mean AO index was negative in most of the snow cover analog years (Figure 6). The forecast discussion concluded that a robust Eurasian snow cover signal may not be a reliable indicator for winter conditions across Europe, even though the historical connection to the seasonal AO index is indisputable. Figure 6. Conditional climatology of December through February temperature anomalies in years since 1967 with above normal October Eurasia snow cover indices (SCE and SAI). In view of a broad array of analogs and agreement among the dynamical models, the WCS forecast stated that the WCS expects a positive NAO phase on average in December and January, but confidence is low for February. In agreement with a positive mean NAO pattern, a trough was expected to affect northwestern Europe, bringing above normal precipitation to the northwest and preventing cold continental air from having widespread or prolonged

7 7 influence ; this forecast was very successful (Figure 7). The snow cover analog was discussed as a risk factor that could favor increased high-latitude blocking and colder conditions in the second half of the winter, particularly in the northeast, but the balance of probabilities was clearly in favor of a warmer outcome over much of Europe. Figure 7. Observed sea-level pressure anomaly for December 2014 February Summary World Climate Service seasonal forecasts rely on a wide variety of predictors that are known to have skill in anticipating seasonal climate anomalies. By incorporating predictability from several independent sources, the WCS approach gravitates toward a consensus solution that usually represents the most probable outcome, and outlier predictors are unable to dominate the forecast. Winter provides a good illustration of the value of this approach, because forecasts that were based solely or mainly on the October Eurasian snow cover or the October Pattern Index were incorrect over much of the Northern Hemisphere.

8 8 The World Climate Service seasonal forecast product suite provides access to: - A broad array of independent predictors, including both dynamical and statistical guidance, and presented in probabilistic form - Objective and unique analog ensembles that compare current or recent conditions to past occurrences and reveal the most common outcomes - Expert subjective interpretation of available guidance and of risks to the forecast - A comprehensive suite of online tools that allows informed users to perform their own analysis and develop their own forecasts For more information, or to obtain a copy of the November 2014 WCS forecast document, contact Kevin Stenson at Kevin.Stenson@meteogroup.com.